Vla

Figure 6. Schematic showing the three orthogonal planes used for SPECT myocardial imaging and segmental nomenclature. The short-axis (SA) slices can be considered as "cucumber" slices through the left ventricle. The horizontal long axis (HLA) slices are similar in orientation to the echocardiographic four chamber view. The vertical long axis (VLA) slices can be thought of as viewing the heart from a lateral projection. (ant-anterior; ap-apex; inf-inferior; lat-lateral; sep-septal)

data spanning the R-R interval. This allows for assessment of wall motion and using commercially available software the determination of the left venticular ejection fraction (Fig. 8). Gated SPECT has become the standard technique for MPI.

Interpretation and Quantification

The interpretative approach to myocardial perfusion imaging is illustrated in Figure 9. A defect at stress which improves or normalizes at rest is termed a reversible defect and indicates ischemia. An infarct will result in diminished or absent uptake of tracer at stress and at rest. This pattern is termed a fixed defect.

The distribution of radiotracer in the heart is assessed qualitatively by examining the tomographic slices in orthogonal planes. It is also possible to generate a polar plot that condenses the three-dimensional information into a two-dimensional image (Fig. 10). Tracer distribution in the myocardium for a group of patients with a low likelihood of CAD can be used to generate a normal database. In a polar plot, segments of the myocardium below the normal range can be highlighted (Fig. 11).

A semi-quantitative method of representing the extent and severity of the perfusion abnormalities at stress has become popular. The heart is divided into 17 or 20 segments (Fig. 12). Each segment is graded on a five-point scale from normal (0) to absent uptake (4). The sum of these values is the 'summed stress score' (SSS) and is a useful index of cardiac risk. In a similar manner a summed rest score (SRS) can be generated as well as a summed difference score (= SSS-SRS).

Figure 7. Normal MPI study in a 54 year old woman referred because of a positive GXT. There is mildly diminished activity anteriorily both at stress and rest (arrow) attributable to breast attenuation. No other fixed or reversible abnormalities are identified. The top row are short axis (SA) images of the left ventricle at stress and the second row are corresponding SA slices at rest going from apex (on the left) towards the base of the heart (on the right). The next two rows are vertical long axis images (VLA) at stress and rest going from septum to lateral wall. The bottom two rows are horizontal long axis (HLA) at stress and rest going from inferior to superior slices. For subsequent MPI studies the same format has been used.

Figure 7. Normal MPI study in a 54 year old woman referred because of a positive GXT. There is mildly diminished activity anteriorily both at stress and rest (arrow) attributable to breast attenuation. No other fixed or reversible abnormalities are identified. The top row are short axis (SA) images of the left ventricle at stress and the second row are corresponding SA slices at rest going from apex (on the left) towards the base of the heart (on the right). The next two rows are vertical long axis images (VLA) at stress and rest going from septum to lateral wall. The bottom two rows are horizontal long axis (HLA) at stress and rest going from inferior to superior slices. For subsequent MPI studies the same format has been used.

Two ancillary findings, increased lung thallium activity and transient ischemic dilatation are useful in defining patients at high risk for subsequent events. Increased lung activity is related to increased LV filling pressure and accumulation of 201Tl in the pulmonary interstitial space is a marker for patients with multivessel coronary disease and/or LV dysfunction. It is assessed quantitatively in the anterior view performed immediately after the injection of 201Tl during stress, by comparing activity in lung to that in the hottest area of the myocardium. The lung/heart ratio is less useful with 99mTc-sestamibi and tetrofosmin, partly because of the 30-60 minute delay before imaging begins. Dilatation of the LV cavity at stress compared to rest is variously termed transient ischemic dilatation or stress-induced dilatation. It is caused by subendocardial ischemia at stress (causing an apparent increase in LV size) or by true dilatation of the LV cavity post-stress.

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