Myocardial imaging with indium- 113m- and technetium-99m-macroaggregated albumin. New procedure for identification of stress-induced regional ischemia

Vulnerable plaque features on coronary CT angiography as markers of inducible regional myocardial hypoperfusion from severe coronary artery stenoses

OBJECTIVE

We explored whether the presence of 3 known features of plaque vulnerability on coronary CT angiography (CCTA)--low attenuation plaque content (LAP), positive remodeling (PR), and spotty calcification (SC)--identifies plaques associated with greater inducible myocardial hypoperfusion measured by myocardial perfusion imaging (MPI).

METHODS

We analyzed 49 patients free of cardiac disease who underwent CCTA and MPI within a 6-month period and were found on CCTA to have focal 70-99% stenosis from predominantly non-calcified plaque in the proximal or mid segment of 1 major coronary artery. Presence of LAP (≤ 30 Hounsfield Units), PR (outer wall diameter exceeds proximal reference by ≥ 5%), and SC (≤ 3 mm long and occupies ≤ 90° of cross-sectional artery circumference) was determined. On MPI, reversible hypoperfusion in the myocardial territory corresponding to the diseased artery was quantified both as percentage of total myocardium (RevTPD(ART)) by an automatic algorithm and as summed difference score (SDS(ART)) by two experienced readers. RevTPD(ART)≥ 3% and SDS(ART)≥ 3 defined significant inducible hypoperfusion in the territory of the diseased artery.

RESULTS

Plaques in patients with RevTPD(ART)≥ 3% more frequently exhibited LAP (70% vs. 14%, p < 0.001) and PR (70% vs. 24%, p = 0.001) but not SC (55% vs. 34%, p = 0.154). RevTPD(ART) increased from 1.3 ± 1.2% in arteries with LAP-/PR- plaques to 3.2 ± 4.3% with LAP+/PR- or LAP-/PR+ plaques to 8.3 ± 2.4% with LAP+/PR+ plaques (p < 0.001); SDS(ART) showed a similar increase: 0.3 ± 0.7 to 2.3 ± 2.8 to 6.0 ± 3.8 (p < 0.001). Using the same LAP/PR categorization, there was a marked increase in the frequency of significant hypoperfusion as determined by both RevTPD(ART)≥ 3% (1/19 to 10/21 to 9/9, p < 0.001) and SDS(ART)≥ 3 (1/19 to 8/21 to 8/9, p < 0.001). LAP and PR, but not SC, were strong predictors of RevTPD(ART) and SDS(ART) in regression models adjusting for potential confounders.

CONCLUSIONS

Presence of low attenuation plaque and positive remodeling in severely stenotic plaques on CCTA is strongly predictive of myocardial hypoperfusion and may be useful in assessing the hemodynamic significance of such lesions.

Dipyridamole cardiac imaging

Dipyridamole cardiac imaging is a useful alternative technique to exercise stress testing in the evaluation of patients with ischemic heart disease. Intravenous dipyridamole is still in the investigational phase, while oral dipyridamole is widely available. The hemodynamic effects of dipyridamole include an increase in coronary blood flow (due to coronary vasodilation) which is in excess of the increase in myocardial oxygen consumption and cardiac output. The disparity in the increase in coronary blood flow relative to the cardiac output results in an increase in myocardial thallium activity and an increase in the myocardial/background activity ratio. The quality of the thallium images is better or similar to that of exercise thallium images. The optimal dose of intravenous dipyridamole is 0.56 mg/kg, and of the oral dose it is 300 to 400 mg, although higher doses may be necessary in some patients. Analysis of the thallium images has been to a large extent based on visual inspection of the planar images. Delayed images are helpful to establish the nature of the perfusion abnormalities (transient or fixed). The process of redistribution is based on disparate rates of washout from the normal and abnormal zones. The sensitivity and specificity of dipyridamole thallium imaging, whether intravenous or oral, have been shown in a number of studies to be quite adequate and comparable to that achieved during exercise thallium imaging. Dipyridamole two-dimensional echocardiography has also been used in the detection of coronary artery disease; transient (new or worsening of preexisting) wall motion abnormalities have been found to be a specific marker of coronary artery disease. Transmural as well as regional coronary steal phenomena have been postulated as the mechanism for dipyridamole-induced regional wall motion abnormalities. Compared to exercise two-dimensional echocardiography, dipyridamole echocardiography provides high-quality studies and in higher proportions of patients. The results of dipyridamole thallium imaging have also been extremely important in identifying high-risk patients after acute myocardial infarction or patients with peripheral vascular disease undergoing elective vascular surgery; the presence of a dipyridamole-induced perfusion abnormality identifies patients at high risk for future cardiac events. Thus, dipyridamole cardiac imaging is helpful in the diagnosis of coronary artery disease and in risk stratification.

