Techniques for the measurement of the local myocardial extraction efficiency for inert diffusible contrast agents such as gadopentate dimeglumine

Clinical Translation of Three-Dimensional Scar, Diffusion Tensor Imaging, Four-Dimensional Flow, and Quantitative Perfusion in Cardiac MRI: A Comprehensive Review

Cardiovascular magnetic resonance (CMR) imaging is a versatile tool that has established itself as the reference method for functional assessment and tissue characterisation. CMR helps to diagnose, monitor disease course and sub-phenotype disease states. Several emerging CMR methods have the potential to offer a personalised medicine approach to treatment. CMR tissue characterisation is used to assess myocardial oedema, inflammation or thrombus in various disease conditions. CMR derived scar maps have the potential to inform ablation therapy—both in atrial and ventricular arrhythmias. Quantitative CMR is pushing boundaries with motion corrections in tissue characterisation and first-pass perfusion. Advanced tissue characterisation by imaging the myocardial fibre orientation using diffusion tensor imaging (DTI), has also demonstrated novel insights in patients with cardiomyopathies. Enhanced flow assessment using four-dimensional flow (4D flow) CMR, where time is the fourth dimension, allows quantification of transvalvular flow to a high degree of accuracy for all four-valves within the same cardiac cycle. This review discusses these emerging methods and others in detail and gives the reader a foresight of how CMR will evolve into a powerful clinical tool in offering a precision medicine approach to treatment, diagnosis, and detection of disease.

Estimating extraction fraction and blood flow by combining first-pass myocardial perfusion and T1 mapping results

Background

Quantifying myocardial perfusion is complicated by the complexity of pharmacokinetic model being used and the reliability of perfusion parameter estimates. More complex modeling provides more information about the underlying physiology, but too many parameters in complex models introduce a new problem of reliable estimation. To overcome the problem of multiple parameters, we have developed a technique that combines knowledge from two different cardiac magnetic resonance (MR) imaging techniques: dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) and T1 mapping. Using extracellular volume (ECV) estimates from T1 mapping may allow more robust model parameter estimates.

Methods

Simulations and human scans were performed. The myocardial perfusion scans used an ungated saturation recovery prepared TurboFLASH pulse sequence. Four short-axis (SA) slices were acquired after a single saturation pulse with a saturation recovery time of ~25 ms before the first slice. Gadoteridol was injected and ~240 frames were acquired over a minute with shallow breathing and no electrocardiograph (ECG) gating. This was followed 20±5 minutes later by an injection of regadenoson to induce hyperemia. The data were acquired using an under-sampled golden angle radial acquisition. Modified look-locker inversion recovery (MOLLI) T1 mapping was performed in 3 slices pre- and post-contrast. The pre- and post-contrast T1 maps were used for ECV estimation. Quantification of perfusion was done using a 4-parameter model with additional information about ECV supplied during model fitting. Phase contrast scans of the coronary sinus (CS) were acquired at rest and immediately after the stress perfusion acquisition to estimate global flow.

Results

Without ECV information, the 5-parameter model fails to converge to a unique solution and often gives incorrect estimates for the perfusion parameters. The myocardial blood flow (MBF) estimates during rest and stress were 0.9±0.1 and 2.3±0.6 mL/min/g, respectively. The extraction fraction estimates were 0.49±0.04 and 0.34±0.05 during rest and stress, respectively.

Conclusions

These results show that it is possible to successfully fit a dynamic perfusion model with an extraction fraction parameter by using information from T1 mapping scans. This hybrid approach is especially important when the 5-parameter model alone fails to converge on a unique solution. This work is a good example of exploiting information overlaps between various cardiac MR imaging techniques.

Cardiac MR perfusion image processing techniques: a survey

First-pass cardiac MR perfusion (CMRP) imaging has undergone rapid technical advancements in recent years. Although the efficacy of CMRP imaging in the assessment of coronary artery diseases (CAD) has been proven, its clinical use is still limited. This limitation stems, in part, from manual interaction required to quantitatively analyze the large amount of data. This process is tedious, time-consuming, and prone to operator bias. Furthermore, acquisition and patient related image artifacts reduce the accuracy of quantitative perfusion assessment. With the advent of semi- and fully automatic image processing methods, not only the challenges posed by these artifacts have been overcome to a large extent, but a significant reduction has also been achieved in analysis time and operator bias. Despite an extensive literature on such image processing methods, to date, no survey has been performed to discuss this dynamic field. The purpose of this article is to provide an overview of the current state of the field with a categorical study, along with a future perspective on the clinical acceptance of image processing methods in the diagnosis of CAD.

Quantification of myocardial blood flow using model based analysis of first-pass perfusion MRI: extraction fraction of Gd-DTPA varies with myocardial blood flow in human myocardium

For the absolute quantification of myocardial blood flow (MBF), Patlak plot-derived K1 need to be converted to MBF by using the relation between the extraction fraction of gadolinium contrast agent and MBF. This study was conducted to determine the relation between extraction fraction of Gd-DTPA and MBF in human heart at rest and during stress. Thirty-four patients (19 men, mean age of 66.5 ± 11.0 years) with normal coronary arteries and no myocardial infarction were retrospectively evaluated. First-pass myocardial perfusion MRI during adenosine triphosphate stress and at rest was performed using a dual bolus approach to correct for saturation of the blood signal. Myocardial K1 was quantified by Patlak plot method. Mean MBF was determined from coronary sinus flow measured by phase contrast cine MRI and left ventricle mass measured by cine MRI. The extraction fraction of Gd-DTPA was calculated as the K1 divided by the mean MBF. The extraction fraction of Gd-DTPA was 0.46 ± 0.22 at rest and 0.32 ± 0.13 during stress (P < 0.001). The relationship between extraction fraction (E) and MBF in human myocardium can be approximated as E = 1 - exp(-(0.14 × MBF + 0.56)/MBF). The current results indicate that MBF can be accurately quantified by Patlak plot method of first-pass myocardial perfusion MRI by performing a correction of extraction fraction.

