Determining the extent to which delayed-enhancement images reflect the partition-coefficient of Gd-DTPA in canine studies of reperfused and unreperfused myocardial infarction

Myocardial glucose suppression may interfere with the detection of inflammatory cells with FDG-PET as suggested in a canine model of myocardial infarction

BACKGROUND

After myocardial infarction, fibrosis and an ongoing dysregulated inflammatory response have been shown to lead to adverse cardiac remodeling. FDG PET is an imaging modality sensitive to inflammation as long as suppression protocols are observed while gadolinium enhanced MRI can be used to determine extracellular volume (ECV), a measure of fibrosis. In patients, glucose suppression is achieved variously through a high fat diet, fasting and injection of heparin. To emulate this process in canines, a heparin injection and lipid infusion are used, leading to similar fatty acids in the blood. The aim of this study was to examine the effect of glucose suppression on the uptake of FDG in the infarcted myocardial tissue and also on the determination of ECV in both the infarcted tissue and in the myocardium remote to the zone of infarction during a long constant infusion of FDG and Gd-DTPA.

RESULTS

Extracellular volume was affected neither by suppression nor the length of the constant infusion in remote and infarcted tissue. Metabolic rate of glucose in infarcted tissue decreased during and after suppression of glucose uptake by lipid infusion and heparin injection. An increase in fibrosis and inflammatory cells was found in the center of the infarct as compared to remote tissue.

CONCLUSION

The decrease in the metabolic rate of glucose in the infarcted tissue suggests that inflammatory cells may be affected by glucose suppression through heparin injection and lipid infusion.

Simultaneous measurements of myocardial glucose metabolism and extracellular volumes with hybrid PET/MRI using concurrent injections of Gd-DTPA and [18F]FDG

BACKGROUND

The aims of this study were to investigate the application of a constant infusion (CI) to mitigate the issue of constantly changing Gd-DTPA contrast levels in a bolus injection for extracellular volume (ECV) measurements by (a) comparing a CI alone to a bolus alone and a bolus followed by CI in healthy myocardium, (b) evaluating the impact of glucose suppression using heparin on ECV.

METHODS

Five healthy canine subjects were imaged to compare three different protocols for injecting Gd-DTPA and FDG: bolus alone, CI alone, bolus followed by CI. Suppression of myocardial glucose uptake was induced using a continuous infusion of 20% lipid at a rate of 0.25 mL·min-1·kg-1 as well as 2000 units of intravenous heparin injected 20 minutes prior to FDG/Gd-DTPA injection.

RESULTS

There was no significant effect on ECV measurement when heparin was used for glucose suppression at equilibrium irrespective of infusion protocol). Measurements of ECV in myocardium, regardless of infusion protocol showed no significant difference at all time points (P = 0.21) prior to washout.

CONCLUSIONS

The suppression of myocardial uptake of [18F]FDG with heparin did not alter the determination of myocardial ECV though a larger sample size may show differences. Further, the infusion protocol (bolus or constant infusion) had no effect on the calculated ECV.

Tracking the progress of inflammation with PET/MRI in a canine model of myocardial infarction

BACKGROUND

Following myocardial infarction, tissue undergoes pathophysiological changes involving inflammation and scar tissue formation. However, little is known about the pathophysiology and prognostic significance of any corresponding changes in remote myocardium. The aim of this study was to investigate the potential application of a combined constant infusion of 18F-FDG and Gd-DTPA to quantitate inflammation and extracellular volume (ECV) from 3 to 40 days after myocardial infarction.

METHODS

Eight canine subjects were imaged at multiple time points following induction of an MI with a 60-minute concurrent constant infusion of Gd-DTPA and 18F-FDG using a hybrid PET/MRI scanner.

RESULTS

There was a significant increase in ECV in remote myocardium on day 14 post-MI (P = .034) and day 21 (P = .021) compared to the baseline. ECV was significantly elevated in the infarcted myocardium compared to remote myocardium at all time points post-MI (days 3, 7, 14, 21, and 40) (P < .001) while glucose uptake was also increased within the infarct on days 3, 7, 14, and 21 but not 40.

CONCLUSIONS

The significant increase in ECV in remote tissue may be due to an ongoing inflammatory process in the early weeks post-infarct.

