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(Circulation. 1997;95:1877-1885.)
© 1997 American Heart Association, Inc.

Articles

Fast ²³Na Magnetic Resonance Imaging of Acute Reperfused Myocardial Infarction

Potential to Assess Myocardial Viability

Raymond J. Kim, MD; Joao A. C. Lima, MD; Enn-Ling Chen, BA; Scott B. Reeder, MS; Francis J. Klocke, MD; Elias A. Zerhouni, MD; Robert M. Judd, PhD

From the Johns Hopkins Medical Institutions (R.J.K., J.A.C.L., S.B.R., E.A.Z.), Baltimore, Md, the University of Pennsylvania (E.-L.C.), Philadelphia, and the Feinberg Cardiovascular Research Institute (F.J.K., R.M.J.), Northwestern University Medical School, Chicago, Ill.

Correspondence to Robert M. Judd, PhD, Feinberg Cardiovascular Research Institute, Northwestern University Medical School, 303 E Chicago Ave, Tarry 12-703, Chicago, IL 60611-3008. E-mail rjudd{at}nwu.edu

	Abstract

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Background The ability of the myocyte to maintain an ionic concentration gradient is perhaps the best indication of myocardial viability. We studied the relationship of ²³Na MRI intensity to viability and explored the potential of fast-imaging techniques to reduce ²³Na imaging times in rabbits and dogs.

Methods and Results Eighteen rabbits underwent in situ coronary artery occlusion and reperfusion. The hearts were then either imaged following isolation and perfusion with cardioplegic solution (n=6), imaged in vivo (n=6), or analyzed for ²³Na content and relaxation times (n=12). Normal rabbits (n=6) and dogs (n=4) were imaged to examine the effect of animal size on ²³Na image quality. ²³Na imaging times were 7, 11, and 4 minutes for isolated rabbits, in vivo rabbits, and in vivo dogs, respectively. Infarcted, reperfused regions, identified by triphenyltetrazolium chloride staining, showed a significant elevation in ²³Na image intensity compared with viable regions (isolated, 42±5%, P<.02; in vivo, 95±6%, P<.001), consistent with increased tissue sodium content. Similarly, ²³Na MR spectroscopy showed that [Na⁺] was higher in nonviable than viable myocardium (isolated, 99±4 versus 61±2 mmol/L; in vivo, 91±2 versus 38±1 mmol/L; P<.001 for both). Image signal-to-noise ratios were higher in dogs than rabbits despite shorter imaging times, primarily due to larger voxels.

Conclusions Following acute infarction with reperfusion, a regional increase in ²³Na MR image intensity is associated with nonviable myocardium. Fast gradient-echo imaging techniques reduce ²³Na imaging times to a few minutes, suggesting that ²³Na MR imaging has the potential to become a useful experimental and clinical tool.

Key Words: magnetic resonance imaging • myocardium • infarction • sodium

	Introduction

Top Abstract Introduction Methods Results Discussion References

 
Normal myocardial function depends on active cellular maintenance of electrochemical gradients. Disruption of these gradients indicates cellular injury or dysfunction. In viable myocytes, [Na⁺]_i is much lower than [Na⁺]_o due to active outward transport of sodium by Na⁺/K⁺-ATPase.^{1 2} During ischemia, [Na⁺]_i rises steadily in a nearly linear fashion, reaching two- to fivefold increases over 20 to 30 minutes.^{3 4 5 6 7} Upon reperfusion, [Na⁺]_i returns to near baseline levels within minutes,^{3 5 6 7} unless the ischemic insult was prolonged and caused irreversible injury, in which case sodium concentrations remain elevated.⁸ This sequence of events suggests that measurement of tissue sodium content can be used to distinguish viable from nonviable myocardium after ischemic injury.

In theory, ²³Na MRI allows for direct noninvasive examination of myocardial sodium content. Unfortunately, practical implementation of sodium MRI has been limited due to several difficulties. First, the overall MR sensitivity of sodium in the human body is roughly 4 orders of magnitude lower than the sensitivity of protons.^{9 10} Second, the short transverse relaxation times of sodium^{11 12} require short echo-time images that can lead to gradient hardware constraints.¹³ Third, in vivo discrimination between intracellular and extracellular sodium signals is problematic since presently used paramagnetic shift reagents chelate calcium and magnesium¹⁴ and can be toxic.^{15 16} Attempts to separate intracellular from extracellular sodium using differences in relaxation characteristics or multiple quantum filter techniques^{7 17} have been hampered by contamination from a component of extracellular sodium that has similar relaxation characteristics to intracellular sodium¹² and gives rise to multiple quantum signals.^{18 19}

Nevertheless, Cannon et al¹⁰ have shown that ²³Na image intensity is elevated in myocardial regions subject to ischemia and reperfusion. Their data suggest that regional determination of myocardial viability may not require differentiation of intra- and extracellular sodium signals. Since myocardial tissue volume is primarily intracellular (75% of the water space²⁰ ), a weighted average of intra- and extracellular sodium is much lower than the extracellular level under normal conditions. For example, assuming that [Na⁺]_i=15 mmol/L, [Na⁺]_o=145 mmol/L, and 77% of myocardial tissue is water space,²⁰ then the composite concentration would equal 37 mmol/L, ie, 0.77x([0.75x15]+[0.25x145])=37. In the extreme case in which all myocytes within a nonviable region failed to maintain a sodium concentration gradient, the tissue sodium concentration would reach the extracellular level, an increase of >200%, ie, [(0.77x145)-37]/37=202%, above viable myocardium. This difference is likely large enough to be detected on ²³Na images.¹⁰

The primary focus of the present study was to establish the relationship of regional changes in sodium image intensity to myocardial viability. Our approach was to correlate regional image intensity, both in isolated hearts and in vivo, with myocardial viability as determined by TTC staining techniques, regional differences in sodium content measured by using ²³Na MRS, and regional differences in sodium T₁ and T₂ relaxation times. In addition, we applied recently developed rapid gradient-echo techniques used for proton imaging^{21 22 23} to the sodium nucleus. The rationale was to explore methods of reducing ²³Na imaging times to a level that would allow ²³Na imaging to become a practical experimental and clinical tool.