Myocardial extravascular extracellular volume fraction measurement by gadolinium cardiovascular magnetic resonance in humans: slow infusion versus bolus

BACKGROUND

Myocardial extravascular extracellular volume fraction (Ve) measures quantify diffuse fibrosis not readily detectable by conventional late gadolinium (Gd) enhancement (LGE). Ve measurement requires steady state equilibrium between plasma and interstitial Gd contrast. While a constant infusion produces steady state, it is unclear whether a simple bolus can do the same. Given the relatively slow clearance of Gd, we hypothesized that a bolus technique accurately measures Ve, thus facilitating integration of myocardial fibrosis quantification into cardiovascular magnetic resonance (CMR) workflow routines. Assuming equivalence between techniques, we further hypothesized that Ve measures would be reproducible across scans.

METHODS

In 10 volunteers (ages 20-81, median 33 yr, 3 females), we compared serial Ve measures from a single short axis slice from two scans: first, during a constant infusion, and second, 12-50 min after a bolus (0.2 mmol/kg gadoteridol) on another day. Steady state during infusion was defined when serial blood and myocardial T1 data varied <5%. We measured T1 on a 1.5 T Siemens scanner using a single-shot modified Look Locker inversion recovery sequence (MOLLI) with balanced SSFP. To shorten breath hold times, T1 values were measured with a shorter sampling scheme that was validated with spin echo relaxometry (TR = 15 sec) in CuSO4-Agar phantoms. Serial infusion vs. bolus Ve measures (n = 205) from the 10 subjects were compared with generalized estimating equations (GEE) with exchangeable correlation matrices. LGE images were also acquired 12-30 minutes after the bolus.

RESULTS

No subject exhibited LGE near the short axis slices where Ve was measured. The Ve range was 19.3-29.2% and 18.4-29.1% by constant infusion and bolus, respectively. In GEE models, serial Ve measures by constant infusion and bolus did not differ significantly (difference = 0.1%, p = 0.38). For both techniques, Ve was strongly related to age (p < 0.01 for both) in GEE models, even after adjusting for heart rate. Both techniques identically sorted older individuals with higher mean Ve values.

CONCLUSION

Myocardial Ve can be measured reliably and accurately 12-50 minutes after a simple bolus. Ve measures are also reproducible across CMR scans. Ve estimation can be integrated into CMR workflow easily, which may simplify research applications involving the quantification of myocardial fibrosis.

Contrast agents used in cardiovascular magnetic resonance imaging: current issues and future directions

Cardiovascular MRI is being increasingly used in the evaluation of ischemic heart disease, cardiac masses, complex congenital heart disease, and morphologic evaluation of the vascular anatomy throughout the body. Many and varied contrast media may be used to increase the sensitivity and specificity of detecting and evaluating various pathologies, and a knowledge of the different mechanisms of action, distributions and safety profiles of these agents is required for safe and effective imaging. This article reviews the currently available magnetic resonance (MR) contrast media, discusses the risks and benefits, and gives illustrated examples of current clinical applications in cardiovascular disease. A literature search covered the period 1990 to the present with the use of multiple databases including MEDLINE, PUBMED, SciSearch and Google Medical. All identified studies containing information relevant to the topic of cardiovascular MRI and cardiovascular MR contrast agents and their uses and properties were evaluated. Evaluation was limited to studies in English. The conclusions were that the use of contrast agents vastly increases the diagnostic yield, sensitivity and specificity of cardiovascular MRI in the non-invasive diagnosis of the full breadth of cardiovascular pathology. The use of contrast MRI for investigating ischemic heart disease, cardiac masses, and congenital heart disease and in angiography is now well established, and the referring physician, cardiologist, or radiologist requires an in-depth knowledge of the safety profiles and correct dosing of commonly prescribed contrast agents. As the number of MR contrast agents on the market continues to increase, knowledge of the basic mechanism of action is vital for keeping abreast of how new and emerging agents will affect clinical practice in the future.

Quantitative analysis of first-pass contrast-enhanced myocardial perfusion MRI using a Patlak plot method and blood saturation correction

The objectives of this study were to develop a method for quantifying myocardial K(1) and blood flow (MBF) with minimal operator interaction by using a Patlak plot method and to compare the MBF obtained by perfusion MRI with that from coronary sinus blood flow in the resting state. A method that can correct for the nonlinearity of the blood time-signal intensity curve on perfusion MR images was developed. Myocardial perfusion MR images were acquired with a saturation-recovery balanced turbo field-echo sequence in 10 patients. Coronary sinus blood flow was determined by phase-contrast cine MRI, and the average MBF was calculated as coronary sinus blood flow divided by left ventricular (LV) mass obtained by cine MRI. Patlak plot analysis was performed using the saturation-corrected blood time-signal intensity curve as an input function and the regional myocardial time-signal intensity curve as an output function. The mean MBF obtained by perfusion MRI was 86 +/- 25 ml/min/100 g, showing good agreement with MBF calculated from coronary sinus blood flow (89 +/- 30 ml/min/100 g, r = 0.74). The mean coefficient of variation for measuring regional MBF in 16 LV myocardial segments was 0.11. The current method using Patlak plot permits quantification of MBF with operator interaction limited to tracing the LV wall contours, registration, and time delays.

