Identification of myocardial reperfusion with echo planar magnetic resonance imaging. Discrimination between occlusive and reperfused infarctions

Cardiac MR imaging: current status and future direction

Coronary artery disease is currently a worldwide epidemic with increasing impact on healthcare systems. Magnetic resonance imaging (MRI) sequences give complementary information on LV function, regional perfusion, angiogenesis, myocardial viability and orientations of myocytes. T2-weighted short-tau inversion recovery (T2-STIR), fat suppression and black blood sequences have been frequently used for detecting edematous area at risk (AAR) of infarction. T2 mapping, however, indicated that the edematous reaction in acute myocardial infarct (AMI) is not stable and warranted the use of edematous area in evaluating therapies. On the other hand, cine MRI demonstrated reproducible data on LV function in healthy volunteers and LV remodeling in patients. Noninvasive first pass perfusion, using exogenous tracer (gadolinium-based contrast media) and arterial spin labeling MRI, using endogenous tracer (water), are sensitive and useful techniques for evaluating myocardial perfusion and angiogenesis. Recently, new strategies have been developed to quantify myocardial viability using T1-mapping and equilibrium contrast enhanced MR techniques because existing delayed contrast enhancement MRI (DE-MRI) sequences are limited in detecting patchy microinfarct and diffuse fibrosis. These new techniques were successfully used for characterizing diffuse myocardial fibrosis associated with myocarditis, amyloidosis, sarcoidosis heart failure, aortic hypertrophic cardiomyopathy, congenital heart disease, restrictive cardiomyopathy, arrhythmogenic right ventricular dysplasia and hypertension). Diffusion MRI provides information regarding microscopic tissue structure, while diffusion tensor imaging (DTI) helps to characterize the myocardium and monitor the process of LV remodeling after AMI. Novel trends in hybrid imaging, such as cardiac positron emission tomography (PET)/MRI and optical imaging/MRI, are recently under intensive investigation. With the promise of higher spatial-temporal resolution and 3D coverage in the near future, cardiac MRI will be an indispensible tool in the diagnosis of cardiac diseases, coronary intervention and myocardial therapeutic delivery.

Pathological mechanism for delayed hyperenhancement of chronic scarred myocardium in contrast agent enhanced magnetic resonance imaging

Objectives

To evaluate possible mechanism for delayed hyperenhancement of scarred myocardium by investigating the relationship of contrast agent (CA) first pass and delayed enhancement patterns with histopathological changes.

Materials and Methods

Eighteen pigs underwent 4 weeks ligation of 1 or 2 diagonal coronary arteries to induce chronic infarction. The hearts were then removed and perfused in a Langendorff apparatus. The hearts firstly experienced phosphorus 31 MR spectroscopy. The hearts in group I (n = 9) and II (n = 9) then received the bolus injection of Gadolinium diethylenetriamine pentaacetic acid (0.05 mmol/kg) and gadolinium-based macromolecular agent (P792, 15 µmol/kg), respectively. First pass T₂^* MRI was acquired using a gradient echo sequence. Delayed enhanced T₁ MRI was acquired with an inversion recovery sequence. Masson's trichrome and anti- von Willebrand Factor (vWF) staining were performed for infarct characterization.

Results

Wash-in of both kinds of CA caused the sharp and dramatic T₂^* signal decrease of scarred myocardium similar to that of normal myocardium. Myocardial blood flow and microvessel density were significantly recovered in 4-week-old scar tissue. Steady state distribution volume (ΔR₁ relaxation rate) of Gd-DTPA was markedly higher in scarred myocardium than in normal myocardium, whereas ΔR₁ relaxation rate of P792 did not differ significantly between scarred and normal myocardium. The ratio of extracellular volume to the total water volume was significantly greater in scarred myocardium than in normal myocardium. Scarred myocardium contained massive residual capillaries and dilated vessels. Histological stains indicated the extensively discrete matrix deposition and lack of cellular structure in scarred myocardium.

Conclusions

Collateral circulation formation and residual vessel effectively delivered CA into scarred myocardium. However, residual vessel without abnormal hyperpermeability allowed Gd-DTPA rather than P792 to penetrate into extravascular compartment. Discrete collagen fiber meshwork and loss of cellularity enlarged extracellular space accessible to Gd-DTPA, resulting in the delayed hyper-enhanced scar.

Delayed contrast enhancement on MR images of myocardium: past, present, future

Differential enhancement of myocardial infarction was first recognized on computed tomographic (CT) images obtained with iodinated contrast material in the late 1970s. Gadolinium enhancement of myocardial infarction was initially reported for T1-weighted magnetic resonance (MR) imaging in 1984. The introduction of an inversion-recovery gradient-echo MR sequence for accentuation of the contrast between normal and necrotic myocardium was the impetus for widespread clinical use for demonstrating the extent of myocardial infarction. This sequence has been called delayed-enhancement MR and MR viability imaging. The physiologic basis for differential enhancement of myocardial necrosis is the greater distribution volume of injured myocardium compared with that of normal myocardium. It is now recognized that delayed enhancement occurs in both acute and chronic (scar) infarctions and in an array of other myocardial processes that cause myocardial necrosis, infiltration, or fibrosis. These include myocarditis, hypertrophic cardiomyopathy, amyloidosis, sarcoidosis, and other myocardial conditions. In several of these diseases, the presence and extent of delayed enhancement has prognostic implications. Future applications of delayed enhancement with development of MR imaging and CT techniques will be discussed.

Quantification of 3D regional myocardial wall thickening from gated magnetic resonance images

PURPOSE

To develop 3D quantitative measures of regional myocardial wall motion and thickening using cardiac magnetic resonance imaging (MRI) and to validate them by comparison to standard visual scoring assessment.

MATERIALS AND METHODS

In all, 53 consecutive subjects with short-axis slices and mid-ventricular 2-chamber/4-chamber views were analyzed. After correction for breath-hold-related misregistration, 3D myocardial boundaries were fitted to images and edited by an imaging cardiologist. Myocardial thickness was quantified at end-diastole and end-systole by computing the 3D distances using Laplace's equation. 3D thickening was represented using the standard 17-segment polar coordinates. 3D thickening was compared with 3D wall motion and with expert visual scores (6-point visual scoring of wall motion and wall thickening; 0 = normal; 5 = greatest abnormality) assigned by imaging cardiologists.

RESULTS

Correlation between ejection fraction and thickening measurements was (r = 0.84; P < 0.001) compared to correlation between ejection fraction and motion measurements (r = 0.86; P < 0.001). Good negative correlation between summed visual scores and global wall thickening and motion measurements were also obtained (r(thick) = -0.79; r(motion) = -0.74). Additionally, overall good correlation between individual segmental visual scores with thickening/wall motion (r(thick) = -0.69; r(motion) = -0.65) was observed (P < 0.0001).

CONCLUSION

3D quantitative regional thickening and wall motion measures obtained from MRI correlate strongly with expert clinical scoring.

