MR imaging of right and left ventricle

Newer MR techniques (gradient echo imaging or echo planar imaging) allow acquisition of short axis (or long axis) slices within 50 ms with adequate image quality to assess quantitative evaluation of left ventricular function.4

The use of end diastolic and end systolic measurements allows calculation of stroke volume, ejection fraction, and ventricular mass. Wall motion abnormalities can be evaluated quantitatively by cine MR imaging (fig 32.4).9 Myocardial tagging allows for detection of very subtle wall motion abnormalities. MRI permits accurate delineation of epicardial and endocardial borders so that wall motion and systolic wall thickening can be analysed quantitatively. Stress MRI (using dobutamine) is emerging as a technique to detect coronary artery disease,10 11 and evaluation of myocardial contractile reserve by stress MRI appears useful for evaluating myocardial viability.1213 Myocardial perfusion imaging is emerging as a valuable tool to assess distribution of myocardial perfusion with high resolution.14 15 The anatomy and function of the right ventricle in particular is notoriously difficult to assess

EBT is a tomographic x ray technique whereby only the structures in a selected slice (tomo-gram) of the patient are imaged sharply. The x ray photons passing through the body are differentially absorbed by the tissue, thus creating object contrast from which an image is reconstructed.

Figure 32.5. Schematic of EBT scanning to reconstruct a three dimensional image. ID, one dimensional; 2D, two dimensional; 3D, 3 dimensional.

Figure 32.6. EBT of coronary arteries with view from top (B), from a more anterior angle (C), and from a lateral angle (D). The left circumflex (CX) artery is totally occluded. A: corresponding coronary angiography. AO, aorta; INT, intermediate coronary branch; LAD, left anterior descending coronary artery; RCA, right coronary artery, RVOT, right ventricular outflow tract.

The EBT scanner is a dedicated ultrafast cardiac scanner, which is able to acquire tomograms within 100 ms.17 This fast acquisition is achieved because, unlike "conventional" computed tomographic (CT) scanners, there is no need to rotate the x ray source around the patient (which is energy and time consuming); rather the patient is positioned within a fixed source detector combination where x rays are produced with an electronically steered electron beam. The acquisition is obtained at a predetermined relative motion free diastolic acquisition period, which is determined by high resolution ECG with triggering usually set at 80% of the RR interval. Breathholding is necessary during acquisition of the tomograms to avoid respiratory motion artefacts. The tomogram thickness is set at 1.5 or 3 mm. Scanning is performed with the patient in supine position on the table. After each tomogram the table increment is set at 1.5 or 2 mm, resulting in contiguous non-overlapping slices or slices with 1 mm overlap. One tomogram is made during each RR interval. To completely cover the heart 40 to 60 transaxial tomograms are made during one breath hold. The data are obtained after injection of 150 ml contrast medium at 4 ml/s through an antecubital vein.

The high contrast in-plane resolution is approximately 0.8 x 0.8 mm (6 line pairs/cm). The radiation exposure is estimated to be one third of that of a diagnostic coronary angio-gram.

The image is constructed from many one dimensional projections, which are used to reconstruct a single slice of data (fig 32.5). A

three dimensional data set is obtained by stacking many two dimensional tomograms from which three dimensional reconstructions are made usually using a surface shaded rendering or a volume rendering technique.

EBT to assess coronary arteries During contrast injection the mean CT density within the coronary arteries is about 165-200 Hounsfield units (HU) while the mean density of the myocardium (85-100 HU) and connective tissue (100 HU) is much lower, thus allowing visualisation of the contrast filled coronary lumen.

So far the results of healthy volunteers and approximately 300 patients have been published. In 75-80% of the cases the image quality is sufficient to allow reliable interpretation of the coronary arteries (fig 32.6).17-19

The sensitivity to detect significant coronary stenosis ranges from 75-90% and the specificity from 80-94%.

The diagnostic accuracy of EBT coronary angiography is highest in the left main artery and proximal and mid parts of the left anterior descending coronary artery, and moderate in proximal and mid parts of the right (RCA) and left circumflex (LCX) arteries. The distal coronary segments cannot be visualised.

Misdiagnosis is caused by cardiac motion artefacts (in particular of the RCA and LCX), inadvertent respiratory motion, overlapping anatomical structures, triggering problems due to irregular heart rhythm, and lumen interpretation problems in cases of severe overlying calcifications. Total coverage of the heart requires a rather long breath hold (for example, with a heart rate of 60 bpm and slice thickness of 3 mm, a 20 second breath hold covers 6 cm from base to apex) which is not always possible in patients.

EBT to assess bypass graft patency Initially EBT was able to establish only the patency of coronary venous and arterial bypass graft patency by assessment of the individual transaxial angiograms. The diagnostic accuracy was high with a sensitivity of 95% and a specificity of 86-97%.17 The recently introduced three dimensional rendering techniques were able to reconstruct the graft completely and thus allow assessment of non-occluding obstructions (fig 32.7). The diagnostic accuracy to detect significant graft obstructions was high, with sensitivity ranging from 92-100% and specificity from 91-100%.

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