Coronary microvascular resistance: methods for its quantification in humans

Coronary microvascular dysfunction is a topic that has recently gained considerable interest in the medical community owing to the growing awareness that microvascular dysfunction occurs in a number of myocardial disease states and has important prognostic implications. With this growing awareness, comes the desire to accurately assess the functional capacity of the coronary microcirculation for diagnostic purposes as well as to monitor the effects of therapeutic interventions that are targeted at reversing the extent of coronary microvascular dysfunction. Measurements of coronary microvascular resistance play a pivotal role in achieving that goal and several invasive and noninvasive methods have been developed for its quantification. This review is intended to provide an update pertaining to the methodology of these different imaging techniques, including the discussion of their strengths and weaknesses.

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.

Magnetic resonance cardiac perfusion imaging-a clinical perspective

Coronary artery disease (CAD) with its clinical appearance of stable or unstable angina and acute myocardial infarction is the leading cause of death in developed countries. In view of increasing costs and the rising number of CAD patients, there has been a major interest in reliable non-invasive imaging techniques to identify CAD in an early (i.e. asymptomatic) stage. Since myocardial perfusion deficits appear very early in the "ischemic cascade", a major breakthrough would be the non-invasive quantification of myocardial perfusion before functional impairment might be detected. Therefore, there is growing interest in other, target-organ-specific parameters, such as relative and absolute myocardial perfusion imaging. Magnetic resonance (MR) imaging has been proven to offer attractive concepts in this respect. However, some important difficulties have not been resolved so far, which still causes uncertainty and prevents the broad application of MR perfusion imaging in a clinical setting. This review explores recent technical developments in MR hardware, software and contrast agents, as well as their impact on the current and future clinical status of MR imaging of first-pass myocardial perfusion imaging.

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

MRI after a constant infusion (CI) of Gd-DTPA has been used to identify the extent of myocardial infarction (MI). However, Gd-DTPA-enhanced "viability" imaging is more commonly performed with a bolus (for "delayed-enhancement" (DE) imaging). This study sought to determine how image delay time and time postinfarction influence the assessment of necrosis by DE. Both infusion and DE imaging was performed in dogs with reperfused (N = 6) or unreperfused (N = 4) MI. Estimates of the partition-coefficient of Gd-DTPA (lambda) with DE were compared with those calculated after 60 min of infusion, and the comparisons were repeated until 4 (reperfused) or 8 (unreperfused) weeks postinfarction. In reperfused animals, the concordance (Rc) between DE and infusion estimates of lambda was > 0.90 for most image delays > 8 min postinjection, for day 0 through week 3, with Rc at day 0 greater than at week 4 (P = 0.022). In unreperfused animals, there was an interaction between image delay time and time postinfarction (P < 0.001): Rc > 0.90 corresponded to longer image delays at week 1 than at weeks 4-8. Therefore, when image delays are selected appropriately, DE images can strongly reflect lambda and identify irreversibly injured myocardium.

The influence of myocardial blood flow and volume of distribution on late Gd-DTPA kinetics in ischemic heart failure

PURPOSE

To determine the mechanism of enhancement of contrast-enhanced MRI (ceMRI) in chronic ischemic myocardium. While ceMRI can identify scar tissue in chronic ischemic myocardium, the mechanism of enhancement is not completely understood.

MATERIALS AND METHODS

A total of 11 patients with ischemic heart failure (ejection fraction [EF] 28 +/- 9%) were imaged with ceMRI and positron emission tomography (PET) to measure myocardial blood flow (MBF). Longitudinal relaxation rate (T1) of blood, normal tissue, and scar tissue defined by ceMRI was determined before and two to 50 minutes after contrast (Look Locker technique), and the partition coefficient (lambda) and volume of distribution (VD) were calculated.

RESULTS

In scar and viable tissue, T1 was significantly different over the whole period after contrast, but not before contrast. However, T1 of scar and blood were similar five to 15 minutes post contrast, making the detection of subendocardial defects difficult. lambda reached an initial steady state in viable tissue, but was delayed (20 minutes) in scar tissue. VD in scar was double that of viable tissue (0.54 +/- 0.01 vs. 0.29 +/- 0.02, respectively) indicating an increased interstitial space. Contrast wash-in kinetics correlated moderately with MBF (r = -0.36), but well with the combination of MBF and VD (r = 0.59).

CONCLUSION

Late myocardial contrast kinetics depend on both MBF and VD; however the increased VD seems to be the main mechanism for the late enhancement effect.

Tissue edema does not change gadolinium-diethylenetriamine pentaacetic acid (Gd-DTPA)-enhanced T1 relaxation times of viable myocardium

PURPOSE

To determine whether tissue edema changes gadolinium-diethylenetriamine pentaacetic acid (Gd-DTPA)-enhanced T1 relaxation times of the viable myocardium.

MATERIALS AND METHODS

A total of 16 isolated pig hearts were divided into four groups (N=4/group) and perfused in a Langendorff apparatus. Gd-DTPA was injected into the aortic perfusion line. Tissue edema was then induced by two hours of simultaneous arterial/venous perfusion (SAVP). Myocardial water content and T1 relaxation times were monitored throughout SAVP. The volumes of the extracellular and intracellular compartments were assessed using 31P MRS-detectable markers, phenylphosphonic acid (PPA) and dimethyl methylphosphonate (DMMP).