	Methods

Top Abstract Introduction Methods Results Discussion References

 
To examine ²³Na image intensity in viable and nonviable regions, in situ myocardial infarction and reperfusion were followed by ²³Na imaging of either isolated (n=6) or in vivo (n=6) rabbit hearts. To examine regional sodium content, ²³Na MRS was performed on tissue samples from all six in vivo rabbit hearts and an additional six isolated rabbit hearts subjected to the same infarction/reperfusion protocol. In four of the six isolated rabbit hearts, ²³Na relaxation times (T₁ and T₂) were also measured in the tissue samples. To evaluate image quality in both small and large animals, in vivo ²³Na images of normal rabbits (n=6) and dogs (n=4) were acquired. All in vivo ²³Na MRI was performed in closed-chest, cardiac-gated animals.

Experimental Preparation
The care and treatment of all animals involved in this study were in accordance with the Position of the American Heart Association on Research Animal Use, adopted November 15, 1984.

Isolated Hearts
New Zealand White rabbits (3.5 to 4.0 kg) were anesthetized with sodium pentobarbital (27 mg/kg IV), intubated, and mechanically ventilated. A median sternotomy was performed, and a reversible snare ligature was placed around an anterior branch of the left coronary artery. After 40 minutes of in situ occlusion followed by 60 minutes of reperfusion, the hearts were rapidly excised and perfused in a retrograde manner with cardioplegic solution at room temperature. An epicardial marker (a 2-mm-diameter polyethylene tube filled with saline) was attached to the RV at the same base-to-apex level as the infarct territory. Pressure was adjusted at the beginning of the experiment to obtain a flow of 10 mL/min (1.0 to 1.5 mL·min^-1·g^-1 as measured with an in-line electromagnetic flowmeter, model 1401, Skalar Medical) and then held constant. Typical perfusion pressures were 35 to 45 mm Hg. The perfusate was not recirculated. Perfusate composition was (in mmol/L) Na⁺ 120, K⁺ 16, Mg²⁺ 16, Cl^- 160, and HCO₃^- 10.²⁴ The perfusate was equilibrated with 95% O₂ and 5% CO₂ to maintain pH at 7.4 to 7.53.²⁵ We have shown^{24 26} that hearts isolated in this manner remain viable. The hearts were hung vertically in a 30-mm-diameter RF volume coil and placed in the magnet.

In Vivo Hearts
New Zealand White rabbits were anesthetized with ketamine 50 mg/kg IM and xylazine 2.5 mg/kg IM, intubated, and mechanically ventilated. A catheter was placed in the femoral artery to monitor systemic pressure. A left thoracotomy was performed at the fifth intercostal space. A deflated 2-mm angioplasty balloon catheter was loosely sutured around an anterior branch of the left coronary artery. An epicardial marker filled with saline was placed over the territory perfused by the artery, and a catheter was placed in the left atrium for injection of 15-µm fluorescent microspheres (Molecular Probes). The chest was then closed in two layers, and the rabbits were placed prone on a 5-cm-diameter double-resonant ²³Na-¹H surface RF coil and placed in the magnet. This approach allowed coronary artery occlusion and reperfusion to be performed closed-chest in the magnet by inflating and deflating the balloon.

MRI and Experimental Protocol
All images were acquired on a GE/Bruker 4.7 T Omega system using a gradient-echo pulse sequence²⁷ that used basic features of GRASS.^{22 23} For isolated hearts, the sequence was run continuously. For in vivo imaging, cardiac-gated, segmented k-space data acquisition was used. To decrease ²³Na imaging times, half-period sinusoid gradients were used for many of the gradient waveforms, including the slice-select gradient.^{27 28} In addition, the slice refocus, phase encode, and readout prephaser gradient lobes were chosen to overlap completely and had a minimum duration determined by the maximum gradient strength and the lobe that required the greatest area. Partial-echo acquisition^{27 29 30} was employed to further reduce TR and TE. Different gradient sets were used for rabbit and dog imaging. For rabbits, the maximum gradient slew rates and amplitudes were 19.5 G·cm^-1·ms ^-1 and 3.9 G/cm, respectively. For dogs, the corresponding values were 6 G·cm^-1·ms ^-1 and 1.2 G/cm.

Isolated Hearts
A test tube filled with normal saline ([Na⁺]=154 mmol/L) was placed adjacent to the heart for signal calibration. LV short-axis ²³Na images were acquired by using the epicardial marker to locate the appropriate slice. Imaging time was 7.1 minutes; TE, 4.6 ms; TR, 13 ms; N_AVG, 256; matrix size, 256x128; and voxel size, 0.6x1.2x4.5 mm. Imaging was performed at the Ernst angle (see "Discussion"), which was determined empirically. After MR imaging, the short-axis slice identified by the epicardial marker (4 mm thick) was incubated in a 1% TTC solution at 37°C to 40°C for 15 minutes. Since TTC forms a red precipitate in the presence of intact dehydrogenase enzyme systems and reducing coenzymes, viable myocardium stains brick red, whereas necrotic areas fail to stain.^{31 32} The TTC-stained myocardial slice was photographed, and the resultant 35-mm slides were digitally scanned for subsequent analysis.

In Vivo Hearts
Femoral artery pressure was used for cardiac gating. Double-oblique, short- or long-axis ¹H images were first acquired by using the epicardial marker to identify the to-be-infarcted territory. The RF coil was then tuned to the ²³Na frequency, and a control ²³Na image was acquired at the same location. ²³Na imaging time was 11 minutes with 16 phase encodes per cardiac cycle (gated to end diastole); TE, 4.6 ms; TR, 13 ms; N_AVG, 256; matrix size, 256x128; and voxel size, 1.25x2.5x6 mm. Heart rate in these anesthetized rabbits was 180 bpm. A control set of microspheres was injected into the left atrium. The balloon catheter was then inflated to produce coronary artery occlusion for 40 minutes, a second set of microspheres was injected, and another ²³Na image was acquired. The balloon was then deflated to allow reperfusion, a third set of microspheres was injected, and another ²³Na image was acquired. After 60 minutes of reperfusion, a final set of microspheres was injected, and a final ²³Na image was acquired. The hearts were then removed and sectioned at the level of the epicardial marker. One side of the heart was stained with TTC to verify the location and extent of infarction. The other side of the heart was used to obtain tissue samples from infarcted and normal regions for spectroscopic analysis of sodium content (see "MRS") and microsphere flow determination.