Myocardial wall thickening from gated Magnetic Resonance images using Laplace's equation

The aim of our work is to present a robust 3D automated method for measuring regional myocardial thickening using cardiac magnetic resonance imaging (MRI) based on Laplace's equation. Multiple slices of the myocardium in short-axis orientation at end-diastolic and end-systolic phases were considered for this analysis. Automatically assigned 3D epicardial and endocardial boundaries were fitted to short-axis and long axis slices corrected for breathold related misregistration, and final boundaries were edited by a cardiologist if required. Myocardial thickness was quantified at the two cardiac phases by computing the distances between the myocardial boundaries over the entire volume using Laplace's equation. The distance between the surfaces was found by computing normalized gradients that form a vector field. The vector fields represent tangent vectors along field lines connecting both boundaries. 3D thickening measurements were transformed into polar map representation and 17-segment model (American Heart Association) regional thickening values were derived. The thickening results were then compared with standard 17-segment 6-point visual scoring of wall motion/wall thickening (0=normal; 5=greatest abnormality) performed by a consensus of two experienced imaging cardiologists. Preliminary results on eight subjects indicated a strong negative correlation (r=-0.8, p<0.0001) between the average thickening obtained using Laplace and the summed segmental visual scores. Additionally, quantitative ejection fraction measurements also correlated well with average thickening scores (r=0.72, p<0.0001). For segmental analysis, we obtained an overall correlation of -0.55 (p<0.0001) with higher agreement along the mid and apical regions (r=-0.6). In conclusion 3D Laplace transform can be used to quantify myocardial thickening in 3D.

Reperfusion revisited: beyond TIMI 3 flow

Therapy for acute myocardial infarction has advanced dramatically since the early 1980s with the use of early intravenous fibrinolytic therapy. Combining low-dose fibrinolysis and platelet lysis appears to provide an additional increase in infarct-related artery (IRA) patency, but the large-scale mortality reduction trials evaluating this strategy are just getting under way. Recently, considerable attention has shifted away from the epicardial arteries to the microvasculature. Contemporary evidence suggests that epicardial patency does not necessarily translate to actual perfusion at the myocardial level. Techniques to evaluate beyond thrombolysis in myocardial infarction (TIMI) epicardial flow are now available and validated. In addition, there are promising treatments for the prevention or alleviation of certain forms of microvascular obstruction. This review attempts to clarify the confusion surrounding epicardial flow and "myocardial malperfusion" and to provide some insight into the next direction in acute myocardial infarction therapeutics.

Noninvasive assessment of myocardial viability in a small animal model: comparison of MRI, SPECT, and PET

Acute myocardial infarction (AMI) research relies increasingly on small animal models and noninvasive imaging methods such as MRI, single-photon emission computed tomography (SPECT), and positron emission tomography (PET). However, a direct comparison among these techniques for characterization of perfusion, viability, and infarct size is lacking. Rats were studied within 18-24 hr post AMI by MRI (4.7 T) and subsequently (40-48 hr post AMI) by SPECT ((99)Tc-MIBI) and micro-PET ((18)FDG). A necrosis-specific MRI contrast agent was used to detect AMI, and a fast low angle shot (FLASH) sequence was used to acquire late enhancement and functional images contemporaneously. Infarcted regions showed late enhancement, whereas corresponding radionuclide images had reduced tracer uptake. MRI most accurately depicted AMI, showing the closest correlation and agreement with triphenyl tetrazolium chloride (TTC), followed by SPECT and PET. In some animals a mismatch of reduced uptake in normal myocardium and relatively increased (18)FDG uptake in the infarct border zone precluded conventional quantitative analysis. We performed the first quantitative comparison of MRI, PET, and SPECT for reperfused AMI imaging in a small animal model. MRI was superior to the other modalities, due to its greater spatial resolution and ability to detect necrotic myocardium directly. The observed (18)FDG mismatch likely represents variable metabolic conditions between stunned myocardium in the infarct border zone and normal myocardium and supports the use of a standardized glucose load or glucose clamp technique for PET imaging of reperfused AMI in small animals.

MDCT of the myocardium: a new contribution to ischemic heart disease

RATIONALE AND OBJECTIVES

Despite the progress made in diagnosis and treatment, cardiovascular diseases remain the main cause of death worldwide.

MATERIALS AND METHODS

Multidetector row computed tomography (MDCT) provides several diagnostic insights, namely assessment of coronary artery anatomy and measurement of left ventricular volume and function. The ability of CT to show myocardial infarcted areas as an enhanced territory was described in the late 1970s in an animal model.

RESULTS

This method found a second wind with the arrival of MDCT technology that led to its clinical application. Several authors describe the ability of MDCT to assess myocardial injury both in animals and humans. The MDCT assessment of myocardial late enhancement is based on the same principle as delayed enhancement MRI.

CONCLUSIONS

The aim of this review is to cover the technical aspects of cardiac MDCT in assessing the myocardium and its potential in diagnosing ischemic heart disease.

Magnetic resonance imaging in the evaluation of non-ischemic cardiomyopathies: current applications and future perspectives

Patients with non-ischemic cardiomyopathy often represent a diagnostic challenge, and correct etiologic diagnosis may influence outcomes. Lately, delayed myocardial enhancement MR imaging has been developed and is currently being used for a growing number of clinical applications. On delayed enhancement MR images, scarring or fibrosis appears as an area of high signal intensity, and the pattern by which this enhancement occurs in the myocardium allows distinction of many different pathologies. In nonischemic cardiomyopathy, the delayed enhancement usually does not occur in a coronary artery distribution and is often midwall rather than subendocardial or transmural. It could also guide myocardial biopsy to an affected area, increasing its yield. Cardiac magnetic resonance imaging has now a definitive role in clinical practice, and its capability to non-invasively provide high resolution images of the heart with good tissue characterization is redefining the understanding of the conditions that can adversely affect the myocardium.

Characterization of myocardial viability using MR and CT imaging

Cardiovascular magnetic resonance (MR) imaging is of proven clinical value for the noninvasive characterization of myocardial viability. Computed tomography (CT) is also being exploited for this indication. Examples of each of these imaging strategies for the assessment of myocardial viability will be provided in this review. Key MRI concepts and practical considerations such as customized MR imaging techniques and tailored imaging protocols dedicated to viability assessment are outlined with the primary focus on recent developments. Clinical applications of MR-based viability assessment are reviewed, ranging from rapid functional cine imaging to tissue characterization using T2-weighted imaging and T1-weighted late-contrast-enhanced imaging. Next, the merits and limitations of state-of-the-art CT imaging are surveyed, and their implications for viability assessment are considered. The final emphasis is on current trends and future directions in noninvasive viability assessment using MRI and CT.

Contrast agents and cardiac MR imaging of myocardial ischemia: from bench to bedside

This review paper presents, in the first part, the different classes of contrast media that are already used or are in development for cardiac magnetic resonance imaging. A classification of the different types of contrast media is proposed based on the distribution of the compounds in the body, their type of relaxivity and their potential affinity to particular molecules. In the second part, the different uses of the extracellular type of T1-enhancing contrast agent for myocardial imaging is covered from the detection of stable coronary artery disease to the detection and characterization of chronic infarction. A particular emphasis is placed on the clinical use of gadolinium-chelates, which are the universally used type of MRI contrast agent in the clinical routine. Both approaches, first-pass magnetic resonance imaging (FP-MRI) as well as delayed-enhanced magnetic resonance imaging (DE-MRI), are covered in the different situations of acute and chronic myocardial infarction.

MR imaging in ischemic heart disease

This article reviews the current MR imaging literature with respect to ischemic heart disease and focuses on the clinical practicalities of cardiac MR imaging today.

