ECR 2003 / C-1074
MR imaging of the ischemic cascade
Authors:
H. J. Lamb; Leiden/NL
DOI:
10.1594/ECR03/C-1074
Methods and Materials
Based on clinical examples, MR imaging of the ischemic cascade will cover evaluation of acute and chronic ischemic heart disease. Applied MR techniques will be explained, concerning 31P-MR spectroscopy for evaluation of myocardial high-energy phosphate metabolism, MR imaging for assessment of global and regional cardiac function,
myocardial perfusion, and delayed enhancement for determination of viability and MR flow for diastolic heart function. Systolic dysfunction ECG-triggered dynamic MRI is the most accurate and reproducible imaging technique currently available to detect myocardial wall motion abnormalities. In this example, the antero-septal akinesia is clearly visible (arrow), when comparing end-diastolic and end-systolic images. The images shown were aqcuired using a balanced FFE technique, which was introduced recently for clinical application, yielding high quality images of cardiac function. Note the high contrast between blood and myocardium, helping automated contour detection te be successful. Planscan short-axis view The short-axis view is planned in the following way. Position a stack of slices perpendicular to the long-axis of the left ventricle, as defined from the mid-mitral point to the left ventricular apex, based on the diastolic (A) and systolic (B) images of the four-chamber view, and the diastolic (C) and systolic (D) two-chamber view. Be sure to include the entire ventricle in end-diastole, as can be checked on the end-diastolic images (A, C). An end-diastolic short-axis image is shown here, acquired during a 12 s breath-hold in expiration using echo-planar MRI (E). Perfusion defect Myocardial perfusion MRI can be used to monitor the passage of a bolus with Gadolinium contrast. Early after injection (1), blood and myocardium are not yet enhanced, and appear dark as a result of a saturation prepulse. Contrast first arrives in the right ventricle (2) as a white signal intensity. After lung passage, also the left ventricle becomes enhanced (3). Finally, normal myocardium is enhanced, whereas underperfused myocardium has a slight delay in contrast enhancement (4). The latter feature is used for detection of ischemic myocardium, for example, in the infero-lateral wall (arrow). Coronary MR Angiography Noninvasive determination of coronary artery stenosis severity is also possible using MRI. Although many technical issues have to be solved, initial results are promising. In a recent publication, with pooled data of several expertise centers, a reasonable sensitivity/specificity was shown for detecting coronary artery stenosis by coronary MRA. Metabolic abnormalities Example of 31P-MR Spectroscopy in a patient with hypertension and left ventricular hypertrophy. Note the relative decrease due to dobutamine stress testing in phosphocreatine (PCr) as compared to adenosine-tri-phosphate (ATP). The change in PCr/ATP from 1.41 to 0.95 suggests demand ischemia of the left ventricle. In patients with acute or chronic myocardial ischemia, 31P-MRS may help to determine reversibility of wall motion defects, thus complementary to functional aspects as determined by cardiovascular MRI. Myocardial infarction Early after injection of Gadolinium contrast, MRI perfusion imaging can be performed. After myocardial wash-out of contrast medium in a normal heart, no contrast is left in the myocardium. After myocardial ischemia, a region can be damaged in such a way that scar tissue is formed, causing a delay for the contrast medium to wash-out completely. Approximately 20 minutes after venous Gadolinium injection MRI of delayed enhancement can be performed. Enhanced areas are believed to correspond to areas wich are irreversibly damaged ("bright-is-dead"). Planscan diastolic flow measurements The mitral valve flow acquisition is planned in the following way. The center of the slice is positioned in the middle of the mitral valve on the end-systolic two- (D) and four-chamber (B) images, and angulated parallel to the mitral valve, also based on the end-diastolic two- (C) and four-chamber (A) images. An end-diastolic image of the mitral valve is shown here (E), the upper image shows the normal, or modulus image, the lower image shows the velocity-encoded image (velocity map). Diastolic dysfunction Typical mitral valve flow curve obtained after tracing the orifice of the valve in all cardiac time frames on the velocity encoded images. The integration of velocity data over time for all pixels enclosed in the traced area results in a volume flow (flux) curve. The early peak filling rate (2) is a result of the pressure difference between the left atrium and left ventricle, and is a passive process. The atrial peak filling rate (5) is a result of left atrial contraction, and is an active process. Another functional parameter which can be derived from the curve is the fastest change in flux between, for example 2 and 3, the so called 'early deceleration' peak. Note the diastase between 3 and 4.