Collimation for imaging the myocardium. II.

Line-source response functions and modulation transfer functions (MTF) were used to compare the spatial resolutions obtained with an Anger camera system and four different nuclides used as myocardial-imaging agents: 99mTc, 123I, 201Tl, and 43K. The measurements were made with a low-energy converging collimator (LECY, a medium-energy converging collimator (MEC), and a pinhole collimator. The MTF values for 99mTc were very similar for all three collimator types, although the LEC collimator gave slightly higher values at hgih spatial frequencies and had 40% greater sensitivity. Iodine-123 was satisfactorily imaged only with the MEC and pinhole collimators, which in turn yielded MTF values comparable to those measured for 99mTc. Thallium-201 produced MTF curves that were similar for the MEC and pinhole collimators; the curve for th LEC collimator was slightly poorer. All three MTF curves for 201 Tl were inferior to those of 99mTc. For imaging with 43K, only the pinhole collimator provided marginally acceptable spatial resolution.

Collimation for imaging the myocardium.

Line-source response functions and modulation transfer functions (MTFs) were used to compare the resolution obtained with four myocardial-imaging agents: 129Cs, 43K, 13N-ammonia, and 81Rb contamination. When an Anger camera with pinhole collimator was used, the order of decreasing resolution was 129Cs, 43K, 13N, and 81Rb. Two techniques were employed to determine the extent to which spatial resolution could be improved. The first, involving the addition of lateral shielding, improved the MTFs for all the agents studied. The second, which utilized a subtraction mode, gave the best overall results. With the second method the MTF curves for 129Cs and 43K became very similar and were superior to both 81Rb and 13N. Both techniques were found useful in improving the spatial resolution of certain myocardial-imaging agents by reducing or eliminating the effects of penetration of the pinhole collimator walls by high-energy photons.

Experimental basis of myocardial imaging with 123I-labeled hexadecenoic acid.

Progress in myocardial perfusion imaging has been slowed by the lack or radiopharmaceuticals with suitable physical and biologic characteristics. Hexadecenoic acid, terminally labeled with 123I, partially overcomes these limitations by providing a compound that concentrates in the myocardium in proportion to relative regional blood flow and carries a gamma-emitter with desirable detection and imaging qualities. After intravenous injection in experimental animals, the clearance half-times of hexadecenoic acid for blood and myocardium are 1.7 and 20 min, respectively. These values compare favorably with 18-carbon fatty-acid analogs labeled with 11C. In acute and chronic infarction, similar distribution patterns are found for hexadecenoic acid and 43K, which indicates that hexadecenoic acid is a suitable substitute for the potassium analogs now in use for myocardial imaging. Because of the high count rates obtainable with 123I-hexadecenoic acid, good-guality images can be acquired in as little as 2-3 min per view. Iodine-123-hexadecenoic acid is potentially a useful radiopharmaceutical for clinical application.

Myocardial images in nonacute coronary and noncoronary heart diseases.

To determine the variables that might affect interpretability of myocardial perfusion images in patients with acute myocardial infarctions, images obtained following intravenous administration of potassium-43 or cesium-129 were evaluated in 68 patients with nonacute coronary or noncoronary heart diseases, who were undergoing cardiac catheterization. Severe coronary arterial disease usually produces no distinctive perfusion defects in the resting state. Remote infarcts likewise tend to remain undetectable unless accompanied by wall-motion disturbances that can be detected by ventriculography. Left ventricular hypertrophy or cardiac dilatation can produce perfusion patterns indistinguishable from the ischemic defects of infarction. Right ventricular hypertrophy can cause image alterations that mimic infarcts in the left ventricle. In patients with acute myocardial infarction, sequential imaging studies with perfusion indicators should be of value in determining the effects of various therapeutic maneuvers on regional myocardial perfusion, but variations caused by conditions other than acute vascular occlusion limit the usefulness of perfusion imaging for diagnosing acute infarction. In suspected acute infarction, perfusion imaging will be used most effectively in conjunction with other imaging or nonimaging procedures that show the presence of damaged or necrotic myocardium. The information derived from this study should be generally applicable to the interpretation of imaging results obtained with the newer indicators of myocardial perfusion now in use or under development.