New concepts in characterization of ischemically injured myocardium by MRI

New concepts regarding the assessment of ischemic myocardial injuries have been addressed in this Minireview using magnetic resonance imaging (MRI). MRI, with its different techniques, brings not only anatomic, but also physiologic, information on ischemic heart disease. It has the ability to measure identical parameters in preclinical and clinical studies. MRI techniques provide the ideal package for repeated and noninvasive assessment of myocardial anatomy, viability, perfusion, and function. MR contrast agents can be applied in a variety of ways to improve MRI sensitivity for detecting and assessing ischemically injured myocardium. With MR contrast agents protocol, it becomes possible to identify ischemic, acutely infarcted, and peri-infarcted myocardium in occlusive and reperfused infarctions. Necrosis specific and nonspecific extracellular contrast-enhanced MRI has been used to assess myocardial viability. Contrast-enhanced perfusion MRI can explore the disturbances in large (angiography) and small coronary arteries (myocardial perfusion) as the underlying cause of myocardial dysfunction. Perfusion MRI has been used to measure myocardial perfusion (ml/min/g) and to demonstrate the difference in transmural myocardial blood flow. Information on no-reflow phenomenon is derived from dynamic changes in regional signal intensity after bolus injection of MR contrast agents. Another development is the near future availability of blood pool MR contrast agents. These agents are able to assess microvascular permeability and integrity and are advantageous in MR angiography (MRA) due to their persistence in the blood. Noncontrast-enhanced MRI such as cine MRI at rest/stress, sodium MRI, and MR spectroscopy also have the potential to noninvasively assess myocardial viability in patients. Futuristic applications for MRI in the heart will focus on identifying coronary artery disease at an early stage and the beneficial effects of new therapeutic agents such as intra-arterial gene therapy. MR techniques will have great future in the drug discovery process and in testing the effects of drugs on myocardial biochemistry, physiology, and morphology. Molecular imaging is going to bloom in this decade.

Magnetic resonance imaging of myocardial infarct

Magnetic resonance imaging offers the unique opportunity to directly visualize the size and location of myocardial infarcts (MIs) with excellent spatial resolution. Because infarct size is the most important determinant of postinfarct outcome, precise determination of infarct size may be valuable to risk stratify patients after acute MI. In addition, infarct imaging may provide direct information on the amount of irreversibly injured myocardium and thus can be used to identify myocardial viability in dysfunctional regions. Acute infarcts can be recognized as hyperintense signal on T2-weighted spin-echo images. This technique, however, does not identify chronic infarcts and may overestimate infarct size by including area at risk. Also, T2-weighted images often have a low signal-to-noise ratio. Contrast-enhanced perfusion imaging provides better-quality images. Extravascular contrast agents such as (Gd-DTPA) gadolinium diethyletriamine-pentaacetic acid identify infarcts as hyperenhanced regions on images acquired late after contrast injection. In addition, these tracers can examine the integrity and permeability of infarct microvasculature on first-pass perfusion images. Necrosis avid tracers and 23Na imaging are other new exciting approaches to identify infarcted myocardium acutely after MI. These techniques, are still investigational, and their value for clinical imaging remains to be established.

Blood pool MR contrast agents for cardiovascular imaging

Currently available magnetic resonance (MR) contrast agents are not confined to the intravascular space because of their small molecular size. These agents produce peak vascular enhancement for only a short period. Conversely, blood pool agents have longer intravascular residence time and higher relaxivity. Therefore these agents provide MR angiography with flexibility, versatility, and accuracy. With blood pool agents, the timing of contrast injection becomes less significant because the optimal imaging window is in tens of minutes rather than seconds. In addition, larger anatomic regions can be imaged optimally. Preliminary evidence appears to support the notion that blood pool agents may play a diagnostic role in coronary, peripheral, and pulmonary angiography. Besides their ability to increase vascular contrast, blood pool agents provide physiologic information, including rate of entry, rate of accumulation, and rate of elimination. MR imaging with blood pool agents also have proven to be of significant value in the assessments of myocardial perfusion and microvascular permeability. In anticipation of broad clinical use, blood pool agents are currently being evaluated in human trails. Examples include gadolinium-chelate that binds in vivo to albumin to form blood pool agents and ultrasmall superparamagnetic iron oxide particles. This review discusses the applications of MR blood pool agents in the cardiovascular system. J. Magn. Reson. Imaging 2000;12:890-898.

Magnetic resonance imaging of regional myocardial perfusion in patients with single-vessel coronary artery disease: quantitative comparison with (201)Thallium-SPECT and coronary angiography

The clinical value of magnetic resonance perfusion imaging (MRI) was investigated by quantitative comparison with (201)thallium-single-photon emission computed tomography ((201)TI-SPECT) and quantitative coronary angiography (QCA). Short-axis imaging was performed during dipyridamole administration in 13 patients with single-vessel coronary artery disease. Using inner and outer contours, the myocardium was divided into 30 contiguous, radial regions. Defining a perfusion defect as a region with less than 90% of maximum (201)TI intensity, nine patients had a matching perfusion defect, two had no defect on both (201)TI-SPECT or MRI, and one had a defect on (201)TI-SPECT but not on MRI. One patient had a defect on both modalities but with inaccurate localization. Three perfusion parameters were investigated: a) maximum contrast enhancement (MCE); b) slope of the signal intensity versus time curve; and c) inverse mean transit time (1/MTT). The sensitivity and specificity of MCE in the detection of perfusion abnormalities with TI-SPECT as the reference method were 71% and 71%, respectively (slope 77% and 61%, 1/MTT 44% and 70%). Furthermore, correlations were calculated per patient for the entire circumference of the short-axis myocardium. Median correlations were as follows: MCE 0.92, slope 0.91, and 1/MTT 0.40. Mismatches between (201)TI defects and defects on MRI resulted in low mean correlations (MCE 0.45, slope 0.46, and 1/MTT 0.26). There was a trend between severity of perfusion defects on MRI (using MCE) and QCA stenosis area (r = -0.56, P = 0.06). Thus, MRI and (201)TI-SPECT demonstrate fair agreement in the assessment of perfusion defects but show moderate correlation when the entire short-axis myocardium is correlated.

MR contrast media for myocardial viability, microvascular integrity and perfusion

Cardiovascular imaging requires an appreciation of rapidly evolving MR imaging sequences as well as careful utilization of intravascular, extracellular and intracellular MR contrast media. At the present time, clinical studies are restricted to the use of extracellular MR contrast media. MR imaging has the potential to noninvasively measure multiple parameters of the cardiovascular system in a single imaging session. Recent advances in fast and ultrafast MR imaging have considerably enhanced the capability of this technique, beyond the assessment of left ventricular wall motion and morphology into visualization of the coronary arteries and measurement of blood flow. During the course of the last several years, multiple strategies for imaging viable myocardium have been developed and validated using MR contrast media. Contrast enhanced dynamic MR imaging provides information regarding microvascular integrity and perfusion. Because these information can be provided noninvasively by MR imaging, repeated measurements can be performed in longitudinal studies to monitor the progression or regression of myocardial injury. Similar studies are needed to examine the effects of newly developed cardioprotective therapeutics. Development of suitable intravascular MR contrast medium may be essential for visualization of the coronary arteries and interventional therapies. MR imaging may emerge as one-stop-shop for evaluating the heart and coronary system. This capability will make MR imaging cost-effective in the first decade of this millennium.