Myocardial T1 mapping and determination of partition coefficients at 3 tesla: comparison between gadobenate dimeglumine and gadofosveset trisodium

Objective

To compare an albumin-bound gadolinium chelate (gadofosveset trisodium) and an extracellular contrast agent (gadobenate dimeglumine), in terms of their effects on myocardial longitudinal (T1) relaxation time and partition coefficient.

Materials and Methods

Study subjects underwent two imaging sessions for T1 mapping at 3 tesla with a modified look-locker inversion recovery (MOLLI) pulse sequence to obtain one pre-contrast T1 map and two post-contrast T1 maps (mean 15 and 21 min, respectively). The partition coefficient was calculated as ΔR1_myocardium /ΔR1_blood , where R1 is 1/T1.

Results

A total of 252 myocardial and blood pool T1 values were obtained in 21 healthy subjects. After gadolinium administration, the myocardial T1 was longer for gadofosveset than for gadobenate, the mean difference between the two contrast agents being −7.6 ± 60 ms (p = 0.41). The inverse was true for the blood pool T1, which was longer for gadobenate than for gadofosveset, the mean difference being 56.5 ± 67 ms (p < 0.001). The partition coefficient (λ) was higher for gadobenate than gadofosveset (0.41 vs. 0.33), indicating slower blood pool washout for gadofosveset than for gadobenate.

Conclusion

Myocardial T1 times did not differ significantly between gadobenate and gadofosveset. At typical clinical doses of the contrast agents, partition coefficients were significantly lower for the intravascular contrast agent than for the extravascular agent.

Cardiac T1 mapping in congenital heart disease: bolus vs. infusion protocols for measurements of myocardial extracellular volume fraction

Myocardial extracellular volume fraction (ECV) reflecting diffuse myocardial fibrosis can be measured with T1 mapping cardiovascular magnetic resonance (CMR) before and after the application of a gadolinium-based extracellular contrast agent. The equilibrium between blood and myocardium contrast concentration required for ECV measurements can be obtained with a primed contrast infusion (equilibrium contrast-CMR). We hypothesized that equilibrium can also be achieved with a single contrast bolus to accurately measure diffuse myocardial fibrosis in patients with congenital heart disease (CHD). Healthy controls (n = 17; median age 24.0 years) and patients with CHD (n = 19; 25.0 years) were prospectively enrolled. Using modified Look-Locker inversion recovery T1 mapping before, 15 min after bolus injection, and during constant infusion of gadolinium-DOTA, T1 values were obtained for blood pool and myocardium of the left ventricle (LV), the interventricular septum (IVS), and the right ventricle (RV) in a single midventricular plane in short axis or in transverse orientation. ECV of LV, IVS and RV by bolus-only and bolus-infusion correlated significantly in CHD patients (r = 0.94, 0.95, and 0.74; p < 0.01, respectively) and healthy controls (r = 0.96, 0.89, and 0.64; p < 0.05, respectively). Bland-Altman plots revealed no significant bias between the techniques for any of the analyzed regions. ECV of LV and RV myocardium measured by bolus-only T1 mapping agrees well with bolus-infusion measurements in patients with CHD. The use of a bolus-only approach facilitates the integration of ECV measurements into existing CMR imaging protocols, allowing for assessment of diffuse myocardial fibrosis in CHD in clinical routine.

Magnetic resonance imaging dynamic contrast enhancement (DCE) characteristics of healed myocardial infarction differ from viable myocardium

PURPOSE

To determine whether healed myocardial infarction alters dynamic contrast-enhancement (DCE) curve shapes as well as late gadolinium-enhancement (LGE).

MATERIALS AND METHODS

Twenty patients with chronic myocardial infarction underwent MR imaging at 1.5 T with blood and myocardial T1 measurements before and after contrast administration for forty minutes. Viable and infarcted myocardial partition coefficients were calculated using multipoint slope methods for ten different DCE sampling intervals and windows. Partition coefficients and coefficients of determination were compared with paired statistical tests to assess the linearity of DCE curve shapes over the 40 min time period.

RESULTS

Calculated partition coefficients did not vary significantly between methods (p=0.325) for viable myocardium but did differ for infarcted myocardium (p<0.001), indicating a difference in infarcted DCE. There was a significant difference between viable and infarcted myocardial partition coefficients estimates for all methods with the exception of methods that included measurements during the first 10 min after contrast agent administration.