RESULTS

Tissue water content in both viable and infarcted myocardium increased significantly during two-hour SAVP. However, Gd-DTPA-enhanced T1 relaxation times of the viable myocardium remained relatively unchanged. Infarcted myocardium, on the other hand, exhibited significant T1 shortening during SAVP. Furthermore, SAVP resulted in significant expansions of both extracellular and intracellular compartments, but the ratio of the volumes of the two compartments remained relatively constant.

CONCLUSION

Tissue edema in the viable myocardium does not increase the relative distribution volume of the contrast agent. As a result, edema does not change Gd-DTPA-enhanced T1 relaxation times of the viable myocardium.

Imaging of myocardial perfusion with magnetic resonance

Coronary artery disease (CAD) is currently the leading cause of death in developed nations. Reflecting the complexity of cardiac function and morphology, noninvasive diagnosis of CAD represents a major challenge for medical imaging. Although coronary artery stenoses can be depicted with magnetic resonance (MR) and computed tomography (CT) techniques, its functional or hemodynamic impact frequently remains elusive. Therefore, there is growing interest in other, target organ-specific parameters such as myocardial function at stress and first-pass myocardial perfusion imaging to assess myocardial blood flow. This review explores the pathophysiologic background, recent technical developments, and current clinical status of first-pass MR imaging (MRI) of myocardial perfusion.

In and Ex Vivo MR Evaluation of Acute Myocardial Ischemia in Pigs by Determining R1 in Steady State After the Administration of the Intravascular Contrast Agent NC100150 Injection

RATIONALE AND OBJECTIVES

To study the dose response in perfused and nonperfused myocardium by measuring relaxation rate (R1) in a steady-state situation after injection of the intravascular contrast agent NC100150 Injection in pigs and whether the dose response differs in vivo and ex vivo.

MATERIALS AND METHODS

The left anterior descending artery was occluded. R1 was measured using a Look-Locker sequence for 2 dose groups (2 mg Fe/kg bw, n = 4, and 5 mg Fe/kg bw, n = 5) and a control group (n = 3).

RESULTS

A significant increase in R1 was found in perfused myocardium after contrast agent injection, in contrast to nonperfused myocardium. There was a significantly larger difference in R1 between perfused and nonperfused myocardium in the 5 mg Fe/kg bw dose group compared with the other 2 groups. The difference in R1 between perfused and nonperfused myocardium was significantly higher in vivo than ex vivo.

CONCLUSION

A nearly linear R1 dose response was found in perfused myocardium in vivo. The dose response ex vivo was less steep possibly due to larger water exchange limitations.

Contrast-enhanced MR imaging of the heart: overview of the literature

The use of magnetic resonance (MR) imaging for cardiac diagnosis is expanding, aided by the administration of paramagnetic contrast agents for a growing number of clinical applications. This overview of the literature considers the principles and applications of cardiac MR imaging with an emphasis on the use of contrast media. Clinical applications of contrast material-enhanced MR imaging include the detection and characterization of intracardiac masses, thrombi, myocarditis, and sarcoidosis. Suspected myocardial ischemia and infarction, respectively, are diagnosed by using dynamic first-pass and delayed contrast enhancement. Promising new developments include blood pool contrast media, labeling of myocardial precursor cells, and contrast-enhanced imaging at very high fields.

Fast magnetic resonance imaging with contrast for soft tissue sarcoma viability

Because dynamic (fast) contrast-enhanced magnetic resonance imaging with its temporal resolution allows evaluation of contrast kinetics of soft tissue sarcomas, its efficacy for defining viable tumor in these neoplasms was studied for three applications: biopsy localization, chemotherapeutic response, and differentiation between recurrence and inflammation after treatment. After conventional T1-weighted and T2-weighted magnetic resonance sequences to localize the lesion, patients had dynamic contrast-enhanced magnetic resonance imaging with fast and ultrafast sequences and postprocessing techniques (subtraction, time-intensity curves, and parametric color-encoding). In 10 of 40 patients, dynamic imaging more precisely defined the most malignant foci of tumor for biopsy than conventional magnetic resonance imaging. After chemotherapy, dynamic imaging distinguished 11 good responders from 21 poor responders. In followup of 196 patients, dynamic imaging detected 42 early enhancing recurrences and excluded recurrent tumor in six late enhancing pseudotumors. Dynamic imaging can differentiate viable tumor from nonviable tumor and inflammation by showing two temporally different phases of contrast enhancement: an early phase correlative with viable tumor at histologic examination, and a late phase when all tissues enhance simultaneously and may be indistinguishable. By showing tumor viability, dynamic contrast-enhanced magnetic resonance imaging can help define biopsy sites, chemotherapeutic response, and presence or absence of recurrences and therefore affect the initial evaluation, treatment, and followup of patients with soft tissue sarcomas.

The use of Gd-DTPA as a marker of myocardial viability in reperfused acute myocardial infarction

At present, accurate assessment of the extent of myocardial viability after acute myocardial infarction is limited due to the spatial resolution of currently available imaging modalities. MR cardiac imaging, with its superior spatial resolution, would be used if viable and infarcted tissue could be separated based on signal intensity. In infarcted tissue, cell membrane breakdown allows the entry of the MR contrast agent Gd-DTPA which is normally extracellular. The increased space for Gd-DTPA distribution (partition coefficient, lambda) in this infarcted tissue results in increased Gd-DTPA concentration and hence increased signal intensity on T1-weighted MR images. In a canine model of ischemia/reperfusion injury, the partition coefficient in infarcted tissue increased as early as 1 min post reperfusion. lambda in infarcted tissue stayed increased over that in normal tissue for at least 8 weeks. The accuracy of contrast-enhanced MRI was confirmed by results of 201Tl SPECT and a cine MRI dobutamine wall motion study in a patient 1 week after an acute myocardial infarction. Thus, contrast-enhanced MRI shows great promise for the non-invasive determination of myocardial viability after acute myocardial infarction.