Continuous determination of regional myocardial blood flow with intracoronary krypton-81m in coronary artery disease

Pacing-induced changes in regional coronary flow were studied continuously with krypton-81m by intracoronary infusion in 25 patients: 21 with 50% or greater diameter narrowing of 1 or more left coronary arteries (group I) and 4 with less than 50% diameter reduction of a left coronary artery (group II). No changes occurred in group II. In group I, krypton-81m perfusion decreased progressively in all areas with more than 70% diameter narrowing, with a simultaneous increase in normal regions. At the end of pacing during angina, krypton-81m perfusion was reduced to 81 +/- 4% of control in areas with 71 to 90% diameter reduction (n = 8) and to 69 +/- 6% in areas with more than 90% diameter narrowing (n = 15). In contrast, in regions with 50 to 70% diameter reduction changes were variable (decrease in 4 regions, increase in 2 and an unchanged distribution in 1 region). Krypton-81m perfusion decreased early, before general signs of ischemia in areas with more than 90% diameter reduction, whereas this decrease occurred later in regions with 71 to 90% diameter narrowing, concurrently with ST-segment changes but before anginal pain. Although all signs of ischemia had disappeared between 2 and 5 minutes after pacing, changes in krypton-81m distribution persisted in most areas for 5 to 15 minutes after pacing. It is concluded that the functional significance of coronary arterial narrowing can be assessed with a continuous intracoronary infusion of krypton-81m. Changes in regional distribution persisted after cessation of pacing-induced ischemia, indicating an ongoing decrease in regional myocardial blood flow.

Regional myocardial blood flow in man during dipyridamole coronary vasodilation

Regional myocardial blood flow before and after intravenous dipyridamole (0.56 mg/kg) was measured during cardiac catheterization in 11 patients using the 133Xe washout technique. Significant increases in heart rate (75 +/- 4 vs 87 +/- 6, p less than 0.004) and decreases in systolic blood pressure (144 +/- 8 vs 131 +/- 7, p less than 0.02) were observed with dipyridamole infusion. However, double product and cardiac output did not differ before or after drug infusion. Regional myocardial blood flow increased from 67 +/- 3 (SEM) to 117 +/- 3 ml/100 mg/min in myocardial segments supplied by nonobstructed coronary arteries. In stenotic coronary arteries, flow increased from 57 +/- 5 to 79 +/- 9 ml/100 mg/min with dipyridamole. We conclude that dipyridamole infusion results in flow differences which discriminate stenotic from nonstenotic coronary arteries.

Validity of contrast hyperemia for clinical assessment of coronary flow reserve: the optimal dose of contrast medium and reproducibility of the technique

The dose-response relation of contrast medium-induced hyperemic response in coronary blood flow (contrast hyperemia) was investigated to determine the optimal dose of contrast medium (CM, Urografin-76) for the assessment of coronary flow reserve in man. The great cardiac venous flow (GCVF) was determined with the continuous thermodilution method during the contrast hyperemia induced by the intracoronary injection of CM of three different doses, ie, 2, 4, and 6 ml/60 kg of body weight, into left coronary artery. Submaximal coronary vasodilation could be obtained by intracoronary injection of 4 ml of CM with minimal changes in systemic hemodynamics. The contrast hyperemia with this dose of CM was reproducible and also closely correlated with that obtained during pacing-induced angina. Thus, we conclude that the contrast hyperemic technique with intracoronary injection of 4 ml of Urografin-76 could be a reliable method to assess the coronary flow reserve.