Reperfused rat myocardium subjected to various durations of ischemia: estimation of the distribution volume of contrast material with echo-planar MR imaging

PURPOSE

To estimate and compare the fractional distribution volume (fDV) of gadodiamide injection and technetium 99m-diethylenetriaminepentaacetic acid (DTPA) in the reperfused myocardium of rat hearts subjected to various durations of ischemia.

MATERIALS AND METHODS

Magnetic resonance (MR) imaging and autoradiography were performed in rats subjected to 20, 30, 40, or 60 minutes of regional ischemia followed by 1 hour of reperfusion. The fDVs of gadodiamide injection and (99m)Tc-DTPA were measured and compared by using inversion-recovery echo-planar imaging and autoradiographic phosphor imaging, respectively.

RESULTS

The mean fDV of both tracers (gadodiamide and (99m)Tc-DTPA) in normal myocardium was 18% +/- 1, whereas that in the entire area at risk increased significantly (P <.05) with 20, 30, 40, and 60 minutes of ischemia to 32% +/- 1, 57% +/- 4, 66% +/- 2, and 68% +/- 2, respectively. The fDV was significantly (P <.05) greater in the core of infarction-78% +/- 4, 89% +/- 5, and 88% +/- 5 with 30, 40, and 60 minutes of ischemia, respectively-than in the normal myocardium or in the area at risk.

CONCLUSION

The fDV of MR contrast material in the periinfarcted rim was significantly (P <. 05) greater than that in the normal myocardium, but significantly less than that in the core of infarcted myocardium.

Early, Complete Infarct Vessel Patency: Arriving at a Gold Standard for Future Clinical Investigation in Myocardial Reperfusion

Early clinical trials of thrombolytic therapy in the setting of acute myocardial infarction (AMI) demonstrated that early angiographic reperfusion correlated with improved survival. This supported the open-artery hypothesis that early reperfusion decreases infarct size, improves left ventricular function, and improves survival. Two subsequent comparative thrombolytic trials showed no difference in left ventricular function or survival between agents with different rates of reperfusion. Additionally, reduction in mortality was demonstrated without improvement in left ventricular function and with the late administration of thrombolytic therapy. Therefore, there was a real question as to the importance of infarct vessel patency, and its relation to clinical outcome. This article discusses the various markers of coronary artery patency, their relation to clinical outcome, and how they reflect perfusion at the tissue level. The coronary angiogram gives a snapshot view of the infarct-related artery (IRA) that does not reflect the dynamic process of vessel reocclusion and recanalization. The patent artery is therefore "open" at only a given time frame, and may undergo cyclic or complete reocclusion. Angiographically characterized flow has been demonstrated to be more clinically meaningful. The GUSTO-I trial was designed to test the open-artery hypothesis. This trial confirmed that improved early IRA patency and optimal (TIMI-3) flow correlated with improved survival. The presence of TIMI-3 flow in the IRA has consistently demonstrated significant improvement in patient morbidity and mortality, and conversely, less than optimal, but still "patent" (TIMI-2) flow in the IRA correlates with clinical outcomes observed in patients with occluded infarct vessels. Even TIMI-3 flow in the IRA does not always confirm perfusion of the myocardium at risk. Therefore, the "patent" IRA can be subsequently compromised by intermittent patency, reocclusion, less than TIMI-3 flow, and a "no-reflow" effect at the tissue level. The development of accurate, reliable non-invasive markers of IRA patency is crucial. This would allow a more selective application of invasive and interventional techniques to restore patency to the IRA. The merits and faults of these noninvasive markers are discussed. The ideal gold standard for establishing the adequacy of therapy in AMI is one that could detect rapid, complete, and sustained coronary reperfusion with adequate myocardial perfusion. Current technologic achievements allow an approach to this ideal; however, as of 1997, the coronary angiogram demonstrating TIMI-3 flow represents the clinically proven standard of optimal therapeutic efficacy.

Does a higher concentration of gadolinium chelates improve first-pass cardiac signal changes?

The purpose of this study was to evaluate first-pass cardiac signal changes with a higher concentrated gadolinium-chelate (gadobutrol) and its influence on bolus geometry. Phantom studies and in vivo first-pass cardiac studies were performed in rabbits (n = 8 experiments) under general anesthesia at 1.0 T using an ultrafast T1-weighted Turbo-fast low-angle shot (FLASH) sequence (TR/TE 4.7/1. 6 msec, alpha = 90 degrees ) with a time resolution of 870 msec. Gadobutrol was injected as an intravenous bolus at two concentrations (0.5 and 1.0 mol Gd/L) and five doses (0.3, 0.15, 0.1, 0.055, and 0.03 mmol Gd/kg bw). The blood-pool gadolinium compound gadopentetate dimeglumine-polylysine (0.15, 0.075, 0.05, and 0.015 mmol Gd/kg bw, 0.5 mol Gd/L) and the standard extracellular gadopentetate dimeglumine (0.1 and 0.05 mmol Gd/kg bw, 0.5 mol Gd/L) served as reference agents. Cardiac signal changes were calculated from serial signal intensity measurements. Maximum signal intensity changes and best peak profiles during first pass of the right and left ventricle were observed with a dose of 0.03 mmol Gd/kg bw gadobutrol using T1-weighted Turbo-FLASH. At the low application volumes used, the higher concentration of 1.0 mol Gd/L gadobutrol did not increase the degree of signal intensity changes or sharpen the bolus profile. First-pass cardiac signal changes using T1-weighted Turbo-FLASH with the new extracellular contrast agent gadobutrol are best observed at a dose of 0.03 mmol Gd/kg bw. There is no advantage to the concentrated formulation (1 mol Gd/L gadobutrol) when using small injection volumes. J. Magn. Reson. Imaging 1999;10:806-812.

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.

Reperfused myocardial infarction as seen with use of necrosis-specific versus standard extracellular MR contrast media in rats

PURPOSE

To measure the difference in size of reperfused myocardial infarction with necrosis-specific (bis-gadolinium-mesoporphyrin [hereafter, mesoporphyrin]) and standard extracellular (gadopentetate dimeglumine) magnetic resonance (MR) contrast media.

MATERIALS AND METHODS

Echo-planar (for T1 measurement) and spin-echo (for infarction size) MR imaging were conducted in 32 rats subjected to reperfused reversible (n = 16) and irreversible (n = 16) myocardial injuries. All animals received gadopentetate dimeglumine 1 hour after reperfusion and underwent imaging. Sixteen rats received mesoporphyrin at 2 hours, the other 16 rats received gadopentetate dimeglumine at 24 hours, and all animals underwent imaging at 24 hours.

RESULTS

Mesoporphyrin produced prolonged (22 hours) reduction in T1 in irreversibly, but not in reversibly, injured myocardium. The size of the mesoporphyrin-enhanced region (37% +/- 4 [SEM] of left ventricular surface area) closely correlated with the true infarction size as measured by means of histomorphometry (36% +/- 3, r = 0.90). The size of the gadolinium-enhanced region overestimated (48% +/- 2 and 43% +/- 1 at 1 and 24 hours of reperfusion, respectively) the size of true infarction (36% +/- 3, P < .05, r = 0.02), but it was close to the size of the area at risk (r = 0.93).