Gd-DTPA bolus tracking in the myocardium using T1 fast acquisition relaxation mapping (T1 FARM)

MRI methods currently used for bolus tracking in the myocardium, such as saturation recovery turbo-fast low-angle shot (FLASH) (srTFL), are limited by signal intensity (SI) saturation at high contrast agent (CA) concentrations. By using T1 fast acquisition relaxation mapping (T1 FARM), a Gd-DTPA bolus (0.075 vs. 0.025 mmol/kg) may be injected without causing saturation. This study tested the feasibility of in vivo T1 FARM bolus tracking under rest/stress conditions in seven beagles with multiple permanently occluded branches of the left anterior descending (LAD) coronary artery. Although it underestimated the myocardial perfusion reserve (MPR) measured ex vivo using radioactive microspheres (mean +/- SEM; 3.60 +/- 0.26), the MPR determined upon application of the modified Kety model (1.86 +/- 0.10) enabled distinction between normal and infarcted tissue. The partition coefficient (lambda) estimated at rest and stress using the modified Kety model underestimated ex vivo radioactive measurements in infarcted tissue (0.25 +/- 0.01 vs. 0.26 +/- 0.01 vs. 0.79 +/- 0.08 ml/g, P < 0.0001) yet was accurate in normal tissue (0.28 +/- 0.01 vs. 0.30 +/- 0.01 vs. 0.33 +/- 0.01 ml/g, P = NS). Thus, although unsuitable for myocardial viability assessment, T1 FARM bolus tracking shows potential for assessment of myocardial perfusion.

Examining a canine model of stunned myocardium with Gd-DTPA-enhanced MRI

It has previously been shown that the distribution volume of Gd-DTPA (lambda) in infarcted, canine myocardium is higher than that of normal tissue. The purpose of this study was to determine whether stunned myocardium exhibits increased lambda. Stunning was produced in beagles by means of 30 min LAD occlusion followed by 3 weeks (n = 4) reperfusion. Gd-DTPA was infused at each imaging session and lambda determined in vivo using a saturation recovery turboFLASH sequence; cine imaging was used to assess ventricular wall thickening (%WT). (201)Tl uptake was used as an independent assessment of viability. %WT data confirmed that the brief insult caused prolonged, yet reversible, regional contractile dysfunction in each animal. %WT was not significantly different from baseline values by 3 weeks post-reflow. Normal (201)Tl uptake confirmed the absence of infarction. The lambda of stunned tissue (lambda = 0.381 +/- 0.030 ml/g) was not elevated above that of normal tissue (lambda = 0.398 +/- 0.027 ml/g, P = NS), at any time point studied, in vivo. These data suggest that an increase in lambda is a specific indicator of irreversible damage.

Measurement of the gadopentetate dimeglumine partition coefficient in human myocardium in vivo: normal distribution and elevation in acute and chronic infarction

PURPOSE

To establish a method for measuring the partition coefficient (lambda) of gadopentetate dimeglumine in humans in vivo, evaluate the spatial and intersubject variation in the lambda of normal myocardium, and compare these values on a regional basis with lambda values of acute and chronic infarcted myocardium.

MATERIALS AND METHODS

Twelve healthy subjects and patients with acute (n = 5) or chronic (n = 5) myocardial infarction underwent magnetic resonance imaging at 1.5 T. Look-Locker images were acquired at four short-axis levels to measure myocardial and blood longitudinal relaxation time at baseline and after a 30-40-minute infusion of gadopentetate dimeglumine. lambda was calculated as DeltaR1(M)/DeltaR1(B, )where M = myocardium, and B = blood.

RESULTS

The magnitude of the estimated lambda in normal myocardium was uniform over the entire myocardium at 0.56 mL/g +/- 0.10 (SD). The lambda values in patients with acute (0.91 mL/g +/- 0.11, P <.001) or chronic (lambda = 0.78 mL/g +/- 0.09, P <.001) infarction were significantly elevated, as compared with those in healthy subjects. A 20% elevation in lambda, as compared with the mean value of a corresponding normal circumferential segment, allowed identification of chronically (sensitivity, 88%; specificity, 96%) or acutely (sensitivity, 100%; specificity, 98%) infarcted segments.

CONCLUSION

Quantification of the lambda in vivo allows differentiation between normal and acutely or chronically infarcted myocardium, with high sensitivity and specificity.

Clinical assessment of myocardial viability using MRI during a constant infusion of Gd-DTPA