Intravenous dipyridamole combined with isometric handgrip for near maximal acute increase in coronary flow in patients with coronary artery disease

Twenty-four patients with coronary artery disease were studied during cardiac catheterization to determine the effects of sustained isometric handgrip exercise and intravenous dipyridamole and their combination on coronary and systemic hemodynamics and measured coronary luminal caliber. During 4 to 5 minutes of 25 percent maximal handgrip, blood pressure and heart rate increased 24 and 19 percent, respectively, coronary sinus flow increased to 1.7 x baseline value, and epicardial coronary arteries constricted to increase predicted flow resistance by 40 percent in 36 diseased arterial segments. After a 4 minute intravenous infusion of dipyridamole (0.56 mg/kg body weight), systemic pressure decreased 8 percent, heart rate increased 23 percent, coronary sinus flow increased to 2.4 x baseline value and coronary luminal caliber was unchanged. During isometric handgrip initiated 6 minutes after the infusion of dipyridamole, systemic pressure and heart rate increased to 14 and 31 percent, respectively, above control values, coronary sinus flow increased to 3.3 x baseline value (3.8 x baseline value in patients with normal anterior perfusion) and stenotic flow resistance increased by 36 percent. The response of coronary flow to the combined stresses was 68 percent greater than the response to dipyridamole alone (p less than 0.02); these flow levels exceed values previously reported for the human coronary circulation. Aminophylline plus nitroglycerin appears to assure patient safety.

Scintigraphic anatomy of coronary artery disease in digital thallium-201 myocardial images

One hundred and eight patients with single and multiple vessel coronary artery disease confirmed by arteriography were evaluated by exercise thallium-201 (201Tl) myocardial scintigraphy to determine the scintigraphic appearances of specific coronary stenoses. In general proximal stenoses caused more widespread, but not necessarily more severe, myocardial tracer deficit than distal stenoses. In particular, proximal dominant right coronary artery disease was specifically associated with extensive inferior wall tracer deficit in the anterior scintigram, whereas proximal left circumflex disease caused similar tracer depletion best visualised in the left lateral scintigram. A triad of uptake defects was caused by left anterior descending coronary artery disease: viz. apical tracer deficit (anterior view) in 71% lesions, septal tracer deficit (left anterior oblique view) in 83% of lesions, and anterolateral wall tracer deficit (left lateral projection) in 72% of lesions. The last defect has been termed a 'diagonal window' because it was associated with independent disease of the main diagonal branch of the left anterior descending coronary artery or with disease in the main left anterior descending artery situated proximal to this branch. Diagonal window tracer deficit was the most useful scintigraphic sign distinguishing proximal from distal disease in the left anterior descending coronary artery. False negative scintigraphic defects occurred more commonly in patients with triple vessel disease and in association with well-developed coronary collateral vessels. Certain scintigraphic patterns of 201Tl myocardial accumulation appear invaluable in the noninvasive localisation of stenoses within specific coronary arteries and thus may be useful in predicting life-threatening coronary artery disease which should be confirmed by definite coronary arteriography. The digital 201Tl myocardial scintigram also provides an independent functional guide to the interpretation of coronary arteriograms and may be helpful in the planning of aortocoronary bypass graft surgery.

Effect of intracoronary glyceryl trinitrate on perfusion distribution in the collateralised human myocardium

The effect on myocardial perfusion distribution of intracoronary glyceryl trinitrate in a dose (60 micrograms) insufficient to cause alterations in systemic blood pressure or heart rate was studied in eight patients with angiographically demonstrated collaterals from the left coronary system to the distal right coronary artery. Double isotope imaging using technetium-99m and iodine-131 labelled albumin macroaggregates allowed each patient to serve as his own control. The reproducibility of the imaging and data handling techniques was shown in 12 control patients. Glyceryl trinitrate caused a significant diminution in the collateral-mediated fractional perfusion while increasing that of the native coronary bed.