Normal Animals
To explore the clinical potential of sodium imaging (see "Discussion"), we acquired in vivo ²³Na images in normal rabbits (n=6) and dogs (n=4). Normal rabbits were imaged by using the same methodology for animals subject to infarction. Normal mongrel dogs (20 to 25 kg) were anesthetized with 35 mg/kg IV sodium pentobarbital and intubated. After a femoral catheter was inserted for cardiac gating, the animals were placed in the left antecubital position on a 15-cm-diameter double-resonant ²³Na-¹H surface coil and placed in the magnet. The same pulse sequence was used for dogs. Double-oblique short-axis ¹H images were first acquired and then followed by ²³Na images at the same location. For the dogs, imaging time was 4 minutes with 32 phase encodes per cardiac cycle (gated to end diastole); TE, 3.9 ms; TR, 8.1 ms; N_AVG, 128; matrix size, 256x128; and voxel size, 3x6x25 mm. Heart rate in these anesthetized dogs was 120 bpm.

Image Analysis
Isolated Hearts
Since the epicardial marker had guided the selection of both the TTC-stained and MR slice, spatial correlation of the two images was undertaken. For each heart, the LV on the digitized TTC-stained image was traced by two independent observers using the software package NIH Image on a Macintosh Quadra. The nonviable (TTC-negative) region was also traced. These outlines were superimposed over the MR image, which was scaled and rotated appropriately to match the LV borders. The TTC-negative outline was then used to draw a comparable region on the MR image. In all cases the region of altered signal intensity on the MR image was similar in size and location to the region of abnormal TTC staining. However, since the LV borders on the MR image were not identical to the TTC image and did not perfectly overlie it, observers were instructed to include myocardial regions with obviously altered signal intensities. ROIs were also selected from remote viable regions of myocardium. Signal intensities were normalized to the saline standard and averaged for the two observers.

In Vivo Hearts
For in vivo hearts subject to infarction, ROIs were placed over the infarcted territory (identified by the external marker and postmortem TTC staining) and an adjacent viable region. In normal animals, ROIs were placed in the anterior myocardium, LV cavity, and posterior myocardium to calculate SNRs at these locations.

All Hearts
SNRs were determined by using Henkelman's method for magnitude images.³³

MRS
Tissue samples (350 to 750 mg) were taken from nonviable and viable regions. The nonviable region, distal to the coronary occlusion site, was easily identified by discoloration and the presence of intramyocardial hemorrhage. The tissue samples were blotted dry to remove surface contamination. The circumferential margins of the samples were trimmed 2 mm in case capillary action may have removed tissue water. Samples were weighed and placed in sealed glass tubes. Na⁺ concentrations of the tissue samples were determined spectroscopically by comparison with the Na⁺ signal of a reference standard. The standard consisted of a sealed glass tube filled with 1 mL of a solution containing 109 mmol/L Na⁺ and 43.5 mmol/L Dy-TTHA.¹⁴ Dy-TTHA was used to shift the ²³Na peak of the standard such that two ²³Na peaks would appear in the spectrum: one peak from the glass tube containing the tissue and one peak from the glass tube containing the Dy-TTHA standard. Care was taken to place the tissue sample and the adjacent standard entirely within the RF coil. ²³Na spectra were acquired by using a 90° pulse (45 µs), a preacquisition delay of 58 µs, a data size of 1 K, an acquisition time of 100 ms, 512 averages, and a repetition rate of 250 ms to allow complete relaxation between pulses. Tissue [Na⁺] was calculated as (area under tissue peak/area under standard peak)x109 mmol/Lx(1 g/tissue sample weight). The results are expressed in millimoles per liter Na⁺ and assume 100% visibility for all sodium signals (see "Sodium Visibility"). The spectroscopic method described above was validated by measuring sodium concentration in five test tubes containing known concentrations of sodium. The r value relating known to measured [Na⁺] was .99.

T₁ and T₂ Determination
Relaxation times were measured for total (intracellular plus extracellular) myocardial sodium. T₁ values were obtained by using standard inversion recovery, with inversion times ranging from 1 to 250 ms (9 data points). A Hahn spin-echo pulse sequence with echo times ranging from 0.5 to 40 ms was used to measure T₂ (11 data points). For both experiments there were 64 averages with a predelay of 250 ms. All signals were analyzed as the area under the peak in the frequency domain. The T₁ data were fit to a single exponential. The T₂ data were fit to a double exponential by using the equation M=M_fast e^{(-TE/T_2fast)}+M_slow e^{(-TE/T_2slow)} (1)

where M_fast (M_slow) represents the magnitude of the signal with time constant T_2fast (T_2slow). The sum of the coefficients M_fast and M_slow was constrained to equal 1.0.

Tissue Water Content
In the isolated hearts, the wet/dry weights of viable and nonviable myocardium were measured by desiccation in a heating oven at 50°C for at least 36 hours to determine if differences in tissue sodium content could by explained by edema.

Statistical Analysis
All results are expressed as mean±SEM. Differences between viable and nonviable myocardium in image intensity, sodium content, tissue water content, and relaxation characteristics were assessed by using unpaired t tests. The hypothesis that image intensities in the same region of the same heart varied before, during, and after coronary artery occlusion was assessed by using repeated-measures ANOVA.³⁴ MRI and MRS results were compared by using an unpaired t test for isolated hearts and a paired t test for in vivo hearts. Values of P<.05 were considered significant.