CONCLUSION

Myocardial partition coefficients calculated from a slope calculation vary in healed myocardial infarction based on the selection of samples due to non-linear DCE curve shapes. Partition coefficient calculations are insensitive to data sampling effects in viable myocardium due to linear DCE curve shapes.

Coronary artery angiography and myocardial viability imaging: a 3.0-T contrast-enhanced magnetic resonance coronary artery angiography with Gd-BOPTA

With improving MR sequence, phase-array coil and image quality, cardiac magnetic resonance imaging is becoming a promising method for a comprehensive non-invasive evaluation of coronary artery and myocardial viability. The study aimed to evaluate contrast-enhanced whole-heart coronary MR angiography (CE WH-CMRA) at 3.0-Tesla for the diagnosis of significant stenosis (≥50%) and detection of myocardial infarction (MI) in patients with suspected coronary artery disease (CAD). CE WH-CMRA was performed in consecutive 70 patients with suspected CAD by using a 3.0-T MR system. A respiratory-gated, electrocardiography-triggered, inversion-recovery, segmented fast low angle shot sequence (TI = 200 ms) was used. Data acquisition began 60 s after the slow injection of Gd-BOPTA (0.2 mmol/kg body weight, at an injection rate 0.3 ml/s). At last, breath-hold 2D-PSIR-SSFP sequence was performed. Diagnostic accuracy of CE WH-CMRA in detecting significant stenosis (≥50%) was evaluated using invasive coronary angiography as the referenced standard. The MI region appearing as high signal intensity visualized on CEWH-CMRA and 2D-PSIR-SSFP images were compared and analyzed. CE WH-CMRA correctly identified 42 of 44 patients with significant CAD. The overall sensitivity, specificity, negative predictive value, positive predictive value and accuracy for diagnosing significant CAD was 83.6, 95.8, 96.0, 82.8 and 93.4% respectively. The MI region detected by WH-CMRA and 2D-PSIR-SSFP were consistent in 10 patients and these segments manifested with transmural or subendocardial enhancement patterns. Only one MI patient was judged inconsistent between WH-CMRA and 2D-PSIR-SSFP, who was confirmed by clinical and electrocardiogram results. The enhancement pattern in this patient was spotted and focal in 2D-PSIR-SSFP, but was dismissed by WH-CMRA. It is feasible to obtain information about coronary artery stenosis and myocardial viability in a single CE WH-CMRA with administration of Gd-BOPTA.

Myocardial extracellular volume fraction from T1 measurements in healthy volunteers and mice: relationship to aging and cardiac dimensions

OBJECTIVES

This study aimed to test the characteristics of the myocardial extracellular volume fraction (ECV) derived from pre- and post-contrast T1 measurements among healthy volunteers.

BACKGROUND

Cardiac magnetic resonance (CMR) T1 measurements of myocardium and blood before and after contrast allow quantification of the ECV, a tissue parameter that has been shown to change in proportion to the connective tissue fraction.

METHODS

Healthy volunteers underwent standard CMR imaging with administration of gadolinium. T1 measurements were performed with a Look-Locker sequence followed by gradient-echo acquisition. We tested the segmental, interslice, inter-, intra-, and test-retest characteristics of the ECV, as well as the association of the ECV with other variables. Juvenile and aged mice underwent a similar protocol, and cardiac sections were harvested for measurement of fibrosis.

RESULTS

In healthy volunteers (N = 32, 56% female; age 21 to 72 years), the ECV averaged 0.28 ± 0.03 (range 0.23 to 0.33). The intraclass coefficients for the intraobserver, interobserver, and test-retest absolute agreements of the ECV were 0.94 (95% confidence interval: 0.84 to 0.98), 0.93 (95% confidence interval: 0.80 to 0.98), and 0.95 (95% confidence interval: 0.52 to 0.99), respectively. In volunteers, the ECV was associated with age (r = 0.74, p < 0.001), maximal left atrial volume index (r = 0.67, p < 0.001), and indexed left ventricular mass. There were no differences in the ECV between segments in a slice or between slices. In mice (N = 12), the myocardial ECV ranged from 0.20 to 0.32 and increased with age (0.22 ± 0.02 vs. 0.30 ± 0.02, juvenile vs. aged mice, p < 0.001). In mice, the ECV correlated with the extent of myocardial fibrosis (r = 0.94, p < 0.001).