This study assessed the accuracy and feasibility of magnetic resonance imaging (MRI) during a constant infusion of gadolinium diethylenetriaminepentaacetic acid (Gd-DTPA) for the determination of myocardial viability in patients with recent acute myocardial infarction (AMI). Nine patients were studied within 10 days of AMI. Rest-redistribution 201Thallium (201Tl) single photon emission computed tomography (SPECT) was used as a gold standard for viability. Using MRI, regional perfusion was assessed using dynamic imaging during a bolus injection of Gd-DTPA and viability was assessed during a continuous infusion. Finally, cine MR images were acquired at baseline, during low-dose dobutamine infusion and after recovery. To assess viability, the left ventricle was divided into 16 segments and signal intensity in corresponding MRI and redistribution SPECT segments were compared. Wall thickening index (WTI) was determined at each step during the dobutamine study. The results revealed that in five patients, reduced perfusion in infarcted regions was observed qualitatively during dynamic first pass imaging. There was a significant inverse correlation between 201Tl uptake and MRI signal intensity, i.e. infarcted tissue (low 201Tl uptake) had increased MR signal intensity. Segments were separated into normal (201Tl uptake > 90%) and infarcted (< 601%). lnfarcted MRI segments had greater signal intensity than normal segments (179 +/- 50 vs. 102 +/- 14%; P < 0.0001). WTI in normal segments increased by 18 +/- 8.5% (P < 0.0001) from baseline to 10 microg/kg per min of dobutamine while infarcted tissue WTI decreased 2.8 +/- 7.2% (P = 0.17). Thus regions of myocardium that were infarcted as defined by reduced 201Tl uptake and absent contractile reserve showed greatly increased MRI signal intensity during a constant infusion of Gd-DTPA. The use of MRI during a constant infusion of Gd-DTPA is accurate and feasible for the determination of myocardial necrosis in a clinical setting.

Contrast-enhanced MRI for the assessment of myocardial viability after permanent coronary artery occlusion

Previous studies in a model of ischemia/reperfusion using a constant infusion of Gd-DTPA have shown that distribution volume (lambda) is increased in infarcted myocardial tissue. This study examined this technique in the setting of permanent coronary artery occlusion. Ten beagles underwent permanent occlusion of a coronary artery for 2 days (N = 3), 1 week (N = 4), or 3 weeks (N = 3). Imaging was performed at 2 days and, depending on the length of occlusion, 1 week, 2 weeks, and 3 weeks to follow changes in lambda in vivo. At sacrifice, (201)Tl was injected and the extent of the hyperenhanced region was compared to pathology. lambda was increased in infarcted tissue by 2 days post occlusion and this increase persisted for 3 weeks. At sacrifice, lambda correlated strongly with (201)Tl uptake (r = -0.86 to -0.95, P < 0.05; i.e., lambda increased in infarcted tissue) and the size of the hyperenhanced region was comparable to pathological infarct size (slope 1.006, r = 0.96, P < 0.0001). Thus, beyond 2 days after coronary occlusion, MRI, during a constant infusion of Gd-DTPA, can assess myocardial viability regardless of the success of reperfusion. Magn Reson Med 44:309-316, 2000.

Kinetic characterization of CMD-A2-Gd-DOTA as an intravascular contrast agent for myocardial perfusion measurement with MRI

Recent developments in magnetic resonance imaging (MRI) using specific contrast media allow the assessment of myocardial perfusion. The purpose of this study was to characterize the intravascular properties of a new macromolecular contrast agent, CMD-A2-Gd-DOTA, to evaluate myocardial perfusion. Two groups of isolated pig hearts perfused at various controlled flows were used. To demonstrate the intravascular properties of CMD-A2-Gd-DOTA, the agent was simultaneously injected with 99mTc-labeled red blood cells in five hearts (group 1). Tracer kinetics of both compounds were assessed by coronary sinus effluent sampling, radioactivity counting and concentration determination in samples for first-pass time curves measurements. Five other hearts (group 2) were studied using a two-slice turboFLASH sequence on a 1.5-T whole-body MRI in order to evaluate first-pass CMD-A2-Gd-DOTA signal intensity (SI) versus time curves. In group 1, for the studied flows ranging from 0.8 to 3.1 ml/min(-1) x g(-1), CMD-A2-Gd-DOTA showed first-pass concentration curves typical of an intravascular contrast agent. In group 2, MRI parameters, i.e., upslope and mean transit time of SI time curves correlated strongly with myocardial perfusion. Within the physiologic range of flows, CMD-A2-Gd-DOTA was able to demonstrate tracer kinetics for in vivo assessment of myocardial perfusion using MRI.

Contrast-enhanced MRI for quantification of myocardial viability

During the past 10 years substantial advances have taken place in magnetic resonance imaging (MRI) capabilities and in contrast media development. Furthermore, knowledge of in vivo contrast media interactions with surrounding water and distribution into tissue has increased, permitting regional quantification of concentration-time profiles in the myocardium. The combination of these advances has substantially improved the capability of contrast-enhanced MRI characterization of myocardial ischemic injury, including its ability to discriminate viable from nonviable zones. Discrimination of viable from nonviable myocardial subregions is important for patient management and for research applications. This review addresses recent progress toward the goal of defining viable and nonviable myocardium based on MRI detection of contrast media effects. J. Magn. Reson. Imaging 1999;10:694-702.

T1 fast acquisition relaxation mapping (T1-FARM): an optimized reconstruction

Maps of spin lattice relaxation time (T1) can be reconstructed directly from magnetic resonance imaging (MRI) k-space data measured with very short data acquisition times, e.g., a data set for a 128 x 128 T1 map can be acquired in less than 3 s using gradients with 10-T/m/s slew rate. In principle, this approach could be extended to quantitate other MRI parameters but current use is limited by the lack of precise, accurate and fast reconstruction. Using theoretical calculations, computer simulations, and experiments we have optimized a parametric reconstruction method using a Leverberg-Marquardt (L-M) algorithm and compared it to the quasi-Newton method originally used. We have found significant improvement using the L-M method provided T1 is solved for directly without linearization. Reconstruction time was reduced by a factor of 60. Computer simulations show that the method has acceptable accuracy even in signals with 5% noise. Optimization included the investigation of the signal-to-noise (S/N) of each k-space data point and its impact on relative error of the reconstruction. This result indicates that rectangular k-space data could be collected for further reduction of data acquisition times. Determination of T1 maps by direct parametric reconstruction of k-space data appears feasible and may stimulate further application of quantitative MRI.