	Results

Top Abstract Introduction Methods Results Discussion References

 
Imaging Results
Fig 1 shows typical ²³Na images in four different isolated hearts with their corresponding TTC-stained slices. Fig 2 shows similar results in a heart imaged in vivo. Fig 3 shows microsphere flow for the in vivo animals, demonstrating successful coronary artery occlusion and reperfusion closed-chest in the magnet. In all cases, the ²³Na signal was sufficient to generate clear images of both LV and RV walls. As expected, the structures with the highest sodium concentration (ie, the ventricular cavity filled with saline perfusate or blood) had the highest image intensity. In comparison, viable myocardium had less signal (48±5% of saline [isolated hearts] and 50±3% of blood [in vivo hearts]), consistent with active transport of Na⁺ out of the myocyte. In nonviable regions, identified by the lack of TTC staining, Na⁺ image intensity was greater than in viable regions both in isolated (Fig 1) and in vivo (Fig 2) hearts, consistent with intracellular accumulation of Na⁺.

View larger version (52K): [in this window] [in a new window]   Figure 1. 23Na MR images (top) of four different isolated rabbit hearts after acute infarction and reperfusion with corresponding TTC-stained sections (bottom). Each image is a short-axis cross-sectional slice with RV at the left and anterior surface at the top. RVs were removed before TTC staining in some specimens (A and D, bottom). Accumulation of perfusate in LV cavities led to bright cavities (A and C, top) unless a basal slice above the perfusate level was imaged (B and D, top). Red arrows indicate saline standard; green arrows, either areas with increased image intensity (top) or TTC-negative, nonviable regions (bottom). Note visual correlation of regions with increased image intensity with nonviable regions.

View larger version (42K): [in this window] [in a new window]   Figure 2. In vivo, long-axis 23Na MR image of a rabbit heart (left) with corresponding TTC-stained section (right). Nonviable region has elevated myocardial 23Na image intensity (arrows).

View larger version (46K): [in this window] [in a new window]   Figure 3. Bar graph of microsphere blood flow in in vivo rabbits demonstrating successful closed-chest occlusion and reperfusion performed in the magnet without moving the animals. Data are expressed as percent of blood flow in remote (viable) regions.

We acquired Na⁺ images of the to-be-infarcted territory before, during, and after coronary artery occlusion in three of the six in vivo animals. In the remaining three animals, our a priori estimate of the location of the to-be-infarcted territory was incorrect, and only postreperfusion Na⁺ image data were acquired. Fig 4 shows in vivo Na⁺ images from the same myocardial slice of one animal before, during, and after coronary occlusion. Before occlusion, image intensity in the to-be-infarcted territory was similar to adjacent viable regions. During occlusion, Na⁺ image intensity within the territory decreased by 24% in this animal. After 1 hour of reperfusion, image intensity within the territory increased by 96%.

View larger version (135K): [in this window] [in a new window]   Figure 4. In vivo long-axis 23Na images before (upper left) and during (upper right) coronary occlusion and 10 minutes (lower left) and 1 hour (lower right) after reperfusion. Apex is to the right. In this animal, 23Na image intensity within the territory (outlined section) decreased by 24% during occlusion and increased by 96% after 1 hour of reperfusion.

Fig 5 summarizes the image intensity results. In isolated hearts, image intensity was 42±5% greater in nonviable than viable myocardium (P<.02). For in vivo hearts, the elevation was 95±6% (P<.001). For the three hearts in which Na⁺ images were acquired before, during, and after coronary artery occlusion, Na⁺ image intensity fell by 22±4% (P<.05) during occlusion and rose by 104±8% (P<.001) after 1 hour of reperfusion.

View larger version (41K): [in this window] [in a new window]   Figure 5. Bar graph summarizes 23Na image intensity in viable (TTC-positive) and nonviable (TTC-negative) myocardium in all isolated rabbit hearts (n=6), all in vivo rabbit hearts (n=6), and in vivo rabbit hearts with images acquired at the same location before, during, and after closed-chest coronary artery occlusion (n=3). Intensity was normalized to either remote (viable) regions or preocclusion intensity.

Spectroscopy Data
Fig 6 shows the composite ([Na⁺]_i+[Na⁺]_o) sodium concentration of nonviable compared with viable myocardium for the isolated and in vivo hearts. Sodium content was significantly higher in nonviable (isolated, 99±4 mmol/L; in vivo, 91±2 mmol/L) than viable (isolated, 61±2 mmol/L; in vivo, 38±1 mmol/L; P<.001 for both) tissue. The elevation in sodium concentration between nonviable and viable myocardium measured by spectroscopy (isolated, 63±8%; in vivo, 142±7%) was larger than the elevation in image intensity measured by MRI (isolated, 42±5%; in vivo, 95±6%; P<.05 for both).

View larger version (44K): [in this window] [in a new window]   Figure 6. Bar graph summarizes sodium content as measured by spectroscopy in isolated hearts and postmortem tissue samples from in vivo hearts. [Na+] values are a composite of [Na+]i and [Na+]o. In each case, nonviable tissue [Na+] was higher than viable tissue [Na+].

Relaxation Parameters
All T₁ relaxation data were well characterized by a single exponential decay. The measured T₁ and T₂ values of the Dy-TTHA standard were nearly the same with each experiment, demonstrating the reproducibility of the measurements. The mean T₂ of the Dy-TTHA standard was near the mean T₁ value, although it was slightly decreased (28.2±0.3 versus 29.2±0.3 ms; P<.001). Likewise, the slow component of T₂ for both nonviable (21.9±1.2 ms) and viable (31.5±0.8 ms) tissue approached the T₁ values (26.2±1.5 and 34.2±0.9 ms, respectively), although they were consistently less (P<.01 for both). The T₁ of nonviable tissue (26.2±1.5 ms) was significantly shorter than that of viable tissue (34.2±0.9 ms; P<.005). Similarly, both the fast and slow components of nonviable tissue T₂ were shorter than viable tissue T₂, although only differences in T_2slow reached significance (T_2fast, 2.2±0.2 versus 3.6±0.6 ms, NS; T_2slow, 21.9±1.2 versus 31.5±0.8 ms, P<.001). The magnitude of the fast component as a percentage of the total signal did not differ significantly between nonviable and viable tissue (26±5% versus 22±1%).

Tissue Water Content
In the isolated hearts, there were no significant differences in tissue water content between nonviable and viable myocardium (84±0.3 versus 85±0.3 percent water by weight, respectively).