CONCLUSIONS

In healthy volunteers, the myocardial ECV ranges from 0.23 to 0.33, has acceptable test characteristics, and is associated with age, left atrial volume, and left ventricular mass. In mice, the ECV also increases with age and strongly correlates with the extent of myocardial fibrosis.

Quantification of extracellular matrix expansion by CMR in infiltrative heart disease

OBJECTIVES

The aim of this study was to perform direct quantification of myocardial extracellular volume fraction (ECF) with T1-weighted cardiac magnetic resonance (CMR) imaging in patients suspected to have infiltrative heart disease.

BACKGROUND

Infiltrative heart disease refers to accumulation of abnormal substances within the myocardium. Qualitative assessment of late gadolinium enhancement (LGE) remains the most commonly used method for CMR evaluation of patients suspected with myocardial infiltration. This technique is widely available and can be performed in a reproducible and standardized manner. However, the degree of extracellular matrix expansion due to myocardial infiltration in the intercellular space has, to date, not been amenable to noninvasive quantification with LGE.

METHODS

We performed 3-T CMR in 38 patients (mean age 68 ± 15 years) who were referred for assessment of infiltrative heart disease and also in 9 healthy volunteers as control subjects. The T1 quantification by Look-Locker gradient-echo before and after contrast determined segmental myocardial partition coefficients. The ECF was obtained by referencing the tissue partition coefficient for gadolinium to the plasma volume fraction in blood, derived from serum hematocrit. Cine CMR and LGE imaging in matching locations were also performed.

RESULTS

Seventeen patients (45%) had cardiac amyloidosis (CA) (biopsy-confirmed or clinically highly probable), 20 (53%) had a non-amyloid cardiomyopathy, and 1 had lysosomal storage disease. Median global ECF was substantially higher in CA patients (0.49) compared with non-amyloid cardiomyopathy patients (0.33, p < 0.0001) and volunteers (0.24, p = 0.0001). The ECF strongly correlated with visually assessed segmental LGE (r = 0.80, p < 0.0001) and LV mass index (r = 0.69, p < 0.0001), reflecting severity of myocardial infiltration. In patients with CA, ECF was highest in segments with LGE, although it remained elevated in segments without qualitative LGE.

CONCLUSIONS

The CMR ECF quantification identified substantial expansion of the interstitial space in patients with CA compared with volunteers. Further studies using this technique for diagnosis and assessment of the severity of myocardial infiltration are warranted.

MRI study of cryoinjury infarction in pig hearts: i. Effects of intrapericardial delivery of bFGF/VEGF embedded in alginate beads

The aim of the study was the testing of sustained intrapericardial delivery of vascular growth factors (GFs) from alginate beads on cryoinjury size and perfusion. In domestic pigs (15-20 kg, n = 21), the left ventricular (LV) anterolateral wall of exposed hearts was cryoinjured using an aluminum rod (25 mm o.d.) cooled in liquid nitrogen. Alginate beads (d = 3.2 ± 0.2 mm), containing human recombinant basic fibroblast GF (bFGF, 50 µg) and vascular endothelial GF (VEGF, 50 µg) + heparin (50 µg) or heparin alone (Con, n = 5), were sutured to the cryoinjured epicardium (GF, n = 5; Con, n = 3 ) or pericardium (GF, n = 3; Con, n = 2), or no beads were implanted (n = 4). Four pigs were sham-operated. Cine and T(1) -weighted MRI was performed in vivo at ~2.5 h and 1, 2, 3 and 4 weeks after injury in a 3T imager. A double bolus of GdDTPA was injected (0.05 and 0.15 mmol/kg) and first-pass and late enhancement kinetics were monitored. After 4-week cryoinjury, following the injection of 5 x 10(6) 15-µm NIR fluorescent microspheres (FMS, 645/680 nm), hearts were sliced and examined with fluorescence imaging. Triphenyltetrazolium chloride (TTC) staining was used to determine infarct areas. Epicardial GF-containing beads were encapsulated within the hypointense 3-4-week infarct tissue. This tissue had a 75% higher LV thickening index, a lower distribution volume for GdDTPA (0.44 ± 0.12 vs 0.68 ± 0.05, p = 0.02), and 25% faster first-pass Gd kinetics relative to control infarctions. TTC staining revealed TTC-positive islands in the core of treated infarcts, which showed higher FMS fluorescence relative to surrounding infarct tissue (0.64 ± 0.14 vs. 0.31 ± 0.14; p < 0.0001) and to control infarcts (0.37 ± 0.09, p < 0.05). GF-beads attached to the pericardium were not effective. We conclude that sustained intrapericardial release of bFGF + VEGF from alginate beads attached to the epicardium facilitated vascular growth in the cryoinjured area.