Regional measurement of the Gd-DTPA tissue partition coefficient in canine myocardium

Quantifiable MRI perfusion studies using the contrast agent Gd-DTPA require measurement or estimation of the tissue partition coefficient (lambda) for tracer kinetic modeling. Radiotracer techniques were used to obtain regional lambda measurements from the left ventricles of five dogs. Measurements were analyzed to determine whether spatial heterogeneity was a major component of lambda variability. No systematic variations were identified in terms of radial position, short-axis slice location, or wall position. The high lambda variability seen in this study and in cited data of others may be due in part to tissue heterogeneity in interstitial volume, plasma volume, and perfusate hematocrit.

The determination of myocardial viability using Gd-DTPA in a canine model of acute myocardial ischemia and reperfusion

The partition coefficient of Gd-DTPA was thought to vary with the amount of cellular membrane damage after an acute myocardial infarction. The relationship between the partition coefficient of Gd-DTPA (lambda) and the uptake of 201Tl (as a marker of tissue viability) was studied 2 h to 3 weeks after reperfusion of a 2-h occlusion to the left anterior descending coronary artery in a canine model. Gd-DTPA was infused as a bolus followed by a prolonged constant infusion, and this infusion protocol was optimized such that the concentration of Gd-DTPA was directly related to lambda. After this infusion, MR images of excised hearts showed regions of increased signal intensity corresponding to increased Gd-DTPA concentration. At all time points, lambda and 201Tl uptake were strongly negatively correlated indicating that lambda is an accurate indicator of myocardial viability. Furthermore, lambda in the infarcted regions was increased relative to normal regions after 2 h of reperfusion and stayed elevated up to 3 weeks. At all time points, lambda in the infarcted and normal regions were significantly different. As well, this data showed a trend that lambda in infarcted regions decreased monotonically from 1 day to 3 weeks. This trend was confirmed with MR imaging by examining the change in signal intensity of in vivo images from 4 days to 3 weeks in two animals. These results suggest that MRI with Gd-DTPA could be used to measure the extent of myocardial damage after an acute myocardial infarction.

Dynamic contrast-enhanced MR imaging of musculoskeletal tumors: basic principles and clinical applications

The purpose of this article is to review the basic principles and clinical applications of dynamic contrast-enhanced MRI in the musculoskeletal system. This method of physiologic imaging provides clinically useful information by depicting tissue vascularization and perfusion, capillary permeability, and composition of the interstitial space. Different imaging, evaluation, and postprocessing techniques are described. The most important applications in the musculoskeletal system are identification of areas of viable tumor for biopsy, tissue characterization, and monitoring of preoperative chemotherapy. Practical guidelines for performing a dynamic contrast-enhanced MR examination are proposed.

Noninvasive assessment of no-reflow phenomenon in a canine model of reperfused infarction by contrast-enhanced magnetic resonance imaging

The aim of this study was to test whether contrast-enhanced magnetic resonance (MR) imaging may assess in vivo the severity of the no-reflow phenomenon in a dog model of infarction with 2-hour coronary occlusion followed by reperfusion (6 hours). Subsecond MR imaging combined with intravenous bolus administration of superparamagnetic iron oxide particles (SPIO) was performed at the fifth hour of reperfusion. An MR index was calculated using the difference of signal-intensity enhancement between ischemic and nonischemic zones during the SPIO first pass. Dogs were separated into two groups according to the severity of ischemia: collateral blood flow in the central ischemic zone at 120 minutes of occlusion (radioactive microsphere technique) < 22.5% of the flow in the nonischemic zone (group I) or > 22.5% (group II). Mean collateral blood flow during occlusion was lower in group I (11.3% +/- 2.9%, n = 7) than in group II (66.8% +/- 19.8%, n = 6, p < 0.05). Mean infarct size was significantly larger in group I (58.6% +/- 4.9% of the area-at-risk, n = 7) than in group II (16.5% +/- 6.5%, n = 6, p < 0.05). For the entire population (n = 13), the infarct size was inversely correlated to the collateral blood flow (r = -0.64, p = 0.02, standard error of estimate = 0.24). The relative rate of enhancement in ischemic myocardium (MR index) was significantly lower in group I (38.1% +/- 10.9%) than in group II (142.8% +/- 32%; p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of bolus volume and dose of gadolinium chelate for first-pass myocardial perfusion MR imaging studies

First-pass MR myocardial perfusion measurements require a well-defined left ventricular (LV) blood pool input function. We used a peripheral intravenous (i.v.) injection of a gadolinium (Gd) chelate to obtain a well-characterized LV time-intensity curve. Using a strongly T1-weighted subsecond MR sequence, we performed cardiac MR imaging after administering an IV bolus injection of one of three different doses of the Gd chelate: a standard dose (0.1 mmol/kg, group I, n = 8); a low dose with two bolus volumes (0.01 mmol/kg, 1/10e bolus volume, group II, n = 7, and 0.01 mmol/kg diluted in saline, same bolus volume as group I, group III, n = 3); and an intermediate dose (0.05 mmol/kg, group IV, n = 5). Unlike in group I (high dose), in groups II and III (low dose), the LV curve had a well-defined first peak, followed by a downslope and a recirculation peak. With the intermediate dose (group IV), a saturation effect still remained on the LV curve. The signal intensity (SI) enhancement of the myocardium was respectively 580 +/- 77% at 0.1 mmol/kg, 362 +/- 95% at 0.05 mmol/kg, and at 0.01 mmol/kg, it was 184 +/- 33% in group II and 272 +/- 8% in group III. In conclusion, with subsecond T1-weighted MR imaging and a low dose of Gd chelate (i.e., 0.01 mmol/kg), the LV input function is a well-defined first step for MR perfusion modeling.