SNR Measurements
In vivo sodium images of normal rabbits were similar to those in animals subjected to infarction (Figs 2 and 4). Fig 7 shows in vivo proton and sodium images of a normal canine heart. Since a surface coil was used for in vivo imaging, the anterior LV myocardium had a higher SNR than the posterior myocardium (rabbits: anterior, 12±1; posterior, 8±1; dogs: anterior, 20±3; posterior, 11±2). We obtained in vivo sodium images with a shorter imaging time in dogs than rabbits (4 versus 11 minutes) and almost twice the SNR, most likely because larger animals allow the use of larger voxels. The imaging parameters in dogs were chosen to allow estimations of voxel sizes, imaging times, and image quality in humans (see "Potential for Clinical Application").

View larger version (62K): [in this window] [in a new window]   Figure 7. In vivo, closed-chest, gated MR images of a canine heart at 4.7 T. A, Short-axis, double-oblique proton image of the heart with chest wall at right. B, Sodium image at the same location. The 23Na image was acquired in 4 minutes.

	Discussion

Top Abstract Introduction Methods Results Discussion References

 
Relationship of Image Intensity to Myocardial Viability and Sodium Content
This is the first study to compare sodium image intensity, both in isolated hearts and in vivo, with TTC-determined myocardial viability and myocardial sodium content by spectroscopy. The data establish that there is a physiological basis for the application of ²³Na MRI to investigate myocardial viability.

Image intensity of nonviable myocardium was 42±5% higher than that of viable myocardium in isolated hearts and 95±6% higher in vivo. Our spectroscopy results showed that nonviable tissue had on average a 63±8% increase in [Na⁺] compared with viable tissue in isolated hearts and a 142±7% increase in vivo, strongly suggesting that the differences in image intensity were due to differences in myocardial [Na⁺]. Similarly, studies have also shown increased tissue [Na⁺] in infarcted, reperfused myocardium by in vitro techniques such as flame emission photometry.^{8 10} Our finding that image intensity differences were smaller than postmortem sodium concentrations could be explained by differences in tissue ²³Na relaxation characteristics. We found that both T₁ and T₂ were shorter in infarcted myocardium. For our imaging pulse sequence, shorter T₁ and T₂ in nonviable myocardium would have opposite effects on image intensity, with shorter T₁ increasing image intensity and shorter T₂ (along with T₂*) decreasing image intensity. Nevertheless, despite the fact that our images (Figs 1, 2, and 4) have some T₁, T₂, and T₂* weighting, the data suggest that regional differences in tissue sodium concentration (Fig 6) are so large that tissue [Na⁺] dominates image intensity. Partial volume effect, in which relatively large imaging voxels contain both nonviable and viable myocardium, is an additional factor that could lead to smaller differences in image intensity compared with differences in tissue [Na⁺].

Earlier, we estimated that a voxel of normal myocardium would have a sodium concentration of 37 mmol/L, assuming 77% of the tissue is water, 75% of the water space is intracellular,²⁰ [Na⁺]_i=15 mmol/L, and [Na⁺]_o=145 mmol/L. The spectroscopy results for the in vivo experiments showed virtually the same value (38±1 mmol/L; Fig 6). In isolated hearts, however, the value was 61±2 mmol/L. The elevation in [Na⁺] in isolated hearts was likely due to edema formation,³⁵ as suggested by our measurement of 85±0.3% tissue water content in isolated hearts compared with 77% in vivo.²⁰ Elevated [Na⁺]_i in the isolated hearts may also have contributed.³⁵ In nonviable myocardium, [Na⁺] was 99±4 mmol/L in isolated hearts and 91±2 mmol/L in vivo. These values are close to the value one would estimate assuming all myocytes in the nonviable region failed to maintain a sodium concentration gradient, namely 112 mmol/L (0.77 · 145=112, which assumes 77% of tissue is water and plasma [Na⁺]=145 mmol/L).

Increases in tissue Na⁺ in nonviable regions, however, require sodium delivery via microvascular perfusion. Jennings et al⁸ have clearly shown that in infarcted tissue without reperfusion several hours may pass before the total tissue sodium rises since electrolyte delivery would depend on slow ion diffusion. Figs 4 and 5 show that ²³Na image intensity actually decreases (22±4%, P<.05) during complete ischemia, perhaps secondary to decreases in vascular and/or interstitial volumes (which contain high [Na⁺]) caused by reduced perfusion. Although "no-reflow"^{36 37 38} zones in the core of the infarct could also limit electrolyte delivery to infarcted myocardium, recent studies suggest that regional no reflow due to microvascular damage or stasis from intravascular neutrophil accumulation is a progressive phenomenon that develops during the reperfusion period in areas that initially received adequate reperfusion.^{36 37}

Potential Limitations
Sodium Visibility
Quadrupolar interactions of the ²³Na nucleus could lead to sufficient homogeneous and/or heterogeneous broadening of the outer transition lines to cause a proportion of ²³Na to be undetectable.¹¹ While some studies have shown intracellular sodium to be totally visible in perfused hearts,^{39 40} others have concluded that a significant proportion is invisible.^{5 41} Although the issue of sodium "visibility" makes it difficult to offer a strictly quantitative interpretation of image intensity and/or spectroscopically determined [Na⁺], our data demonstrate a clear relationship between ²³Na image intensity and myocardial viability.

Effects of Extracellular Sodium on ²³Na Image Intensity
In the present study, the composite sodium signal was obtained without differentiation between intracellular and extracellular signals. In general, relating ²³Na image intensity to myocardial viability is considerably complicated by the contribution of extracellular Na⁺ to image intensity. For example, because [Na⁺]_o is normally much greater than [Na⁺]_i, even a small increase in extracellular volume due to edema may significantly elevate ²³Na image intensity even though [Na⁺]_i remains near normal levels. However, in vivo elevations in myocardial image intensity of nearly 100% and postmortem [Na⁺] of nearly 150%, as found in the present study, would be difficult to explain without postulating substantial intracellular accumulation of Na⁺.