CONCLUSION

The sizes of hyperenhanced regions displayed by using mesoporphyrin and gadopentetate dimeglumine differed from each other. The difference in size of the hyperenhanced region demarcated by mesoporphyrin and gadopentetate dimeglumine may provide an estimation of potentially salvageable myocardium.

First-pass myocardial perfusion imaging and equilibrium signal changes using the intravascular contrast agent NC100150 injection

In this phase I clinical study, the new ultrasmall superparamagnetic iron oxide contrast agent, NC100150 Injection (Nycomed AS, Oslo, Norway, a part of Nycomed Amersham), was assessed for first-pass magnetic resonance myocardial perfusion studies and its ability to produce equilibrium signal changes, as a possible indicator of myocardial blood volume. Data were acquired in 18 healthy male volunteers at 0.5 T and 1.5 T. At both field strengths, first-pass studies using T1-weighted sequences were acquired. Long TE spin-echo echoplanar imaging (EPI) was used at 0.5 T and short TE fast low-angle shot (FLASH) imaging at 1.5 T. With both sequences, T1 effects dominated the images for low doses, and time intensity curves potentially suitable for perfusion analysis were generated. At higher doses, T2 and T2* effects were observed. At 1.5 T, these predominantly affected the blood pool signal; however, at 0.5 T the myocardial signal was also involved, reflecting the relative T2 and T2* sensitivity of the spin-echo EPI sequence as a result of the long TE and long readout window, respectively. Equilibrium changes were assessed at both field strengths using T1-weighted FLASH sequences and in addition at 1.5 T using T2*-weighted gradient-echo EPI. With the T1-weighted images at both field strengths, signal changes were observed in all subjects; however, no dose-response relationship could be shown. With the T2*-weighted EPI there was significantly lower signal (P < 0.05) with the 3 and 4 mg/kg doses than with the 2 mg/kg dose. In conclusion, NC100150 Injection is useful for first-pass myocardial perfusion using T1-weighted sequences; however, low doses in combination with short TE sequences are required to minimize sensitivity to T2* effects. Equilibrium signal changes can also be induced in the myocardium. More work is required to optimize the imaging sequences and dose of NC100150 Injection for first-pass studies and also to determine whether the equilibrium signal changes can be used to measure myocardial blood volume changes in ischemic heart disease.

Magnetic resonance imaging comes of age

MAGNETIC RESONANCE IMAGING OF THE heart started in the 1970's, over the subsequent 25 years, enormous advances have been made in both the hardware and software of magnetic resonance imaging machines so that we can now obtain fast, detailed and accurate images of the heart and great vessels. In chronological terms, therefore, we can say that cardiac magnetic resonance imaging has come of age. In this article we will highlight some of the clinical applications of different magnetic resonance imaging techniques, as well as some recent developments. We will demonstrate that, in clinical investigation of congenital heart disease, magnetic resonance imaging has truly come of age.

Assessment of myocardial viability by MRI

Assessment of myocardial viability has become an important issue in patients presenting with either acute myocardial infarction or presenting with chronic ischemic left ventricular dysfunction. In patients with viable myocardium recovery of left ventricular function can be anticipatedm, spontaneously in patients with acute myocardial infarction or following revascularization in patients with ischemic cardiomyopathy. In contrast, patients without viable tissue are not likely to improve in left ventricular function. Currently, nuclear imaging techniques and dobutamine stress echocardiography are used for assessment of viability; recent studies with magnetic resonance imaging (MRI) have however demonstrated the potential usefulness of this technique for the assessment of viability. Various parameters, derived from resting MRI, can be used as markers of myocardial viability, including the end-diastolic wall thickness, systolic wall thickening and signal intensity without contrast-enhancement. Other studies have combined the information from resting MRI with the assessment of contractile reserve during dobutamine stimulation. Finally, recent studies have evaluated the use of contrast-enhanced MRI to detect viable myocardium. All of these parameters are potentially useful and MRI provide an alternative approach for the assessment of viable myocardium.

Magnetic resonance perfusion imaging in ischemic heart disease

This review explores the present status of contrast media available for myocardial perfusion studies, the magnetic resonance (MR) sequences adapted to multi-slice first-pass acquisitions, and the issue of myocardial perfusion quantification. To date, only low molecular weight paramagnetic gadolinium chelates have been used in clinical protocols for myocardial perfusion. With the availability of fast MR acquisition techniques to follow the first-pass distribution of the contrast agent in the myocardium, the bolus tracking technique represents the more widely used protocol in MR perfusion studies. On T1-weighted imaging, the ischemic zone appears with a delayed and lower signal enhancement compared with normally perfused myocardium. Visual analysis of the image series can be greatly improved by image post-processing to obtain relative myocardial perfusion maps. With an intravascular tracer, myocardial kinetics are in theory easier to analyze in terms of perfusion. In experimental studies, different intravascular or blood pool MR contrast agents have been tested to measure quantitative perfusion parameters. If a simple flow-limited kinetic model is developed with MR contrast agents, one important clinical application will be the evaluation of the functional consequence of coronary stenoses, ie, non-invasive evaluation of the coronary reserve.

Myocardial "low reflow" assessed by Dy-DTPA-BMA-enhanced first-pass MR imaging in a dog model

The aim of this study was to determine whether the use of a magnetic resonance (MR) susceptibility contrast medium, dysprosium diethylenetriamine pentaacetic acid-bismethylamide (Dy DTPA-BMA; Sprodiamide), may characterize myocardial perfusion abnormalities in a dog model of 90 minutes of coronary occlusion followed by 24 hours of reperfusion (no-reflow phenomenon installed). First-pass MR imaging after an intravenous bolus administration of the contrast agent was performed at the end of reperfusion. Signal intensity analysis on MR imaging, planimetry of pathological data, and blood flow determination were obtained by reference methods for comparison. Dogs were separated into two groups according to the level of collateral blood flow level (group I, <22.5 % of the flow in the non-ischemic zone; group II, >22.5 % of the flow in the non-ischemic zone). Signal intensity-time curves in the ischemic and non-ischemic left ventricle walls were extracted. Mean collateral blood flow was lower during occlusion in group I (9.8 +/- 5.4%, n = 5) than in group II (38 +/- 12.5%, n = 7, P < 0.05). Mean infarct size (expressed as a percentage of the area at risk) was significantly larger in group I (low collateral blood flow; 25.3 +/- 14.6%) than in group II (high collateral blood flow; 5.8 +/- 1.1%, P < 0.05). After rapid injection, a transient decrease of signal intensity induced by Dy DTPA-BMA was observed in both remote and ischemic myocardium but more markedly in remote normally perfused myocardium. Hence, during the transit of a susceptibility-type contrast agent, ischemic myocardium after ischemia and reperfusion appeared as a relative high signal intensity area. First-pass MR imaging with susceptibility contrast agent demonstrated the no- or low-reflow phenomenon. However, the behavior of the myocardial signal intensity-time-related curves did not allow distinction between the two groups of dogs.