Assessment of coronary stenoses with myocardial perfusion imaging during pharmacologic coronary vasodilatation. IV. Limits of detection of stenosis with idealized experimental cross-sectional myocardial imaging

Because atherosclerosis may be reversible, a routine noninvasive screening test for the reliable diagnosis of mild coronary arterial lesions would allow potential prevention of coronary events in specific patients through intensive dietary management, drug therapy and physical training. To determine the minimal coronary stenosis detectable with myocardial perfusion imaging techniques, standardized stenoses ranging from 31.4 to 72.5 percent diameter narrowing were applied to the left circumflex coronary artery of 12 open chest dogs. Indium-113m-labeled human albumin microspheres were injected into the left atrium under control conditions and technetium-99m human albumin microspheres during maximal coronary vasodilatation induced with intravenous dipyridamole. Hearts were removed, sliced into 1 cm thick cross sections and imaged under a gamma camera. The results demonstrate that 40 percent diameter coronary stenoses can be identified by imaging relative subendocardial underperfusion during pharmacologic coronary vasodilatation. An imaging technique sensitive enough to identify mild coronary lesions for diagnostic screening purposes requires (1) a potent stimulus for coronary vasodilatation, such as intravenous dipyridamole; (2) an imaging agent taken up by the myocardium in proportion to coronary flow at flow rates up to four or more times resting coronary flow so that differences in regional maximal flows caused by mild stenoses can be identified; and (3) cross-sectional tomographic myocardial imaging to visualize relative endocardial-epicardial perfusion, the most sensitive indicator of the hemodynamic effects of coronary stenoses, and to exclude from the imaging field the interfering activity of lung, background and overlying heart structures.

Intraoperative evaluation of the functional significance of coronary collateral vessels in patients with coronary artery disease

Coronary hemodynamics were studied intraoperatively in 65 patients undergoing aortocoronary bypass grafting. Poststenotic coronary pressure and graft flow hyperemia were measured. Patients without coronary collateral vessels on arteriography (class A) were compared with patients with collateral vessels (class B). Patients in class A were grouped according to the angiographically determined degree of coronary stenosis. Eight of these patients with moderate coronary stenosis underwent intraoperative studies with transient complete coronary occlusion and were classified in the "acute" occlusion group. In class B all patients had complete coronary occlusion with good retrograde filling of the distal segment. In class A patients there was good correlation between the degree of stenosis and poststenotic pressure or hyperemic response. Stenosis had to be at least 80 percent before it produced a significant pressure gradient or graft flow hyperemia. In class B patients (those with complete "chronic" coronary occlusion), poststenotic pressure was significantly greater than in the class A patients with "acute" occlusion, significantly less than in the class A groups with 71 to 80 percent and 81 to 90% stenosis but not significantly different from values in the class A group with 91 to 99 percent stenosis. The hyperemic response was significantly less than in the "acute" occlusion group of class A, significantly greater than in the class A groups with 71 to 80 percent and 81 to 90 percent stenosis, but not significantly different from values in the class A group with 91 to 99 percent stenosis. It is concluded that (1) under basal conditions a coronary stenosis must be at least 80 percent to be hemodynamically significant, and (2) well developed collateral vessels produce in a completely occluded coronary artery hemodynamic changes that simulate those of a 90 percent coronary stenosis without collateral vessels.

Myocardial imaging with thallium-201 at rest and during exercise. Comparison with coronary arteriography and resting and stress electrocardiography

Myocardial imaging with intravenous thallium-201 (201Tl) was performed at rest and following maximal treadmill exercise in 101 patients with suspected coronary artery disease. Results were interpreted from Polaroid scintiphotos by three independent observers with complete interobserver agreement in 79%. Of 25 patients with no or insignificant coronary artery disease (less than 50% diameter stenosis), one (4%) had a resting 201Tl image defect, one (4%) had an exercise 201Tl defect, none had an ECG Q wave, and four (16%) had exercise ST-segment depression. Among 76 patients with coronary artery disease (greater than or equal to 50% diameter stenosis), 58 (76%) had a defect on either the rest or exercise 201Tl image. The proportion of patients with an exercise image defect (50/76, 66%) was greater than the proportion with exercise ST depression alone (34/76, 45%; P less than 0.02). Overall, 69 of the 76 (91%) patients with coronary artery disease had either a positive rest or exercise myocardial image and/or a positive rest (ECG Q waves) or exercise (ST depression) electrocardiogram. This exceeded the proportion with only rest or exercise electrocardiographic abnormalities (50/76, 65%; P less than 0.001). We conclude that rest and exercise myocardial imaging with 201Tl is easily accomplished with readily available imaging equipment. The image data enhanced the diagnostic sensitivity of stress electrocardiography, and provided spatial identification of the abnormal segment(s) of myocardium.