Evaluation of the acute effects of distal coronary microembolization using multidetector computed tomography and magnetic resonance imaging

The purpose of this study was to test the potential of clinical imaging modalities, 64-slice multidetector computed tomography (MDCT) and 1.5T magnetic resonance imaging (MRI) for qualitative and quantitative evaluation of acute microinfarcts and to determine the effects of <120 μm microemboli on left ventricular function, perfusion, cardiac injury biomarkers, arrhythmia, and cellular and vascular structures. Under X-ray fluoroscopy, 40-120 μm (16 mm(3) ) microemboli were delivered to embolize the left anterior descending (LAD) coronary artery of nine pigs. MDCT/MRI were performed at 72 h in a single session. Microinfarcts were visible in six of nine animals on delayed contrast-enhanced MDCT/MR images but measurable in all animals using semiautomated threshold methods. Other MDCT and MRI sequences demonstrated decline in left ventricular ejection fraction, regional strain and perfusion in visible and invisible microinfarcted regions. Microemboli caused significant elevation in cardiac injury enzymes and arrhythmias. Various sizes of microinfarcts appeared microscopically as distinct aggregates of macrophages replacing myocardium. Semiautomated threshold methods are necessary to measure and confirm/deny the presence of myocardial microinfarcts. This study offers support for alternative applications of MDCT/MRI in assessing clinical cases in which microemboli <120 μm escape protective devices during percutaneous coronary interventions. Although microembolization resulted in no mortality, it caused left ventricular dysfunction, perfusion deficit, cellular damage increase in cardiac injury enzymes, and arrhythmias.

Quantification of diffuse myocardial fibrosis and its association with myocardial dysfunction in congenital heart disease

BACKGROUND

the etiology of ventricular dysfunction in adult congenital heart disease (ACHD) is not well understood. Diffuse fibrosis is a likely common final pathway and is quantifiable using MRI.

METHODS AND RESULTS

patients with ACHD (n=50) were studied with cardiac MRI to quantify systemic ventricular volume and function and diffuse fibrosis. The fibrosis index for a single midventricular plane of the systemic ventricle was quantified by measuring T1 values for blood pool and myocardium before and after administration of gadolinium (0.15 mmol/kg) and then adjusted for hematocrit. Results were compared to healthy volunteers (normal controls, n=14) and patients with acquired heart failure (positive controls, n=4). Patients studied (age, 37±12 years; female sex, 40%) included 11 with a systemic right ventricle (RV), 17 with tetralogy of Fallot, 10 with cyanosis, and 12 with other lesions. The fibrosis index was significantly elevated in patients with ACHD compared to normal controls (31.9±4.9% versus 24.8±2.0%; P=0.001). Values were highest in patients with a systemic RV (35.0±5.8%; P<0.001) and those who were cyanotic (33.7±5.6%; P<0.001). The fibrosis index correlated with end-diastolic volume index (r=0.60; P<0.001) and ventricular ejection fraction (r=-0.53; P<0.001) but not with age or oxygen saturation in patients who were cyanotic. Late gadolinium enhancement did not account for the differences seen.

CONCLUSIONS

patients with ACHD have evidence of diffuse, extracellular matrix remodeling similar to patients with acquired heart failure. The fibrosis index may facilitate studies on the mechanisms and treatment of myocardial fibrosis and heart failure in these patients.

Assessment of acute myocardial infarction: current status and recommendations from the North American society for Cardiovascular Imaging and the European Society of Cardiac Radiology

There are a number of imaging tests that are used in the setting of acute myocardial infarction and acute coronary syndrome. Each has their strengths and limitations. Experts from the European Society of Cardiac Radiology and the North American Society for Cardiovascular Imaging together with other prominent imagers reviewed the literature. It is clear that there is a definite role for imaging in these patients. While comparative accuracy, convenience and cost have largely guided test decisions in the past, the introduction of newer tests is being held to a higher standard which compares patient outcomes. Multicenter randomized comparative effectiveness trials with outcome measures are required.

