Quantitative aspects of myocardial perfusion imaging

Automatic registration and alignment on a template of cardiac stress and rest reoriented SPECT images

Single photon emission computed tomography (SPECT) imaging with 201Tl or 99mTc agent is used to assess the location or the extent of myocardial infarction or ischemia. A method is proposed to decrease the effect of operator variability in the visual or quantitative interpretation of scintigraphic myocardial perfusion studies. To effect this, the patient's myocardial images (target cases) are registered automatically over a template image, utilizing a nonrigid transformation. The intermediate steps are: 1) Extraction of feature points in both stress and rest three-dimensional (3-D) images. The images are resampled in a polar geometry to detect edge points, which in turn are filtered by the use of a priori constraints. The remaining feature points are assumed to be points on the edges of the left ventricular myocardium. 2) Registration of stress and rest images with a global affine transformation. The matching method is an adaptation of the iterative closest point algorithm. 3) Registration and morphological matching of both stress and rest images on a template using a nonrigid local spline transformation following a global affine transformation. 4) Resampling of both stress and rest images in the geometry of the template. Optimization of the method was performed on a database of 40 pairs of stress and rest images selected to obtain a wide variation of images and abnormalities. Further testing was performed on 250 cases selected from the same database on the basis of the availability of angiographic results and patient stratification.

Blood flow of human soft tissue sarcomas measured by thallium-201 scanning: prediction of tumor response to radiation

Thallium-201 chloride (201T1) has been used to determine regional perfusion in the myocardium and in tumors. This study was done to determine the potential prognostic importance of lesion tracer uptake to regression, local control, and rate of distant metastasis in 14 patients with neoplasms of soft tissue. Most patients had planned resections following preoperative radiation therapy. Minimum follow-up was 4 years. The ratio of nuclide uptake in the tumor to surrounding normal tissue was used as an estimate of relative blood flow. Tumors with acute volume responses (greater than or equal to 50% at the completion of X irradiation) had lower 201T1 uptake indicating lower relative blood flow than tumors that failed to have a volume reduction [1.63 +/- 0.30 (n = 9) vs 3.49 +/- 0.41 (n = 5) 201T1]. All patients had local tumor control. Patients with high uptake tumors tended to develop metastases at a higher frequency, although this was not statistically significant (p = 0.10). We conclude that 201T1 scans are a safe, non-invasive method of estimating tumor perfusion which can be useful to predict acute response to radiation, and may help to identify patients who will ultimately develop distant metastases.

201Tl myocardial perfusion in the management of the transplanted heart

A semi quantitative method with thallium has been applied in the management of cardiac transplants. In all, 142 scans were performed in 20 patients and were arranged in 3 groups that represent respectively all controls performed to the transplants (G1), and a selection of the scans obtained during the 1st rejection episode in each patient (G2) and the 2nd if present (G3). A heart/lung index was calculated through early and late images (IA, IB), obtained 5 min and 4 h post injection of thallium. A decrease in IA was detected during acute rejection in all groups. This fall was statistically significant from the mild to moderate rejection degree in G1 (P less than 0.01), and the mild rejection in G3 (P less than 0.001). IB was not modified with rejection. Comparing the biopsies with and without edema, in the absence of rejection, it has been proved that the intra myocardial edema can produce a decrease in IA (P less than 0.001). We conclude that IA is a simple and sensitive index to evaluate the graft, although intra myocardial edema can reduce its specificity.

Digital subtraction angiographic assessment of the coronary arteries and coronary flow reserve

Although intravenous digital subtraction angiography was originally intended as a means of performing less invasive peripheral angiography, this less invasive approach has not proven feasible for coronary artery studies. Digital imaging has, however, proven helpful for the immediate replay, enhancement and quantification of coronary arteriography and enables the performance of regional blood flow (coronary flow reserve) analysis. Flow analysis is clinically helpful in determining the hemodynamic significance of individual coronary stenoses, which cannot always be assessed even using quantitative stenosis measurements. One method of assessing flow reserve by digital means uses parametric images to display the timing (color coded) and density (intensity coded) of the contrast bolus as it transverses the regional myocardial circulation. Analysis of baseline and hyperemic condition parametric images provides quantitative regional flow reserve information.

A comparison of the technetium-labeled myocardial agents DiArs and DMPE to 201Tl in experimental animals

The biodistribution, kinetics, imaging characteristics and blood flow correlations of 99mTc labeled DiArs and DMPE were studied. The mice biodistribution were compared to 201Tl in mice and dogs, respectively. The myocardial kinetics of these agents were evaluated in normal and ischemic myocardium using miniaturized endocardial detectors. DiArs had a lower myocardial concentration (9.4 +/- 0.8% dose/g at 1 min), than DMPE (11.7 +/- 1.1% dose/g at 1 min) but both were considerably less than 201Tl (23-26% dose/g at 1 min). The kinetic characteristic of both technetium labeled agents suggested that redistribution into ischemic myocardium would not take place, since the clearance rate from normal and ischemic myocardium was similar for both the DiArs and DMPE . The clear visualization of the canine myocardium after i.v. injection demonstrated the superiority of DMPE over DiArs and the potential use of these agents, if proven, to behave the same in human trials.

PET tomographic imaging of the human heart, pancreas, and liver with nitrogen-13 derived from [13N]-L-glutamate

The first commercial version of the Massachusetts General Hospital (MGH) positron camera was used to image the relative distributions of 13N (T1/2 = 9.96 min) in the human heart, pancreas, and liver after intravenous administration of [13N]-L-glutamate. The instrument was operated in two imaging modes. Conventional 2-dimensional (2-D) focal-plane images showed high concentrations of 13N in the heart and pancreas, and lower levels in the liver. Five PET tomographic transverse section (3-D) images were made through the heart and three through the pancreas. Our results suggest that further studies designed to gain an improved understanding of the biochemistry of [13N]-L-glutamate aided by PET imaging, especially with newer instrumentation, are worthy of further investigation.

Slow late myocardial clearance of thallium: a characteristic phenomenon in coronary artery disease

We extended the quantitative seven-pinhole method to follow the dynamics of thallium redistribution after exercise. We observed a pattern of slow late thallium clearance that appears to be characteristic of myocardium supplied by obstructed coronary arteries. In 28 subjects, quantitative thallium scintigrams (seven-pinhole tomography, circumferential count profiles) and blood samples for thallium concentration were taken immediately, 2 hours and 4 hours after maximal treadmill exercise. Twenty subjects had coronary artery disease (CAD) and eight were normal. The rate of thallium clearance from the blood (TCB) was compared with the rate of thallium clearance from each segmental region of myocardium (TCM, derived by ratioing corresponding segments of the absolute circumferential count profiles) between the 2- and 4-hour images. In seven of the eight normal subjects, TCM exceeded TCB in all regions of all images (specificity 88%). Seventeen of the 20 CAD patients had at least one region where TCM was less than TCB (sensitivity 85%). Of the 13 patients with multivessel CAD, 11 had multiple regions with TCM less than TCB. Using this criterion, we detected 31 of 39 obstructed coronary arteries. Of the 37 regions that were abnormal by this analysis, 30 corresponded to obstructed coronary arteries. In contrast, while conventional circumferential count profile analysis also was abnormal in 17 of the 20 CAD patients, it diagnosed multivessel CAD in only five of the 13 patients that had it. These results show that slow late thallium clearance from myocardium is characteristic of regions of myocardium supplied by diseased coronary arteries and that observation of this phenomenon may improve diagnostic sensitivity for the presence of multivessel CAD.