Regional myocardial perfusion abnormalities on xenon-133 imaging in patients with angina pectoris and normal coronary arteries

With use of semiselective xenon-133 injections and gamma camera recording, myocardial scintigrams were obtained in a series of 20 patients with angina pectoris, abnormal exercise electrocardiograms and normal coronary arteries. Ten patients (Group I) exhibited localized perfusion defects and the other 10 (Group II) a hjemogenous uptake of the tracer. Group I was characterized by more past myocardial infarctions and, most significantly, by male preponderance (P less than 0.001). Computer analysis of regional xenon-133 washout curves revealed that every patient in Group I had a reduced flow rate in the area of the perfusion defect (P less than 0.001). A comparison of this group with 26 patients with similarly abnormal scintigrams but coronary arterial obstruction revealed that myocardial perfusion was 16 to 18 percent greater in the group with normal coronary arteries. In three patients of this group, myocardial perfusion rates were not augmented by atrial pacing in contrast to the response in patients with coronary arterial obstruction. The data demonstrated localized perfusion abnormalities in half of the patients with angina pectoris and normal coronary arteries and constitute evidence that a metabolic disorder is not the sole mechanism for ischemia in this syndrome.

Noninvasive detection of subcritical coronary arterial narrowings with a coronary vasodilator and myocardial perfusion imaging

Myocardial perfusion imaging after administration of the potent coronary vasodilator ethyl adenosine-5'-carboxylate, which increases flow to normal areas in excess of that to areas supplied by subcritically stenosed vessels, was investigated as a nonischemia-producing stimulus for detecting subcritical coronary stenosis. Preliminary studies in 10 dogs with reactive hyperemia were performed with thallium-201 and potassium-43 to determine which tracer was a better indicator of increased flow. Neither agent was a linear indicator of increased flow caused by reactive hyperemia but thallium-201, because of its imaging characteristics, was selected as a flow indicator after administration of ethyl adenosine. Five dogs were studied after placement of a subcritical stenosis on the left circumflex coronary artery. Strontium-85 microspheres were injected into the left atrium after placement of the stenosis to verify that changes in resting blood flow were only minimal. Thereafter, intravenous administration of ethyl adenosine was followed by injection of chromium-51-labeled microspheres into the left atrium and intravenous administration of thallium-201. The mean ratio of left circumflex to left anterior descending coronary arterial flow was 0.96 +/- 0.16 for the control experiment after subcritical stenosis; after administration of the vasodilator the ratio of activity levels in the two arteries was 0.43 +/- 0.09 with the chromium-51 microspheres and 0.56 +/- 0.07 with thallium-201. Imaging performed in three additional dogs after injection of microspheres in the presence of subcritical stenosis revealed a normal pattern, whereas imaging after administration of the vasodilator and thallium-201 revealed a perfusion deficit. In two additional dogs without subcritical stenosis, thallium was administered after injection of ethyl adenosine to determine that the drug alone did not cause perfusion deficits. The perfusion scans in these two dogs were normal. These studies suggest that a coronary vasodilator and thallium-201 myocardial imaging can be used to detect subcritical coronary stenosis.

Myocardial imaging with intravenously injected thallium-201 in patients with suspected coronary artery disease: analysis of technique and correlation with electrocardiographic, coronary anatomic and ventriculographic findings

