eMedicine Specialties > Radiology > Cardiac

Coronary Artery Disease: Imaging

Author: Justin D Pearlman, MD, PhD, ME, MA, Director of Advanced Cardiovascular Imaging, Professor of Medicine, Professor of Radiology, Adjunct Professor, Thayer Bioengineering and Computer Science, Dartmouth-Hitchcock Medical Center
Contributor Information and Disclosures

Updated: Nov 3, 2009

Radiography

Findings

Coronary angiography shows where vessels originate, how they branch, whether they have obstructions or dissections or thrombi, the degree of any obstructions, and which territories they supply. Some key questions answered during an examination of the anatomy include the following:

• Does a coronary artery pass between the aorta and pulmonary artery where it may get pinched?
• Does a segment tunnel under a myocardial bridge?
• Which pathway supplies the posterior surface? Is it the right, left circumflex, or both? That is, is it right dominant, left dominant, or cdominant?
• Does the LAD wrap around the apex to supply the distal diaphragmatic surface?
• What vessel supplies the AV node? Is its blood supply impaired?
• If an infarct is present, which is the infarct-related artery?
• If abnormal wall motion is seen, which branch obstruction accounts for it?
• Are any bypass-graft vessels present? If so, where do they originate (left internal mammary, saphenous vein graft from anterior aortic root)? Are they long or short, where do they connect, and how (end to side, side to side)?

The caliber of vessels may be estimated by comparing them with the known diameter of the catheter if it appears on the image. The reviewer should take into account the fact that magnifications differ at different distances from the source to the intensifier with x-ray projection angiography.

After describing the anatomy, note the location, percent narrowing, and character of all focal obstructions (stenoses).

• For each lesion, is it concentric (symmetric) or eccentric (1 sided)?
• Is it long or short?
• Does it abut a branch vessel (which may be lost after intervention)?
• Is it calcified?
• Is any thrombus demonstrated?
• Is evidence of intimal tear demonstrated?
• Is evidence of vessel spasm demonstrated?
• Is diffuse narrowing demonstrated?

The flow of contrast agent–labeled blood offers useful information. TIMI criteria may be applied to determine whether the distribution of contrast material is TIMI 0 (incomplete, fails to fill branches and distal part of the vessel), TIMI 1 (slow but complete), or TIMI 2 (brisk and complete). When imaging is performed at a rate of 30 frames per second, the number of frames it takes for a vessel to completely fill may be assessed. The normal number is approximately 21 frames. Filling takes longer in patients with disease than in healthy people, not only in the diseased vessel but also in normal vessels.

Consider how findings may affect possible interventions and report them accordingly. Clinically significant narrowing in the left main coronary artery is a medical emergency because of the amount of myocardium at risk. Other patterns of disease can pose similar risk; examples are proximal disease in both the LAD and a dominant right or left circumflex vessel.

• What is the caliber of distal vessels that may support a bypass graft?
• Are they calcified?
• Is any stenosis near a branch point (such that balloon angioplasty of the lesion may obstruct a branch artery)?
• How long is the left main coronary artery?
• How much myocardium is at risk?

Examine images for ancillary findings.

• Which calcifications move with the heart?
• Is the mitral valve annulus calcified?
• Is the aortic root or the aortic valve calcified?
• Are valve rings, bypass vessel rings or clips, stents, sternal wires, or other evidence of prior surgeries noted?
• If pacer wires are noted, where do they end?
• Does evidence exist of chamber enlargement, aneurysm, cardiac displacement, abnormal pulmonary venous return, unusual persistence of fetal structures, or other variants?

If left ventriculography is performed, examine LV function for the EF, regional wall-motion abnormalities, and valve integrity. Hypokinesis indicates educed motion, akinesis indicates no motion, and dyskinesis indicates reversed motion, such as ballooning outward during systole. Note any leakage of contrast material back into the left atrium and any restriction of the valve leaflets.