Detection of acute myocardial ischemia using first-pass dynamics of MnDPDP on inversion recovery echoplanar imaging

Previous studies used manganese N,N'-bis-(pyridoxal 5-phosphate)ethylenediamine-N,N'-diacetic acid (MnDPDP) to detect myocardial ischemia at a dose of 0.4 mmol/kg with spin echo imaging. The purpose of this study was to detect acute myocardial ischemia using MnDPDP at a dose range near that approved for hepatobiliary imaging (0.005 mmol/kg) in conjunction with inversion recovery echoplanar imaging (IR EPI). Regional ischemia was produced in 26 rats by occluding the left coronary artery for 20-30 minutes before imaging. Consecutive 32 IR EP images (inversion time [TI]/TR/TE 700/2000/10 msec) were obtained to monitor the first pass of MnDPDP at four incremental doses (0.005, 0.01, 0.02, or 0.04 mmol/kg, n = 6-8). MnDPDP produced dose-dependent enhancement of left ventricular blood and normal myocardium, but not ischemic myocardium. Quantitative analysis revealed a difference in signal intensities (P<0.05) between normal and ischemic myocardium at the time of peak enhancement in all groups. However, differential enhancement between normal and ischemic myocardium produced clear visual delineation of the ischemic region only at doses > or =0.01 mmol/kg. In conclusion, acute myocardial ischemia can be detected with IR EPI using doses close to the clinically approved dose of MnDPDP.

Assessment of myocardial function and perfusion in a canine model of non-occlusive coronary artery stenosis using fast magnetic resonance imaging

Magnetic resonance (MR) functional and perfusion imaging were employed in a canine model of coronary artery stenosis (n = 6) for the quantification of functional and perfusion deficits before and after dipyridamole administration. Left anterior descending and circumflex (LCX) coronary blood flow were measured continuously after placing Doppler flowmeters. Inversion recovery gradient echo images during the transit of MR contrast medium gadolinium-benzyloxypropionictetraacetate dimeglumine (Gd-BOPTA/Dimeg) and fast breath-hold cine MR images were acquired at baseline, during LCX stenosis in basal state, and during LCX stenosis with vasodilation (dipyridamole 0.5 mg/kg). The extent of the functional defect and perfusion defect was expressed as percent of left ventricle (LV) circumference. During stenosis (LCX flow: 62.6 +/- 5.6% of baseline) the extent of the functional defect was slightly larger than the perfusion defect (11.0 +/- 1.8% versus 6.3 +/- 1.70% of LV circumference, respectively; P < 0.01). During vasodilation the extent of the functional defect was considerably smaller than the perfusion defect (25.3 +/- 2.5% versus 35.3 +/- 3.5%; P < 0.01). Thus, the sizes of ischemic regions displayed by MR perfusion defect and functional defect differ from each other.

Depiction of reperfused myocardial infarction using contrast-enhanced spin echo and gradient echo magnetic resonance imaging

RATIONALE AND OBJECTIVES

The authors used gadolinium (Gd) chelate as a T1, T2, and T2* enhancing agent in reperfused myocardial infarction to compare the appearance of reperfused myocardial infarction on spin echo and gradient echo magnetic resonance (MR) sequences.

METHODS

Rats (n = 28) were subjected to reperfused myocardial infarction and received no contrast medium, 0.2, 0.5, or 1.0 mmol/kg Gd DTPA-BMA. Spin echo and gradient echo MR images of the excised hearts (n = 7 rats per group) were acquired using 2.0 T system: repetition time (TR)/echo time (TE) = 300/20 ms for T1-weighted spin echo, TR/TE = 4000/80 ms for T2-weighted spin echo, and TR/TE = 600/10, 15, 20, and 30 ms for gradient echo imaging. Regional T2 and T2* relaxation times were measured. Triphenyl tetrazolium chloride was used to verify regional infarction.

RESULTS

Unenhanced spin echo images failed to distinguish infarcted from normal myocardium. On Gd DTPA-BMA enhanced T1-weighted spin echo images, infarction was depicted as a high-intensity region "hot spot." On the other hand, the infarcted region was visualized as a low-signal region "cold spot" on Gd DTPA-BMA enhanced T2-weighted images. Changes in signal intensity and T2 relaxation time on T2 weighted images were dose dependent. On gradient recalled echo images, the infarcted region was discriminated from normal myocardium by a dark boundary zone, which was visible only at 1.0 mmol/kg. The presence of infarction was documented in every heart.

CONCLUSIONS

The contrast between normal and infarcted myocardium was affected greatly by the dose and imaging parameters. The results indicate that spin echo and gradient echo images have greatly differing sensitivities to extracellular gadolinium chelates. Changes in myocardial T2 relaxation time, but not T2*, correlated well with the dose.

Magnetic resonance imaging evaluation of myocardial perfusion

Noninvasive qualitative/quantitative assessment of myocardial perfusion is considered to be fundamental in the management of known and suspected coronary artery disease patients, as shown by the widespread utilization of thallium-201- and technetium-99m-labeled agents in myocardial single-photon emission computed tomography (SPECT) scintigraphy for diagnostic as well as prognostic purposes. Recently, the availability of subsecond ultrafast magnetic resonance imaging (MRI) sequences (FLASH, TurboFLASH, EPI) has provided new avenues for assessing myocardial perfusion by MRI in conjunction with contrast-agent bolus administration (contrast-enhanced first-pass MRI). MRI contrast agents can be classified into relaxation agents (T1 "positive") and susceptibility agents (T2 star [T2*] "negative"). All the commercially available MRI contrast agents used in clinical practice are relaxation agents employing the T1 shortening effect of metal ions like gadolinium (paramagnetism), thus producing a tissue signal-intensity increase on T1-weighted images (positive enhancement). On the other hand, T2* agents induce mainly susceptibility effects, i.e., rapid dephasing of spins with resultant signal loss on T2*-sensitive sequences (negative enhancement). Unfortunately, both relaxation and susceptibility agents are, by definition, "extracellular" contrast media, as they rapidly diffuse into the interstitial space, thus hampering the simple application of indicator-dilution kinetics for myocardial perfusion assessment. Blood pool agents are therefore needed to obtain predictable relations between the concentration of contrast medium in the myocardium and the change in signal intensity. In addition, newer MRI techniques for tissue perfusion quantitation have been recently reported, based on blood-sensitive sequences, thus without intravenous contrast administration.

Influence of severity of myocardial injury on distribution of macromolecules: extravascular versus intravascular gadolinium-based magnetic resonance contrast agents

OBJECTIVES

This study sought to 1) compare the distribution of extravascular (573 Da) and intravascular (92 kDa) magnetic resonance (MR) contrast agents in reperfused infarcted myocardium, and 2) investigate the effect of injury severity on these distribution patterns.

BACKGROUND

Myocardial distribution of low and high molecular weight contrast agents depends on vascular permeability, diffusive/convective transport within the interstitium and accessibility of the intracellular compartment (cellular integrity).

METHODS

To vary the severity of myocardial injury, 72 rats were subjected to 20, 30, 45 or 75 min (n = 18, respectively) of coronary artery occlusion. After 2 h of reflow, the animals received either 0.05 mmol/kg of gadolinium-diethylenetriaminepentaacetic acid-bismethylamide (Gd-DTPA-BMA) (n = 24), (Gd-DTPA)30-albumin (n = 24) or saline (control group, n = 24). Three minutes after injection, the hearts were excised and imaged (spin-echo imaging parameters: repetition time 300 ms, echo time 8 ms, 2-tesla system), followed by triphenyltetrazolium chloride staining for infarct detection and sizing.