Myocardial imaging was performed after intravenous injection of thallium-201 at rest in 50 patients with suspected coronary artery disease and the results were compared with electrocardiographic, ventriculographic and coronary arteriographic findings. The thallium-201 myocardial images were of good quality and compared favorably with images previously obtained with intracoronary particle injection. Myocardial to background ratios averaged 2:1, a considerable improvement over ratios reported with potassium-43. There was complete intra- and interobserver agreement in the interpretation of images in 90 and 82 percent of cases, respectively. Major disagreement occurred in less than 5 percent of cases. Overall, 15 (30 percent) had an abnormal, 10 patients (20 percent) a borderline abnormal and 25 patients (50 percent) a normal myocardial image. Of patients with electrocardiographic Q waves, 91 percent had an image defect. Of 39 patients without Q waves, 13 percent had an image defect. All 30 patients with a normal or borderline abnormal thallium-201 image had a normal ventricular contraction pattern. All patients with a segmental ventriculographic abnormality had an image defect. In all cases, the area of electrocardiographic or ventriculographic abnormality corresponded to the area of the thallium-201 image defect. The systolic ejection fraction was depressed (0.49 +/- 0.18 [mean +/- standard deviation]) in patients with an image defect compared with that in patients with a normal image (0.64 +/- 0.06, P less than 0.005). Coronary arterial lesions were present and usually of high grade in all patients with an abnormal image; however, the presence of high grade coronary stenosis or occlusion as such correlated with image defects only to the extent that prior myocardial infraction was associated. Thus, satisfactory myocardial images at rest appear to be obtained with intravenously administered thallium-201 and electrocardiographic, ventriculographic and coronary arteriographic data suggest that image defects denote regions of prior myocardial infarction.

Regional myocardial blood flow during hyperemia induced by contrast agent in patients with coronary artery disease

Regional myocardial specific blood flow (regional specific flow) was measured at rest and during contrast hyperemia after the intracoronary injection of xenon-133. The changes in regional specific flow were transient, resulting in some compromise in one of the underlying restraints of the inert gas washout method, namely, the presence of a steady state. Therefore, to determine the clinical utility of this technique, regional specific flow values obtained with this method were correlated with the presence and severity of coronary artery disease as assessed from the coronary arteriogram and left ventriculogram. Regional specific flow during contrast hyperemia was 186+/- 11 (mean +/- 1 standard error of the mean) ml/min per 100 g in control patients and 115+/-5 in patients with coronary artery disease. There was an inverse relation between regional specific flow during contrast hyperemia and the percent coronary stenosis when the stenosis was 40 percent or greater (r = 0.70, P less than 0.001). Regional specific flow was significantly less in patients with asynergy (77 +/- 10 ml/min per 100 g) than in patients with normal ventricular function (105 +/- 5) distal to coronary stenoses of greater than 75 percent. Thus regional specific flow measured during contrast hyperemia using the xenon washout technique and the Anger camera differentiated patients with normal coronary arteriograms from those with coronary artery disease. With this technique, good correlation was shown between regional specific flow and the percent coronary stenosis and presence of ventricular wall abnormalities. The information obtained with this method may provide prognostic information concerning suitability for surgical intervention.

Definition of myocardial perfusion by continuous infusion of krypton-81m

Krypton-81m, a radionuclide emitting a 190 kev gamma ray, has a half-life of 13 seconds. It is a radioisotope of an inert water-soluble noble gas and is produced at a constant rate by spontaneous decay of rubidium-81 in an 81Rb-81mKr generator-delivery system. Delivery is through a minibore Teflon catheter that can be threaded through a standard no. 7 or 8F angiographic catheter. The generator is eluted by 5 percent dextrose-in-water, delivered by infusion pump at 1.5 ml/min, and the eluate is infused intraarterially directly into any organ. Delivery and decay reach equilibrium within 2 minutes, producing a heterogeneous distribution that is proportional to the perfusibility of the tissue concerned and the time required to reach it. The ultrashort halflife of the radionuclide rapidly eliminates activity when delivery ceases; thus, experiments can be sequentially repeated at brief intervals. The radiation hazard for easily imaged doses is negligible. Preliminary studies in open chest dogs were visualized by scintillation camera, stored on digital data disk, processed, and displayed in dual channel, dual color mode on a video system. Images of myocardial perfusion defined relative levels of perfusion, collateral circulation between coronary arterial branches, equilibrium time of diffusible perfusion of the myocardium, focal defects in induced occlusion, collateral circulation to occlusion, and reactive hyperemia after release of induced coronary occlusion. The system and technique appear applicable to human subjects.