Assessment of myocardial perfusion using contrast-enhanced MR imaging: current status and future developments

Excellent inherent tissue contrast is one of the great promises of clinical magnetic resonance (MR) imaging, but functional information is relatively limited. However, MR imaging complemented by the administration of contrast agents can provide such functional assessment. The perfusion status of the myocardium is one of the most important functional information in cardiovascular imaging. Because the clinical acceptance of a contrast agent is measured by its ability to improve patient outcome and to guide therapy, it is unlikely that detection of myocardial infarction, the final stage of ischemic heart disease, should be the target for contrast media development. It would obviously be better if occult regional myocardial perfusion deficits could be reliably detected. The current article was prepared to help the clinical radiologist to keep pace with new strategies for myocardial enhancement and their potential clinical applicability for detection of early perfusion deficits. Several techniques for noninvasive measurement of myocardial perfusion are currently evolving which have the potential to be introduced into routine MR imaging. Most investigators favor a first-pass analysis of the contrast agent bolus through the myocardium using ultrafast sequences. However, such a technique may require clinical introduction of a blood pool agent. There are good reasons to favor T1-weighted sequences over susceptibility imaging in such first-pass studies. In the future, assessment of myocardial perfusion status using contrast-enhanced MR imaging may be done producing perfusion maps with high spatial resolution (e.g., 256 x 128), with sequences available on most scanners without special hardware requirements (e.g., IR-Turboflash, keyhole imaging), and requiring only a short period of time for examination (approximately 3 min).

Measurement of kinetic perfusion parameters of gadoteridol in intact myocardium: effects of ischemia/reperfusion and coronary vasodilation

Quantitation of myocardial perfusion is feasible using contrast enhanced magnetic resonance imaging. A method to quantitate myocardial blood flow is provided by the Kety model modified to account for a diffusable tracer such as gadoteridol. In the present study, perfusion parameters of the modified Kety model (partition coefficient and extraction efficiency) were determined for gadoteridol in intact myocardium using a constant flow, isolated, perfused heart model. Perfusion conditions included hearts with normal perfusion, hearts made globally ischemic for 20 min then perfused normally, and hearts whose coronary flow was more than doubled with 9 microM adenosine. T1 relaxation times were rapidly measured at 0.5 T following step increases in perfusate gadoteridol concentration and at steady state. Both the partition coefficient and extraction efficiency were found to be significantly increased in ischemic/reperfused hearts compared to normal. While flow rates in adenosine hearts were too high for accurate extraction efficiency determination using this technique, the partition coefficient was no different between adenosine and normally perfused hearts. The method described in this article allowed the kinetic parameters of the modified Kety model to be determined in intact heart using NMR relaxation time measurements as the basis of the calculation.

Regional myocardial blood volume and flow: first-pass MR imaging with polylysine-Gd-DTPA

The authors investigated the utility of an intravascular magnetic resonance (MR) contrast agent, poly-L-lysine-gadolinium diethylenetriaminepentaacetic acid (DTPA), for differentiating acutely ischemic from normally perfused myocardium with first-pass MR imaging. Hypoperfused regions, identified with microspheres, on the first-pass images displayed significantly decreased signal intensities compared with normally perfused myocardium (P < .0007). Estimates of regional myocardial blood content, obtained by measuring the ratio of areas under the signal intensity-versus-time curves in tissue regions and the left ventricular chamber, averaged 0.12 mL/g +/- 0.04 (n = 35), compared with a value of 0.11 mL/g +/- 0.05 measured with radiolabeled albumin in the same tissue regions. To obtain MR estimates of regional myocardial blood flow, in situ calibration curves were used to transform first-pass intensity-time curves into content-time curves for analysis with a multiple-pathway, axially distributed model. Flow estimates, obtained by automated parameter optimization, averaged 1.2 mL/min/g +/- 0.5 (n = 29), compared with 1.3 mL/min/g +/- 0.3 obtained with tracer microspheres in the same tissue specimens at the same time. The results represent a combination of T1-weighted first-pass imaging, intravascular relaxation agents, and a spatially distributed perfusion model to obtain absolute regional myocardial blood flow and volume.

Measurement of the extraction efficiency and distribution volume for Gd-DTPA in normal and diseased canine myocardium

We have previously shown that the myocardial Gd-DTPA concentration ([Gd-DTPA]t(t)) after a bolus injection of Gd-DTPA can be predicted by the Modified Kety Equation (MKE). If [Gd-DTPA]t(t) can be determined by MRI and the data fit to the MKE, then the distribution volume (lambda) of Gd-DTPA and the myocardial flow (F) times the extraction efficiency (E), i.e., the FE product, can be determined. Therefore F can only be quantified if E is known. We measured the global E in vivo in normal canine myocardium, and measured E and lambda, in vitro, locally in normal, acute ischemic (n = 5; coronary artery occlusion < 4 h), infarcted (n = 4; coronary artery occlusion, 6 days) and reperfused (n = 4; coronary artery occlusion 2 h, and reperfusion 2 h and 6 days) myocardium. Results indicate that E differs with F and with individuals and consequently, F cannot be quantified using the MKE unless the local E is also determined in vivo.