Gradations in Myocardial Injury
Although the focus of the present study was to differentiate between viable and nonviable myocardium, it is possible that there are gradations of myocardial injury such that a border zone of damaged but viable myocytes may surround an infarcted zone. Since these border zones may have intermediate image intensities, we performed our ²³Na ROI analysis unblinded using superimposed TTC-guided outlines to strictly compare only completely infarcted, nonviable regions with remote normal regions. Clearly, further studies will be essential to clarify the relationship between ²³Na image intensity and reversible levels of injury in viable myocytes.

Specialized Pulse Sequences for Sodium Imaging
The relatively small sodium MR signal is primarily due to low tissue concentrations (15 to 145 versus 110 000 mmol/L for protons) and is the chief difficulty in producing ²³Na MR images of the heart. In addition, the short transverse relaxation of sodium (especially T_2fast) can lead to further signal loss unless specialized, short TE imaging techniques are used. Investigators have employed a variety of techniques to allow sodium imaging despite the low inherent signal. DeLayre et al⁴² produced gated ²³Na images of isolated, perfused rat hearts in 15 minutes at 8.45 T using projection-reconstruction,⁴³ but the low SNR did not allow visualization of the myocardium. Cannon et al¹⁰ obtained ²³Na images of ex vivo canine hearts after coronary occlusion and reperfusion at 2.7 T and clearly demonstrated increased signal intensity in nonviable regions. A three-dimensional Fourier transform technique employing two spin echoes was used to increase SNR, but imaging times were 3 to 4 hours. ²³Na imaging of the in vivo human heart has been described by Ra et al,⁴⁴ who used a specialized RF coil and three-dimensional projection-reconstruction at 1.5 T. Unfortunately, with a total scan time of 70 minutes, poor image quality did not allow the myocardium to be clearly visualized.

Our study is the first to apply recently developed fast-imaging techniques, originally developed for proton imaging, to the sodium nucleus. Using this approach, ²³Na imaging times were reduced to a few minutes, with a sufficient SNR to examine regional differences in myocardial sodium content. The main features that allowed a reduction in imaging time are (1) gradient echoes, (2) fractional echoes, (3) extremely short TRs, and (4) imaging at the Ernst angle. Although the T₁ of sodium can allow a short TR for signal averaging, previous studies have not attempted partial flip-angle, gradient-echo imaging with an extremely short TR. We hypothesized that signal could be gained by fast gradient-echo imaging since the short T₁ of sodium would allow large tip-angle excitations even for fast pulse repetition times. For example, consider a spoiled-GRASS sequence, the theoretical signal intensity of which is given by

(2)

where is the flip angle.²² Fig 8 (solid lines) shows the solution to this equation for a TR of 13 ms and T₁ values similar to those of protons and sodium at 4.7 T (1400 and 30 ms, respectively). Note that at the peak of the sodium curve the gradient-echo sequence collects 48% of the total theoretical ²³Na signal every 13 ms compared with only 8% for ¹H, a sixfold increase. Computer simulation of the phenomenological Bloch equations^{27 45} for steady-state coherent sequences such as GRASS showed a similar increase in signal (Fig 8, dashed lines). Furthermore, compared with classic spin-echo ²³Na imaging with one phase encode per cardiac cycle, gradient-echo techniques allow the acquisition of multiple phase encodes per cardiac cycle (16 for in vivo rabbits) and therefore significantly improve the time efficiency of data collection.

View larger version (26K): [in this window] [in a new window]   Figure 8. Plot of steady-state signal (percent of maximum, %Mo) for spoiled-GRASS (SPGR; ) and GRASS (...) as a function of flip angle for T1 values characteristic of protons and sodium at 4.7 T (1400 and 30 ms, respectively). The curves assume TE=0 and TR=13 ms. The GRASS curves also assume a uniform phase dispersion of isochromats and T2 values of 60 and 20 ms for protons and sodium, respectively. Note that the angle at which signal is maximized (Ernst angle) is much larger for sodium (50° vs 8° for spoiled-GRASS; 59° vs 16° for GRASS). The observation that peak signal is approximately sixfold higher for sodium compared with protons suggests that fast gradient-echo techniques are much more efficient for sodium than for proton imaging.

Potential for Clinical Application
For ²³Na MRI to be clinically useful, it would be necessary to acquire sodium images of the heart with voxel dimensions a few millimeters on each side and imaging times of a few minutes. Superficially, these requirements would appear very difficult to meet in light of the fact that the sodium MR signal is 10 000 times smaller than that of protons. The results of this study, however, suggest that the combination of approaches used here may result in an increase in signal sufficient to achieve the requirements for clinical sodium imaging.

First, we increased signal by working at a higher field strength (4.7 T) than conventional scanners (1.5 T). If we assume that noise is dominated by losses in the RF receiver coil, then the SNR increases with frequency to the 7/4 power.⁴⁶ If noise is dominated by sample losses, then the SNR increases only linearly.⁴⁶ Assuming an intermediate frequency dependence of 3/2 power, the SNR is fivefold higher at 4.7 T than at 1.5 T. Second, voxel volume was at least 15-fold higher than that routinely used with proton imaging, corresponding to a 15-fold increase in signal. Third, we signal averaged 256 echoes. Since the SNR varies with the square root of the number of averages, we obtained an additional 16-fold increase in signal. Finally, if we add a sixfold increase in signal due to the use of fast-imaging techniques applied to the sodium nucleus (Fig 8), we find that we have improved the SNR by nearly 4 orders of magnitude (5x15x16x6=7200).

The results of our in vivo canine ²³Na imaging experiments (Fig 7) are consistent with the above simple analysis. We purposely used a surface coil that was too large for the dog but reasonable for humans (15 cm), chose voxel sizes similar to those that might be useful clinically (3x6x25 mm), and acquired ²³Na images in 4 minutes. Image SNR was similar to routine clinical proton images (20±3 in anterior myocardium), strongly suggesting that existing high-field (4 T), whole-body magnets could be used to produce ²³Na MR images of the human heart with modest trade-offs in imaging time (minutes) and spatial resolution (voxel dimensions ³15=2.5 times larger than protons).

Summary
We conclude that fast gradient-echo imaging techniques applied to the sodium nucleus can produce images of the heart that allow direct assessment of myocardial viability on a regional basis. We further conclude that these techniques can reduce ²³Na imaging times to a few minutes, suggesting that ²³Na MR imaging has the potential to become a useful experimental and clinical tool.