At the time of coronary angiography, the same set of tools can be used to examine other vessels (eg, renal and carotid arteries).²⁷

Degree of Confidence

X-ray angiography (XRA) is the standard for identifying the coronary anatomy and stenoses. In select cases, alternative imaging may appear superior, but be careful to distinguish between high-quality or good-looking pictures and the reliability of the results. X-ray angiography may provide a false-negative result if a branch vessel is occluded at its origin, if the disease is asymmetrical, or if the lesion is cracked, such that the contrast agent can extend close to the full diameter of the vessel even though the vessel cross-sectional area is severely reduced (eg, a star-shaped lesion).

It is possible to miss a lesion that is hidden behind another vessel, but that problem is generally resolved by angled views and by moving the camera (panning) during image collection. If the significance of an obstruction is unclear by XRA, intravascular ultrasound (IVUS) or a flow wire may be used to clarify its spatial extent in relation to the vessel lumen or its impact on flow down a particular branch vessel. A vasodilator may be delivered to assess flow reserve. X-ray angiography is not a good detector of small vessel disease, epitomized by cardiac syndrome-X.

Stress EKG predictive accuracy can be as low as 50%, but it rises above 75% if combined with proBNP blood testing.²² Stress imaging accuracy for detection of coronary artery disease (CAD) ranges from 70-90% if the target stress level is achieved while off antianginal medications.

Treadmill or bicycle stress testing is generally preferred, followed by dobutamine stress testing, then adenosine combined with low level exercise. Adenosine or dipyridamole alone is less reliable. Chest pain during a dipyridamole stress test is not uncommon in the absence of CAD.²⁸ Target heart rate (peak HR) for exercise or dobutamine stress testing is 85% of the age-predicted maximum (85% of peak systolic BP × peak HR). Animal studies have shown that the rate-pressure product is a better predictor of the stress levels that should induce detectible ischemia. A 50% blockage should be detected with more than 50% confidence above a rate-pressure product of 20 kilotorr/min and with more than 85% confidence above 25 kilotorr/min.

False Positives/Negatives

Balloon angioplasty can disrupt an obstruction so that the vessel appears to recover its full diameter when, in fact, the cross-sectional area is improved only minimally and insufficiently. 3D imaging can be used to examine contrast-agent attenuation and the percentage narrowing. On occasion, this condition may be identified by looking at the lesion on different views or by performing IVUS or optical CT.

The introduction of a catheter or a wire can cause intimal dissection (a tear in the lining of a vessel), which may be mistaken for vascular spasm, thrombosis, or a long stenosis on cursory examination. A tissue flap in the endothelial lining may alternate between an open position and an obstructive one, mimicking a spasm; however, it is not responsive to nitrates. The distinction may be a matter of life or death. If clinically significant, stent placement, bypass, placement of a perfusion catheter, or other emergency treatment is typically required to treat a dissection. Sudden obstruction due to a dissection can be deadly, and it does not respond to medications.

Myocardial bridges, or small bands of muscle overlying a vessel, may be mistaken for stenoses; however, these are not amenable to angioplasty. The obstruction from a myocardial bridge is smooth and eccentric. Observation throughout the cardiac cycle shows that the obstruction occurs during systole.

Computed Tomography

Findings

CT imaging of the coronary arteries is achievable with fast CT and EBT systems triggered or gated by ECG to accumulate data when the heart is in diastole. 64-section multidetector-row CT is the newest technology.^29,30,31,32

With a section thickness of 1 or 0.5 mm or less, the coronary anatomy is laid out in a 3D volume. Image processing can greatly facilitate visualization of the course of vessels and branches and the presence and degree of stenoses. The coronary-artery tree may be viewed as a solid rendering of the surface of the heart, but portions may be obstructed from view.