Cardiac magnetic resonance imaging of myocardial contrast uptake and blood flow in patients affected with idiopathic or familial dilated cardiomyopathy

Idiopathic dilated cardiomyopathy (IDC) is characterized by left ventricular (LV) enlargement with systolic dysfunction, other causes excluded. When inherited, it represents familial dilated cardiomyopathy (FDC). We hypothesized that IDC or FDC would show with cardiac magnetic resonance (CMR) increased myocardial accumulation of gadolinium contrast at steady state and decreased baseline myocardial blood flow (MBF) due to structural alterations of the extracellular matrix compared with normal myocardium. CMR was performed in nine persons affected with IDC/FDC. Healthy controls came from the general population (n = 6) or were unaffected family members of FDC patients (n = 3) without signs or symptoms of IDC/FDC or any structural cardiac abnormalities. The myocardial partition coefficient for gadolinium contrast (lambda(Gd)) was determined by T1 measurements. LV shape and function and MBF were assessed by standard CMR methods. lambda(Gd) was elevated in IDC/FDC patients vs. healthy controls (lambda(Gd) = 0.56 +/- 0.15 vs. 0.41 +/- 0.06; P = 0.002), and correlated with LV enlargement (r = 0.61 for lambda(Gd) vs. end-diastolic volume indexed by height; P < 0.01) and with ejection fraction (r = -0.80; P < 0.001). The extracellular volume fraction was higher in IDC patients than in healthy controls (0.31 +/- 0.05 vs. 0.24 +/- 0.03; P = 0.002). Resting MBF was lower in IDC patients (0.64 +/- 0.13 vs. 0.91 +/- 0.22; P = 0.01) than unaffected controls and correlated with both the partition coefficient (r = -0.57; P = 0.012) and the extracellular volume fraction (r = -0.56; P = 0.019). The expansion of the extracellular space correlated with reduced MBF and ventricular dilation. Expansion of the extracellular matrix may be a key contributor to contractile dysfunction in IDC patients.

MRI evaluation of microvascular obstruction in experimental reperfused acute myocardial infarction using a T1 and T2 preparation pulse sequence

PURPOSE

To investigate a T1 and T2 preparation pulse sequence to evaluate microvascular obstruction (MO) in a porcine model of reperfused acute myocardial infarction (AMI).

MATERIALS AND METHODS

A total of 14 pigs with reperfused AMI underwent MRI examinations at baseline and three to four hours after reperfusion. MRI scans included a left ventricular functional study, T1 and T2 measurement on a 1.5T MRI system. At reperfusion, first-pass myocardial perfusion (FPMP) images were obtained after bolus injection of gadopentetate dimeglumine followed by an intravenous drip. Delayed contrast-enhanced MRI (DE-MRI) and T1 measurements were performed 30 and 45 minutes, respectively, after the bolus, during a constant infusion of gadopentetate dimeglumine.

RESULTS

In 11 pigs MO was hypoenhanced in FPMP and DE-MRI. In later T1 preparation difference images postcontrast, MO was hyperenhanced while delayed hyperenhanced (DHE) regions appeared dark. MO areas on DE-MRI and T1 images were comparable. T1 reduction (%) postcontrast in MO was small compared to measurements from DHE regions (P < 0.0001) and similar to those from control segments (P = 0.66). Precontrast T1 and T2 values at reperfusion from MO and DHE regions were larger than in control regions.

CONCLUSION

Using T1 preparation under a constant gadopentetate dimeglumine (Gd-DTPA) infusion, delayed imaging at 30 to 45 minutes demonstrates MO as a positive contrast with larger T1 values. Elevated T1 and T2 values in MO precontrast may also help to differentiate them from both control and DHE regions.