	Selected Abbreviations and Acronyms

 

Dy-TTHA = dysprosium triethylenetetramine–hexaacetic acid GRASS = gradient-recalled acquisition in the steady state LV = left ventricle MRS = magnetic resonance spectroscopy [Na+]i = intracellular sodium concentration [Na+]o = extracellular sodium concentration RF = radio frequency ROI = region-of-interest RV = right ventricle SNR = signal-to-noise ratio TE = echo time TR = relaxation time TTC = triphenyltetrazolium chloride

	Acknowledgments

 
This work was supported by a biomedical engineering research grant from the Whitaker Foundation (R.M.J.), the Frank T. McClure Fellowship in Cardiovascular Research (R.M.J.), NIH-NHLBI R29-HL53411 (R.M.J.), NIH-NHLBI T32-HL07227 (R.J.K.), and a visiting scientist award to R.J.K. from the Feinberg Cardiovascular Research Institute, Northwestern University Medical School, Chicago, Ill. The authors wish to thank Elliot R. McVeigh, PhD, for contributing important parts of the imaging pulse sequence used in this study, and Alice Wyrwicz, PhD, Ying J. Shen, MD, and the rest of the staff at the Center for MR Research in Evanston, Ill, for their help with the in vivo rabbit experiments.

Received July 29, 1996; revision received November 14, 1996; accepted November 25, 1996.

	References

Top Abstract Introduction Methods Results Discussion References

 

1. Gadsby DC. The Na/K pump of cardiac myocytes. In: Zipes DP, Jalife J, eds. Cardiac Electrophysiology: From Cell to Bedside. Philadelphia, Pa: WB Saunders Co; 1990:35-50.
   
2. Sperelakis N. Origin of the cardiac resting potential. In: Berne RM, Sperelakis N, Geiger SR, eds. Handbook of Physiology, Section 2: The Cardiovascular System. Bethesda, Md: American Physiological Society; 1979:187-267.
   
3. Pike MM, Kitakaze M, Marban E. ²³Na-NMR measurements of intracellular sodium in intact perfused ferret hearts during ischemia and reperfusion. Am J Physiol. 1990;259:H1767-H1773. [Abstract/Free Full Text]
   
4. van Echteld CJA, Kirkels JH, Eijgelshoven MHJ, van der Meer P, Ruigrok TJC. Intracellular sodium during ischemia and calcium-free perfusion: a ²³Na NMR study. J Mol Cell Cardiol. 1991;23:297-307. [Medline] [Order article via Infotrieve]
   
5. Malloy CR, Buster DC, Castro MMCA, Geraldes CFGC, Jeffrey FMH, Sherry AD. Influence of global ischemia on intracellular sodium in the perfused rat heart. Magn Reson Med. 1990;15:33-44. [Medline] [Order article via Infotrieve]
   
6. Tani M, Neely JR. Deleterious effects of digitalis on reperfusion-induced arrhythmias and myocardial injury in ischemic rat hearts: possible involvements of myocardial Na⁺ and Ca²⁺ imbalance. Basic Res Cardiol. 1991;86:340-354. [Medline] [Order article via Infotrieve]
   
7. Hutchison RB, Huntley JJA, Zhou HZ, Ciesla DJ, Shapiro JI. Changes in double quantum filtered sodium intensity during prolonged ischemia in the isolated perfused heart. Magn Reson Med. 1993;29:391-395. [Medline] [Order article via Infotrieve]
   
8. Jennings RB, Sommers HM, Kaltenbach JP, West JJ. Electrolyte alterations in acute myocardial ischemic injury. Circ Res. 1964;14:260-269. [Abstract/Free Full Text]
   
9. Narayana PA, Kulkarni MV, Mehta SD. NMR of ²³Na in biological systems. In: Partain CL, Price RR, Patton JA, Kulkarni MV, James AE Jr, eds. Magnetic Resonance Imaging. Philadelphia, Pa: WB Saunders Co; 1988:1553-1563.
   
10. Cannon PJ, Maudsley AA, Hilal SK, Simon HE, Cassidy F. Sodium nuclear magnetic resonance imaging of myocardial tissue of dogs after coronary artery occlusion and reperfusion. J Am Coll Cardiol. 1986;7:573-579. [Abstract]
    
11. Berendsen HJC, Edzes HT. The observation and general interpretation of sodium magnetic resonance in biological material. Ann N Y Acad Sci. 1973;204:459-485. [Medline] [Order article via Infotrieve]
    
12. Foy BD, Burstein D. Characteristics of extracellular sodium relaxation in perfused hearts with pathologic interventions. Magn Reson Med. 1992;27:270-283. [Medline] [Order article via Infotrieve]
    
13. Ra JB, Hilal SK, Cho ZH. A method for in vivo MR imaging of the short T₂ component of sodium-23. Magn Reson Med. 1986;3:296-302. [Medline] [Order article via Infotrieve]
    
14. Chu SC, Pike MM, Fossel ET, Smith TW, Balschi JA, Springer CS Jr. Aqueous shift reagents for high-resolution cationic nuclear magnetic resonance, III: Dy(TTHA)^3-, Tm(TTHA)^3-, and Tm(PPP)₂^7-. J Magn Reson. 1984;56:33-47.
    
15. Endre ZH, Allis JL, Radda GK. Toxicity of dysprosium shift reagents in the isolated perfused rat kidney. Magn Reson Med. 1989;11:267-274. [Medline] [Order article via Infotrieve]
    
16. Anderson SE, Adorante JS, Cala PM. Dynamic NMR measurement of volume regulatory changes in amphiuma RBC Na⁺ content. Am J Physiol. 1988;254:C466-C474. [Abstract/Free Full Text]
    
17. Pekar J, Leigh JS Jr. Detection of biexponential relaxation in sodium-23 facilitated by double-quantum filtering. J Magn Reson. 1986;69:582-584.
    
18. Jelicks LA, Gupta RK. Double-quantum NMR of sodium ions in cells and tissues: paramagnetic quenching of extracellular coherence. J Magn Reson. 1989;81:586-592.
    