Proper viewing of each coronary-artery branch should include clean views in which the LV blood pool, aortic root, and all extracardiac structures are removed, and vascular projections are limited to the zones that include the vessel of interest and a margin for partial-volume effects. Do not rely on threshold-based renderings, which can cause false-stenosis and false-obstruction and which can cause an intravascular thrombus to be missed. The use of a pair of volumes before and after the administration of contrast material for elastic matching³³ greatly facilitates the evaluation by automatically isolating the coronary tree without thresholding.²³

CT also enables superb evaluation of blood delivery. In principle, CT combined with catheterization permits accurate definition of the extent of collateral-dependent myocardium.²³

Pizzuto et al found that transthoracic Doppler echocardiography can improve the diagnostic accuracy of multidetector computed tomography (MDCT) for detecting left anterior descending (LAD) coronary artery stenosis. In 144 consecutive patients, coronary anatomy was assessed with MDCT, and echocardiography was used to calculate coronary flow reserve (CFR), by measuring the ratio of hyperemic to baseline peak flow velocity; results of both methods were verified with invasive coronary angiography. In a univariate model, the prediction of significant LAD stenosis was slightly, but significantly, better with coronary flow reserve (sensitivity 90%, specificity 96%, positive predictive value 84%, negative predictive value 97%, diagnostic accuracy 94%, chi-square = 97.5) than with MDCT (sensitivity 80%, specificity 93%, positive predictive value 71%, negative predictive value 95%, diagnostic accuracy 90%, chi-square = 63.2). When the findings from transthoracic Doppler echocardiography and MDCT agreed, thediagnosticaccuracy increased (96%; chi-square = 86.1, p <0.0001). In the 13 patients missed by MDCT, transthoracic Doppler echocardiography proved 100% accurate at predicting significant LAD stenosis.³⁴

Degree of Confidence

The ability of MRI and CT to depict the anatomy and the absence of notable obstructions is improving rapidly, but it is not uniform. The value of MRI and CT must be assessed in a truly double-blind fashion for each center until standardized, reliable methods are widely established. Whether MRI and CT results match in terms of the percentage of stenosis is relatively unimportant. Most important is whether MRI and CT reliably depict normal tissue and culprit lesions and, then, whether they establish the severity and the territories supplied by the culprit vessel. Both MRI and CT offer the significant advantage of direct assessment of the zones of impaired blood delivery.

False Positives/Negatives

MRI shows calcifications as black or signal voids, whereas CT shows calcifications as white and similar to contrast-filled blood. These appearances can influence the estimation of stenoses. Heavy calcification causes a beam-hardening artifact on CT that can interfere with visualization. Stents cause a local disturbance stronger on MRI than on CT. Also, with 3D MRI or CT, be certain to understand how the images account for local curvature in and out of the imaging planes. In finding the best plane to show a vessel, radiologists can mistake a local curve that is out of plane for an apparent stenosis. Proper image processing resolves this problem.

Magnetic Resonance Imaging

Findings

Coronary MRI has improved from the early methods² and equipment sufficient to identify normal proximal coronary arteries and courses, but it is not a clinical replacement for XRA apart from ruling out aberrant coronary origins, demonstrating graft or native vessel patency, or follow-up on specific lesions.  Coronary MRI may be performed by using a 3D volume, but the trade-off in time and resolution favors imaging in selective planes that address each branch of interest. As a 3D volume, MRIs may show the coronary tree in a way similar to the methods described for CT. Background tissue may be suppressed with fat saturation, tissue saturation, magnetization transfer, and/or T2 preparation (90°-180°-180°- ... -180°-90°).³⁵

The vessel-plane approach is as follows: Any desired target plane can be obtained by specifying 3 points to include in the plane, by drawing the lines of intersection with 2 previous images at different angles, or (commonly) by drawing a single line of intersection with a previous image that is perpendicular to the desired view. For example, to obtain a short-axis view of the coronary sinus, first obtain a long-axis view of the LV parallel to the septum and perpendicular to the AV groove, then prescribe a plane in the AV groove perpendicular to that view passing through the 2 observed points of intersection on the first view with the coronary sinus, seen as bright dots anterior and posterior to the mitral valve.