RESULTS

Histomorphometric and MR infarct size (expressed as percent of slice surface) correlated well: r = 0.96 for Gd-DTPA-BMA; r = 0.95 for (Gd-DTPA)30-albumin. On Gd-DTPA-BMA-enhanced images, reperfused myocardial infarctions were homogeneously enhanced. The ratio of signal intensity of infarcted/ normal myocardium increased with increasing duration of ischemia (overall p < 0.0001, analysis of variance [ANOVA]), indicating an increase in the distribution volume of Gd-DTPA-BMA in postischemic myocardium. On (Gd-DTPA)30-albumin-enhanced images, reperfused infarctions consisted of a bright border zone and a less enhanced central core. The extent of the core increased with increasing duration of ischemia (overall p value < 0.0001, ANOVA).

CONCLUSIONS

At 2 h of reperfusion, the distribution of MR contrast agents in postischemic myocardium is 1) specific for extravascular and intravascular agents, and 2) modulated by the duration of ischemia.

Myocardial perfusion imaging: clinical experience and recent progress in radionuclide scintigraphy and magnetic resonance imaging

In the past 20 years, radionuclide scintigraphy has proven to be a sensitive clinical tool in the assessment of myocardial perfusion abnormalities. Magnetic resonance imaging may also be used to study myocardial perfusion, but its potential value still has to emerge in the clinical setting. This review addresses the potential and achievements of both methods in clinical cardiology.

Distinction between open and occluded infarct-related arteries using contrast-enhanced magnetic resonance imaging

Ultrafast contrast-enhanced magnetic resonance imaging can be used to distinguish open and closed infarct-related arteries. An open artery is characterized by a faster rise and fall in signal intensity.

Quantitative analysis of cardiovascular MR images

The diagnosis of cardiovascular disease requires the precise assessment of both morphology and function. Nearly all aspects of cardiovascular function and flow can be quantified nowadays with fast magnetic resonance (MR) imaging techniques. Conventional and breath-hold cine MR imaging allow the precise and highly reproducible assessment of global and regional left ventricular function. During the same examination, velocity encoded cine (VEC) MR imaging provides measurements of blood flow in the heart and great vessels. Quantitative image analysis often still relies on manual tracing of contours in the images. Reliable automated or semi-automated image analysis software would be very helpful to overcome the limitations associated with the manual and tedious processing of the images. Recent progress in MR imaging of the coronary arteries and myocardial perfusion imaging with contrast media, along with the further development of faster imaging sequences, suggest that MR imaging could evolve into a single technique ('one stop shop') for the evaluation of many aspects of heart disease. As a result, it is very likely that the need for automated image segmentation and analysis software algorithms will further increase. In this paper the developments directed towards the automated image analysis and semi-automated contour detection for cardiovascular MR imaging are presented.

Alterations in T1 of normal and reperfused infarcted myocardium after Gd-BOPTA versus GD-DTPA on inversion recovery EPI

This study tested whether Gd-BOPTA/Dimeg or Gd-DTPA exerts greater relaxation enhancement for blood and reperfused infarcted myocardium. Relaxivity of Gd-BOPTA is increased by weak binding to serum albumin. Thirty-six rats were subjected to reperfused infarction before contrast (doses = 0.05, 0.1, and 0.2 mmol/kg). delta R1 was repeatedly measured over 30 min. Gd-BOPTA caused greater delta R1 for blood and myocardium than did Gd-DTPA; clearance of both agents from normal- and infarcted myocardium was similar to blood clearance; plots of delta R1 myocardium/delta R1 blood showed equilibrium phase contrast distribution. Fractional contrast agent distribution volumes were approximately 0.24 for both agents in normal myocardium, 0.98 and 1.6 for Gd-DTPA and Gd-BOPTA, respectively, in reperfused infarction. The high value for Gd-BOPTPA was ascribed to greater relaxivity in infarction versus blood. It was concluded that Gd-BOPTA/Dimeg causes a greater delta R1 than Gd-DTPA in regions which contain serum albumin.

Normal myocardial perfusion assessed with multishot echo-planar imaging

A new magnetic resonance imaging strategy is presented for accessing myocardial perfusion. Most previous work has relied on using T1-weighted fast gradient-echo imaging to monitor dynamically the signal changes during the passage of a contrast media bolus. However, the gradient-echo approach is limited by an inability to image the entire heart with adequate temporal resolution. This paper focuses on a electrocardiogram-gated multishot echo-planar imaging sequence, using the simple strategy of using the intrinsic T1 weighting produced by a repetition time equal to the heart period. To quantitate the sequence's performance with respect to normal myocardial perfusion, seven volunteers were imaged, each with three different doses of the contrast medium gadolinium diethylenetriamine pentaacetic acid (Gd-DTPA). The first-pass dynamics of the contrast were quantified in 13 regions per heart for each examination. In all volunteers, the complete heart could be covered, with five to seven slices, every two heartbeats. Enhancement was homogeneous throughout the left ventricular myocardium, with an enhancement of approximately 50% for the optimum contrast dose of 0.05 mmol/kg Gd-DTPA.

Quantification of the extent of area at risk with fast contrast-enhanced magnetic resonance imaging in experimental coronary artery stenosis

Fast magnetic resonance (MR) imaging techniques have the capability of demonstrating regions of ischemia caused by stenosis. The size of the potentially ischemic area determines the importance of the stenosis. The purpose of this study was to determine the relative values of relaxivity-enhancing and magnetic-susceptibility MR contrast media in detecting and sizing the area at risk in dogs. Eight dogs were subjected to critical left circumflex coronary artery (LCX) stenosis. Sixty sequential inversion-recovery- and driven-equilibrium-prepared fast gradient recalled echo images were acquired during bolus administration of 0.03 mmol/kg gadodiamide or 0.4 mmol/kg sprodiamide in basal and vasodilated (dipyridamole-stress) states. The size of the area at risk was measured and compared with that measured post mortem. In the basal state, gadodiamide and sprodiamide equivalently altered the signal intensities of nonischemic myocardium and the territory of stenosed coronary artery. Dipyridamole produced a significant increase in left anterior descending coronary artery flow with a decrease in LCX flow. The hypoperfused region was observed as a low-and high-signal intensity region after administration of gadodiamide and sprodiamide, respectively. The size of the hypoperfused region was slightly smaller with gadodiamide (37.4% +/- 2.8%) and sprodiamide (34.0% +/- 2.2%) than the true area at risk measured post mortem (41.8% +/- 2.2%; p < 0.05). Dipyridamole perfusion MR imaging with relaxivity or susceptibility contrast media is a noninvasive method to identify and quantify the area at risk in the territory of a stenotic coronary artery. Changes in myocardial signal intensity on fast gradient recalled echo images reflect the augmentation of flow and volume induced with dipyridamole and are consistent with the "steal phenomenon."