19. Hutchison RB, Malhotra D, Hendrick RE, Chan L, Shapiro JI. Evaluation of the double-quantum filter for the measurement of intracellular sodium concentration. J Biol Chem. 1990;265:15506-15510. [Abstract/Free Full Text]
    
20. Polimeni PI. Extracellular space and ionic distribution in rat ventricle. Am J Physiol. 1974;227:676-683.
    
21. Haase A, Frahm J, Matthaei D, Hanicke W, Merboldt KD. FLASH imaging: rapid NMR imaging using low flip angle pulses. J Magn Reson. 1986;67:258-266.
    
22. Haacke EM, Tkach JA. Fast MR imaging: techniques and clinical applications. AJR Am J Roentgenol. 1990;155:951-964. [Abstract/Free Full Text]
    
23. Chien D, Edelman RR. Ultrafast imaging using gradient echoes. Magn Reson Q. 1991;7:31-56. [Medline] [Order article via Infotrieve]
    
24. Judd RM, Atalay MK, Rottman JR, Zerhouni EA. Effects of myocardial water exchange on T1 enhancement during bolus administration of MR contrast agents. Magn Reson Med. 1995;33:215-223. [Medline] [Order article via Infotrieve]
    
25. Resar JR, Livingston JZ, Yin FCP. In-plane myocardial wall stress is not the primary determinant of coronary systolic flow impediment. Circ Res. 1992;70:583-592.[Abstract/Free Full Text]
    
26. Resar JR, Judd RM, Halperin HR, Chacko VP, Weiss RG, Yin FCP. Direct evidence that coronary perfusion affects diastolic myocardial mechanical properties. Cardiovasc Res. 1993;27:403-410. [Abstract/Free Full Text]
    
27. Reeder SB. Development of High Speed, High Resolution Magnetic Resonance Imaging and Tagging Techniques. Baltimore, Md: Johns Hopkins University; 1993. MS thesis.
    
28. Pauly J, Nishimura D, Macovski A. A k-space analysis of small-tip-angle excitation. J Magn Reson. 1989;81:43-56.
    
29. Foo TKF, Shellock FG, Hayes CE, Schenck JF, Slayman BE. High-resolution MR imaging of the wrist and eye with short TR, short TE, and partial-echo acquisition. Radiology. 1992;183:277-281. [Abstract/Free Full Text]
    
30. Harms SE, Flamig DP, Fisher CF, Fulmer JM. New method for fast MR imaging of the knee. Radiology. 1989;173:743-750. [Abstract/Free Full Text]
    
31. Fishbein MC, Meerbaum S, Rit J, Lando U, Kanmatsuse K, Mercier JC, Corday E, Ganz W. Early phase acute myocardial infarct size quantification: validation of the triphenyl tetrazolium chloride tissue enzyme staining technique. Am Heart J. 1981;101:593-600. [Medline] [Order article via Infotrieve]
    
32. Klein HH, Puschmann S, Schaper J, Schaper W. The mechanism of the tetrazolium reaction in identifying experimental myocardial infarction. Virchows Arch [Pathol Anat]. 1981;393:287-297.
    
33. Henkelman RM. Measurement of signal intensities in the presence of noise in MR images. Med Phys. 1985;12:232-233. [Medline] [Order article via Infotrieve]
    
34. Glantz SA. Primer of Biostatistics. New York, NY: McGraw Hill; 1987:64-100.
    
35. Barclay JA, Hamley EJ, Houghton H. Electrolyte content of rat heart atria and ventricles. Circ Res. 1960;8:1264-1267. [Abstract/Free Full Text]
    
36. Ambrosio G, Weisman HF, Mannisi JA, Becker LC. Progressive impairment of regional myocardial perfusion after initial restoration of postischemic blood flow. Circulation. 1989;80:1846-1861. [Abstract/Free Full Text]
    
37. Jeremy RW, Links JM, Becker LC. Progressive failure of coronary flow during reperfusion of myocardial infarction: documentation of the no reflow phenomenon with positron emission tomography. J Am Coll Cardiol. 1990;16:695-704.[Abstract]
    
38. Judd RM, Lugo-Olivieri CH, Arai M, Kondo T, Croisille P, Lima JAC, Mohan V, Becker LC, Zerhouni EA. Physiological basis of myocardial contrast enhancement in fast magnetic resonance images of 2-day-old reperfused canine infarcts. Circulation. 1995;92:1902-1910. [Abstract/Free Full Text]
    
39. Jelicks LA, Gupta RK. Multinuclear NMR studies of the Langendorff perfused rat heart. J Biol Chem. 1989;264:15230-15235. [Abstract/Free Full Text]
    
40. Fossel ET, Hoefeler H. Observation of intracellular potassium and sodium in the heart by NMR: a major fraction of potassium is `invisible.' Magn Reson Med. 1986;3:534-540. [Medline] [Order article via Infotrieve]
    
41. Pike MM, Frazer JC, Dedrick DF, Ingwall JS, Allen PD, Springer CS Jr, Smith TW. ²³Na and ³⁹K nuclear magnetic resonance studies of perfused rat hearts: discrimination of intra- and extracellular ions using a shift reagent. Biophys J. 1985;48:159-173. [Abstract/Free Full Text]
    
42. DeLayre JL, Ingwall JS, Malloy C, Fossel ET. Gated sodium-23 nuclear magnetic resonance images of an isolated perfused working rat heart. Science. 1981;212:935-936. [Abstract/Free Full Text]
    
43. Lauterbur PC. Image formation by induced local interactions: examples employing nuclear magnetic resonance. Nature. 1973;242:190-191.
    
44. Ra JB, Hilal SK, Oh CH, Mun IK. In vivo magnetic resonance imaging of sodium in the human body. Magn Reson Med. 1988;7:11-22. [Medline] [Order article via Infotrieve]
    
45. Reeder SB, McVeigh ER. The effect of high performance gradients on fast gradient echo imaging. Magn Reson Med. 1994;32:612-621. [Medline] [Order article via Infotrieve]
    
46. Hoult DI, Lauterbur PC. The sensitivity of the zeugmatographic experiment involving human samples. J Magn Reson. 1979;34:425-433.
    

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