Other points regarding MRI to evaluate CAD are the following:

• A transverse stack of images covering the aortic root depicts the origin of the RCA and the left main coronary artery. The typical section thickness should be 3 mm or less. A bright- or dark-blood technique can be applied with the use of single frames or with a dynamic movie series.
• An additional distal transverse image shows a cross-section of the RCA, LAD, and LCX.
• From 2 points along the proximal vessel and from 1 point from the distal vessel, a plane that captures the desired segment is selected. The plane may be adjusted to be thick enough to encompass out-of-plane bends. As an alternative, it may be subdivided into a stack of thin imaging planes for a localized 3D stack of images.
• The course of the RCA in the AV groove can quickly be ascertained from a 4-chamber long-axis view of the heart by obtaining 1 preliminary image perpendicular to the AV groove and parallel to the septum through the mid RV. This provides 2 points of intersection with the RCA: 1 anterior and 1 posterior in the AV groove. Prescribing a plane through those 2 points from the long axis image gives the desired view.
• The posterior descending artery requires a different imaging plane, as do the LAD, LCX, and major branches. The course of the LCX in the AV groove is assessed in a way similar to that used for imaging the RCA, by acquiring a scout image parallel to the septum to identify 2 points to include in one final short-axis image. However, in this case, the scout image should be laterally displaced to the outer third, because the distal LCX is often hard to identify.
• The authors routinely identify the proximal course of the coronary arteries in young patients who have had syncope to look for aberrant origins. A complete absence of abnormalities suggests a good prognosis.
• MRI with contrast is an excellent method to identify myocardial scar (infarction) as small as 1% of the myocardium, which is a very strong prognostic factor³⁶ , while also assessing perfusion and precise function of left and right ventricles. It can also be combined with stress testing and coronary imaging for a "one stop shop."
• MRI is the preferred test for right ventricular injury or infarction.
• Apparent stenosis must be distinguished from an out-of-plane bend.
• A signal void from flow disturbance may exaggerate apparent stenosis.
• MRI is well established as a means to assess the patency of a bypass graft.

Degree of Confidence

MRI offers high sensitivity to changes in wall function, eg, wall thickening and radial motion.²⁴ MRI may be useful in identifying and quantifying impaired blood delivery and wall function in response to interventions.^{5,37,38,39,40,41,42,43} Such applications are perhaps more vital than visualizing the percentage of stenosis.

Confidence in the data depends on the speed and quality of the imaging method, the cooperation of the patient (shallow regular breathing or several matching breath holds), the accuracy of EKG triggering or gating, and the anatomic knowledge and judgment of the person directly supervising data collection.

Usual EKG signal in MRI is markedly distorted by competing signals from movement in a magnetic field and by moving magnetic fields, particularly from blood flow in the great vessels, called the magnetohydrodynamic effect. That distortion makes it difficult to perform electrographic safety monitoring for ischemic changes.

Cardiac MRI with the vessel-chasing approach requires highly informed decision making as the data are being acquired. If the operator acquiring the data understands what the x-ray angiogram demonstrates, the views may be manipulated for the best match. This consideration is not necessarily positive, because the operator may exaggerate the agreement.

The ability of MRI and CT to identify anatomy and the absence of clinically significant obstructions is improving rapidly, but it is not uniform. The value of MRI and CT must be assessed in a truly double-blind fashion for each center until standardized and reliable methods are widely established. Whether MRI and CT results match in terms of the percentage of stenosis is relatively unimportant. Most important is whether MRI and CT reliably depict normal tissue and culprit lesions and, then, whether they help in establishing their severity and in depicting the territories supplied by the culprit vessel. Both MRI and CT offer the notable advantage of enabling direct assessment of the zones with impaired blood delivery.