Rationale and radiopharmaceuticals for myocardial imaging.

Static radionuclide imaging procedures are now available for evaluating regional myocardial perfusion and for detecting acute myocardial infarction. Thallium-201, a radiopharmaceutical that possesses many of the characteristics of potassium analogues, at present is receiving the greatest attention as a regional blood flow indicator. Ischemic lesions appear as areas of decreased tracer uptake. Unfortunately, this agent is expensive, is in limited supply, and has a photopeak that is low for optimum imaging. Positive infarct images can be obtained with various Technetium-99m chelates. Pyrophosphate appears to be the best of the technetium compounds studied to date, although the mechanism of uptake of the chelates has not yet been fully elucidated. Therefore, quantitative measurements of infract size are not justified. As perfusion imaging and infarct imaging provide useful, complementary data, a dual tracer approach to evaluating patients with suspected coronary artery disease and/or myocardial infarction, is probably justifiable.

Application of delayed lung imaging following radioaerosol inhalation.

Lung scintigraphy following radioaerosol inhalation reveals evidence of airway disease and regions of decreased ventilation. In obstructive pulmonary diseases, aerosol deposition can be so excessive that lung parenchyma is not visualized immediately after radioaerosol administration. Most of the tracer which is deposited is cleared within hours, but the aerosol reaching the alveoli is retained with a half-life in excess of 2 weeks. If imaging is repeated 8-24 hours after inhalation, abnormal bronchi will usually be cleared of tracer, and regional ventilation defects can be visualized.

Myocardial extraction of labeled long-chain fatty acid analogs (38509).

The single circulation myocardial extraction of terminally iodinated hexadecenoic acid (77 plus or minus 11.0%) is approximately two times the extraction of 18 carbon fatty acids (33 plus or minus 5.2%) prepared by iodination of double bonds. The results compare favorably with natural 28 carbon steraic (70 plus or minus 7.5%) and oleic (61 plus or minus 7.8%) acids labeled with carbon-11 in the carboxyl group. It is concluded that terminally radioiodinated long chain fatty acids can be used as substitues for in vivo investigations of fatty acid distribution and in particular may be useful as a regional mycardial blood flow indicator.

Collimator and xenon-127 for ventilation studies.

In comparison with xenon-133, the more energetic emissions from xenon-127 suffer less attenuation by tissues and should therefore provide images with better resolution in ventilation studies. However, the better resolution of the low-energy collimator used with xenon-133 may outweigh such advantages of the more energetic emission of xenon-127, which requires a medium or high-energy collimator.

The health physics of xenon-127.

Dosimetry calculations were performed for clinical ventilation studies in which xenon-127 was used in place of xenon-133. A graphical method for obtaining the maximum permissible concentrations for a facility that uses both isotopes is presented. Shielding requirements are also described.

Nuclear imaging, tomographic nuclear imaging, and gray scale ultrasound in the evaluation of the porta hepatis.

Twenty-six patients with an eqivocal portal defect on conventional nuclear images were evaluated with a multiplane tomographic nuclear scanner and gray scale ultrasound. In most cases the correct determination (normal or abnormal) for an area of decreased uptake could be made from the multiplane tomographic scans. When an abnormality was thus confirmed, further characterization was provided by ultrasound.

Myocardial imaging with 123I-hexadecenoic acid.

123I-hexadecenoic acid is a terminally iodinated, 17-carbon fatty acid analog which is rapidly degraded in the myocardium. By determining regional myocardial distribution patterns and clearance rates, it may become useful as a single agent for estimating regional myocardial perfusion and for distinguished viable ischemic tissue from infarcted tissue. The high count rates obtainable with the iodine label permit acquisition of qualitative multiprojection images in only 3 min. per view, or quantifiable single projection high count images in 10 min. Ischemic defects may be observed in anginal patients without subjecting them to stress.