Quantitative myocardial distribution volume from dynamic contrast-enhanced MRI

The objective of this study was to investigate if dynamic contrast-enhanced magnetic resonance imaging (MRI) can be used to quantitate the distribution volume (v(e)) in regions of normal and infarcted myocardium. v(e) reflects the volume of the extracellular, extravascular space within the myocardial tissue. In regions of the heart where an infarct has occurred, the loss of viable cardiac cells results in an elevated v(e) compared to normal regions. A quantitative estimate of the magnitude and spatial distribution of v(e) is significant because it may provide information complementary to delayed enhancement MRI alone. Using a hybrid gradient echo-echoplanar imaging pulse sequence on a 1.5T MRI scanner, 12 normal subjects and four infarct patients were imaged dynamically, during the injection of a contrast agent, to measure the regional blood and tissue enhancement in the left ventricular (LV) myocardium. Seven of the normal subjects and all of the infarct patients were also imaged at steady-state contrast enhancement to estimate the steady-state ratio of contrast agent in the tissue and blood (Ct/Cb) - a validated measure of v(e). Normal and infarct regions of the LV were manually selected, and the blood and tissue enhancement curves were fit to a compartment model to estimate v(e). Also, the effect of the vascular blood signal on estimates of v(e) was evaluated using simulations and in the dynamic and steady-state studies. Aggregate estimates of v(e) were 23.6+/-6.3% in normal myocardium and 45.7+/-3.4% in regions of infarct. These results were not significantly different from the reference standards of Ct/Cb (22.9+/-6.8% and 42.6+/-6.3%, P=.073). From the dynamic contrast-enhanced studies, approximately 1 min of scan time was necessary to estimate v(e) in the normal myocardium to within 10% of the steady-state estimate. In regions of infarct, up to 3 min of dynamic data were required to estimate v(e) to within 10% of the steady-state v(e) value. By measuring the kinetics of blood and tissue enhancement in the myocardium during an extended dynamic contrast enhanced MRI study, v(e) may be estimated using compartment modeling.

Delayed hyperenhancement patterns in occlusive and reperfused myocardial infarcts during different healing stages

PURPOSE

To investigate delayed hyperenhancement during different healing stages of myocardial infarct with and without reperfusion in a pig model.

MATERIALS AND METHODS

Twelve pigs were studied. Six pigs were subjected to permanent right coronary artery (RCA) occlusion, and the other six were subjected to transient 90-minute RCA occlusion. In vivo MRI including first-pass perfusion and myocardial delayed enhancement (MDE) was performed at four, eight, 12, and 24 hours, and eight days after the artery occlusion. The MDE scan was repeated four times, two to 32 minutes after contrast administration.

RESULTS

For all six pigs without reperfusion, the four-hour MRI showed transmural perfusion defect and no delayed hyperenhancement; from eight hours on, delayed hyperenhancement appeared and its spatial extent gradually increased until it became transmural hyperenhancement at eight days. However, in all six pigs with reperfusion, hyperenhancement was present at every stage from four hours to eight days. Pathology confirmed the existence of infarct at various stages of healing irrespective of reperfusion status.

CONCLUSION

Occlusive hyperacute infarct did not exhibit any delayed hyperenhancement, which indicates that perfusion plays an important role in delayed enhancement at the hyperacute stage.

Feasibility of the single-bolus strategy for measuring the partition coefficient of Gd-DTPA in patients with myocardial infarction: independence of image delay time and maturity of scar

The partition coefficient of Gd-DTPA (lambda) is elevated in infarcted relative to normal myocardium. Although MRI following an infusion of Gd-DTPA allows for the quantification of lambda, infarct imaging is more routinely performed using a bolus. In this study we sought to determine how image delay time and time postinfarction influence the estimation of lambda by the bolus strategy. Both infusion and bolus imaging were performed twice in the same group of patients (N = 9): once at 3-4 weeks and again 6 months after reperfusion therapy for myocardial infarction (MI). Bolus estimates of lambda were compared with those calculated after 60 min infusion, and comparisons were repeated at 6 months. The lambda of infarcted myocardium was significantly greater than that of normal tissue, irrespective of either the technique used or the time postinfarction (P < 0.0001, for each). The concordance (Rc) between bolus and infusion estimates of lambda was >0.83 for all image delays >4 min postinjection, and Rc at 2 min (0.78 +/- 0.04) was significantly less than Rc determined for longer image delay times (P = 0.009). Rc did not change with time postinfarction (P = 0.604). Thus, the bolus strategy can be used to provide estimates of lambda that are stable from 1-6 months postinfarction and independent of image delay time.