False Positives/Negatives

In an apparent stenosis, be certain that it is not a partial-volume artifact or a velocity-shear effect. Because local differences in velocity can cause a signal void, estimates of stenosis may be exaggerated.

Magnetic susceptibility artifacts may produce signal voids. Stents, clips, and wires cause local disturbances.

The presence of pacemaker wire is considered a relative contraindication to MRI because the rapidly changing magnetic fields may induce a voltage that can trigger an arrhythmia, induce a burn, or shorten the battery life. Also, when the patient enters and leaves the magnet, the magnetic reed switch on most pacemakers will switch it to fixed mode, and the temperature may rise in metal devices. For example, a pacemaker generator may warm by 1-2°C. However, with informed consent, careful pulse monitoring, and a readiness to promptly abort a pulse sequence if an arrhythmia is induced, patients with pacers have undergone MRI with no apparent consequence and no change in their pacer thresholds. In the dozen reports of mishaps related to pacemakers and MRI, none were caused by MRI.

On MRIs, calcification is depicted as a black area or signal void, whereas CT shows calcifications as white, similar to blood filled with contrast agent. These appearances can influence the estimation of stenoses. Also, with 3D MRI or CT, be certain to understand how the images account for local curvature in and out of the imaging planes. In finding a best MRI plane for showing a vessel, radiologists can mistake a local curve that is out of plane for an apparent stenosis. Proper image processing resolves this problem.

With MRI, flow disturbances that cause velocity shear (range of phases in each picture element or pixel resulting from different rates of motion of blood) cause a local decrease in signal intensity, which may create or exaggerate an apparent stenosis.

Ultrasonography

Findings

Echocardiography can be used to identify the left main coronary artery. In some patients, much of the RCA and LAD can be viewed; however, in most patients, the imaging window is inadequate for useful coronary imaging from outside the chest.

In the catheterization laboratory, IVUS may be performed to examine the coronary arteries from the inside and to characterize plaque. However, the diameter of the device limits the ability to pass through tight stenoses. Also, the injection of a sonographic contrast agent (eg, agitated Renografin) into the coronary arteries, combined with transthoracic or esophageal ultrasonography, can be useful in identifying perfusion territories.

Nuclear Imaging

Findings

Nuclear medicine study does not depict the coronary arteries, but it does demonstrate various metabolites useful in identifying perfusion defects and tissue viability. Thallium-201 and technetium-99m sestamibi are widely used and may be combined to shorten the study of myocardial uptake of radioactive tracer at rest and during stress. Although a rest-and-stress thallium study takes more than 4 hours, a combined study performed with thallium and sestamibi may be completed in less than 2 hours.

By using PET, a rest-and-stress study with rubidium-82 may be completed in 30 minutes, because the agent has a half-life of less than 5 minutes. A defect during stress that is not evident at rest indicates a zone of induced ischemia. A defect at rest and also during stress indicates persisting metabolic dysfunction, either from infarction (scar) or hibernation (prolonged dysfunction). PET with ammonia, fluorinated glucose, or other agents may be used to determine if the tissue with a defect at rest is viable.

Degree of Confidence

Nuclear medicine tests for CAD improve the predictive accuracy over that of stress tests alone, to approximately 90%. The utility of these tests depends on the previous probability of disease and on whether they are being used to identify CAD or to clarify the pathophysiology of known disease.

False Positives/Negatives

Breast attenuation may cause an apparent defect on radionuclide images. Attenuation correction and multiplanar imaging mitigate the problem.

Unusual motion, such as that from a bundle branch block or coughing during imaging, may cause false-positive results. A persisting defect is commonly interpreted as a fixed defect or a scar, but it may represent prolonged yet still-reversible ischemic impairment of tracer uptake.

The low resolution of nuclear medicine studies compared with that of other modalities may result in false-negative results. Also, global disease may be missed because defects are generally identified by comparing them to regions with high uptake of the tracer.

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