Comparison of dobutamine transesophageal echocardiography and dobutamine magnetic resonance imaging for detection of residual myocardial viability

A dobutamine-induced contraction reserve in akinetic but viable myocardium, observed by echocardiography or magnetic resonance imaging (MRI), is a reliable indicator of myocardial viability. However, the comparative diagnostic accuracy of these 2 techniques is unknown. Therefore, 43 patients with myocardial infarction (infarct age > or = 4 months) and regional akinesia underwent dobutamine transesophageal echocardiography (TEE) and dobutamine MRI (10 microg dobutamine/ min/kg). Both imaging techniques were compared with the reference standard 18F-fluorodeoxyglucose positron emission tomography (FDG PET). An infarct region was considered viable if a dobutamine contraction reserve could be assessed visually by TEE or quantitatively by MRI in > or = 50% of segments graded "a" or dyskinetic at rest. Infarct regions were graded viable by PET if FDG uptake was > or = 50% of the maximal FDG uptake in a region with normal wall motion by left ventriculography. A dobutamine contraction reserve was found in 21 of 43 patients (49%) by TEE and MRI. A viable infarct region by FDG PET was diagnosed in 26 of 43 patients (60%). FDG uptake and dobutamine TEE were concordant in 36 of 43 patients (84%) and dobutamine MRI and FDG PET were concordant in 38 of 43 patients (88%). Sensitivity and specificity of dobutamine TEE and dobutamine MRI for FDG PET-defined myocardial viability were 77% versus 81% and 94% versus 100%, respectively. Both imaging techniques yielded similar results for the detection of myocardial viability as defined by FDG uptake, with a slightly higher sensitivity and specificity for the quantitatively evaluated dobutamine contraction reserve by MRI.

MR imaging characterization of postischemic myocardial dysfunction ("stunned myocardium"): relationship between functional and perfusion abnormalities

Stunned myocardium has been detected in patients treated successfully with thrombolytic agents. The hypothesis of this study was that fast gradient echo (GRE) imaging could be used to characterize the regional functional and perfusion abnormalities that are indicative of myocardial stunning. This study was designed to monitor and correlate the extent of wall thickness and perfusion abnormalities as determined by fast (segmented k space) cine and contrast enhanced GRE imaging, respectively. Dogs were subjected to left circumflex (LCX) coronary artery occlusion (15 min) followed by 30-minute reperfusion (n = 8). Perivascular flow probes were used to continuously measure flow in left anterior descending (LAD) and LCX coronary arteries. Short-axis inversion recovery prepared fast GRE and cine images were acquired at baseline, at occlusion, and at 1, 10, and 30 minutes of reflow. Regional signal intensity and percent systolic wall thickening were determined at 26 equally spaced circumferential positions to compare the extent of functional and perfusion abnormalities. During occlusion and reperfusion, the ischemic region was demonstrated on contrast-enhanced images as a hypointense and hyperintense region, respectively. During occlusion, the extent of the perfusion defect (32% +/- 2% of the circumference of the equatorial slice) correlated closely (r = .74) with the extent of contractile dysfunction (35% +/- 2%). After reperfusion, there was transient recovery in the percent wall thickening (26% +/- 4% vs 36% +/- 4% normal), coinciding with the reactive hyperemic response, but this was followed by a significant decline in wall thickening at 10 minutes (19% +/- 4%) and 30 minutes (12% +/- 2%). Fast MR imaging may be useful to monitor postischemic myocardial abnormalities after thrombolytic therapy and the response to pharmacologic interventions.

The use of gadolinium-BOPTA on magnetic resonance imaging in brain infection

RATIONALE AND OBJECTIVES

The use of gadolinium (Gd)-BOPTA as a magnetic resonance contrast agent for central nervous system disease was studied in a canine brain abscess model.

METHODS

A Streptococcus faecalis brain abscess was evaluated in five dogs at 1.5T. Imaging was performed during the late cerebritis stage, at 5 to 7 days after surgery. Magnetic resonance scans were acquired before and at 1, 5, 15, 30, 45, and 60 minutes after contrast administration, using a dose of 0.1 mmol/kg. Scans also were acquired both before and after contrast injection with the implementation of magnetization transfer.

RESULTS

Lesion enhancement, quantified by region-of-interest measurement, peaked at 5 minutes after contrast injection. Both the increase in lesion enhancement from 1 to 5 minutes after injection and the decrease from 5 to 15 minutes after injection, although small, were statistically significant (P < 0.004 and P < 0.03, respectively). The application of magnetization transfer improved lesion enhancement, as measured by signal difference/noise, by 39%. This result also was statistically significant (P < 0.001).

CONCLUSIONS

In intraparenchymal brain infection, Gd-BOPTA provides effective lesion enhancement when used at a dose of 0.1 mmol/kg. Further research is needed to compare the magnitude of enhancement achieved with Gd-BOPTA, which has weak protein binding and both hepatobiliary and renal excretion, with that with Gd chelates, which have pure renal excretion.

Comparison of ultrafast dipyridamole magnetic resonance imaging with dipyridamole SestaMIBI SPECT for detection of perfusion abnormalities in patients with one-vessel coronary artery disease: assessment by quantitative model fitting

The value of ultrafast MRI for detection of myocardial perfusion abnormalities in patients with coronary artery disease (CAD) was assessed in 10 patients with stable angina pectoris and angiographically proven one-vessel CAD using double-level short-axis ultrafast MRI with bolus injection of gadolinium-DTPA and tomographic technetium-99m SestaMIBI imaging (SPECT) during dipyridamole-induced coronary hyperemia. Abnormally perfused regions were assessed with SPECT and MRI in all (100%) patients. Agreement in localization between arteriography and SPECT was 80%; between arteriography and MR, 70%; and between SPECT and MR, 90%. The signal intensity increase after the bolus injection of gadolinium-DTPA using a linear fit, and the slope of gadolinium-DTPA wash-in using double exponential model fitting were significantly different between abnormally and normally perfused regions. These preliminary results demonstrate the potential of dipyridamole ultrafast MR to monitor stress-induced flow maldistribution in patients with single vessel CAD.

Assessment of acute myocardial infarction in man with magnetic resonance imaging and the use of a new paramagnetic contrast agent gadolinium-BOPTA

To assess the feasibility of and characterize the new paramagnetic contrast agent gadolinium-BOPTA/dimeglumine (Gd-BOPTA) to detect acute myocardial infarctions with MR imaging, 24 patients (53.3 +/- 8.3 yr) were examined 9.3 +/- 3.6 days after a first myocardial infarction. Short-axis T1-weighted and T2-weighted MR imaging was performed at three slice levels. T1-weighted images were obtained before, immediately after, 15, 30, and 45 min after injection. Patients received either 0.05 or 0.1 mmol/kg body weight Gd-BOPTA. Images were qualitatively and quantitatively analyzed. Two patients showed no signs of infarction on T2-weighted images as opposed to contrast-enhanced T1-weighted images. Contrast-to-noise ratio was not affected by the dosage level. Signal intensity (SI) of normal to infarcted myocardium was significantly improved by both dosages (p < .0005) but a dosage of 0.05 mmol/kg produced significantly higher SI inf/norm (1.42 +/- 0.07 vs. 1.34 +/- 0.06, respectively, p = .015). SI of normal and infarcted myocardium enhanced immediately after administration of 0.05 mmol/kg (29.3 +/- 5.1% and 53.8 +/- 9.6% respectively), which decreased thereafter to 5.3 +/- 4.8% and 40.2 +/- 8.5% respectively, at 45 min (p < .002 for normal myocardium). SI enhancement immediately after 0.1 mmol/kg Gd-BOPTA showed no decrease within the first 45 min. Gd-BOPTA enables the detection of myocardial infarction. Optimal infarct delineation is achieved from 15 to 45 min after administration of 0.05 mmol/kg body weight Gd-BOPTA. Gd-BOPTA at 0.05 mmol/kg does improve image quality as measured by contrast-to-noise ratio and SI enhancement as compared to 0.10 mmol/kg.