64-slice CT monitors coronary artery health

August 1, 2006
Anselmo Alessandro Palumbo, MD
Anselmo Alessandro Palumbo, MD

,
Filippo Cademartiri, MD
Filippo Cademartiri, MD

,
Gabriel P. Krestin, MD, PhD
Gabriel P. Krestin, MD, PhD

State-of-the-art scanners enable noninvasive assessment of bypass grafts and native vasculature for patency and arterial disease

Treatment options for multivessel coronary artery disease include medical therapy, percutaneous coronary intervention, and, above all, coronary artery bypass grafting. Current U.S. guidelines regard surgery as the treatment of choice for three-vessel disease and/or in cases involving the left main coronary artery.1

Coronary artery bypass grafting (CABG) is the most common major operation in the U.S., where approximately 300,000 such procedures are performed every year. The comparable figure for Europe is 220,000, with Germany accounting for 75,000 of these operations, Italy for 25,000, and the U.K. for 15,000.

Trends in medicine are moving toward minimization of the invasiveness of procedures. Use of "keyhole" techniques, for example, makes it possible to perform safe and effective surgery, while also cutting costs and reducing patient discomfort. Similarly, the advent of multislice CT is enabling coronary artery disease to be detected reliably and noninvasively.2

All grafts, but especially saphenous vein grafts, have a larger diameter and less residual motion than do native coronaries. This makes them better suited to visualization with CT angiography. Patients undergoing CABG are also generally older than the average surgery recipient, present with comorbidities, and are more likely to have conditions such as valvular heart disease or ventricular dysfunction. These factors increase the risk of complications from cardiac catheterization, and this patient group may thus benefit from noninvasive coronary angiography.

TECHNICAL TIPS

We perform noninvasive assessments of CABGs using 64-slice CT. Our state-of-the-art CT scanner provides an isotropic voxel of 0.4 mm3 and a temporal window of 165 msec (using a single 180 degrees algorithm reconstruction). Gantry rotation of 330 msec allows a table feed of 11.84 mm/sec together with a shortened temporal window. The increased number of slices, coupled with increased temporal resolution, means that scan times can be reduced to around 14 to 16 seconds. This is an important factor when seeking patient compliance for breath-hold scanning of large body volumes.

Scanning begins at the level of the aortic arch to depict the entire graft. Satisfactory results are gained when high tube current (850 to 950 mAs) is matched with a properly triggered bolus tracking technique.3

Bypass grafts have a large diameter, are relatively motionless, and tend to have little or no calcified deposits. While these conditions all favor CTA, patient preparation should not be neglected. Image quality can be affected by high and/or irregular heart rates. The best candidate for noninvasive CTA is stable with no contrast allergies, not suffering from severe renal failure, and has a heart beating at less than 70 bpm in sinus rhythm. Administration one hour prior to examination of oral beta blockers and, eventually, benzodiazepines to reduce heart rate is mandatory for a diagnostic result.

Intravenous administration of a 100 mL bolus of nonionic contrast through an antecubital vein allows visualization of the coronary tree. Intravascular enhancement can be maximized by using contrast with a high iodine concentration and/or injection at a high flow rate (320 to 400 mgI/mL injected at 4 to 5 mL/sec). Subsequent administration of a 40 to 50-mL saline bolus at the same flow rate, using a dual-head injector, will push the contrast bolus.4

Proper image reconstruction is another important step. An effective slice thickness of 0.75 mm with an increment interval of 0.4 mm will provide 400 to 500 images. Well-chosen parameters and a good-quality native image data set will allow optimal postprocessing. Volume-rendered reconstructions are often used to show the anatomic configuration of the coronary tree (Figure 1). Such reconstructions can help when following coronary artery bypass conduits and extrapolating the supposed course of an occluded bypass. A focal bump on the ascending aorta, for instance, usually corresponds with the residual anastomosis of the occluded venous graft to the aorta. Similarly, a road map of metallic clips can represent the path of an occluded internal mammary artery.

Curved multiplanar reconstructions (C-MPRs) along the lumen center line will detect coronary disease on bypass grafts and native coronary arteries. A multiplanar approach is mandatory for proper evaluation of coronary vessels and grafts. The operator or software should follow the vessel of interest throughout its course and search for abnormalities. Viewing planes should be aligned to the main axis of the vessel, and multiple views with this approach should be taken. C-MPRs along the central lumen line will show the entire vessel within a single image.

Stenosis should be visualized in a plane orthogonal to the main axis of the vessel. When doubtful images are seen on one data set, additional reconstruction at different time intervals during the cardiac cycle should be used. If an image along the vessel or inside the vessel lumen remains the same throughout the cardiac cycle, the chances are good that it is showing a real lesion rather than an artifact.

Evaluation of calcified native vessels can improve by use of a sharper convolution filter. Images will be noisier, but increasing contrast resolution will reduce blooming artifacts from calcium or metallic clips.

Modern CT scanners provide a wide range of options for manipulating raw data. It is possible to obtain any phase of the cardiac cycle and any plane within the scan volume. This freedom and flexibility explain the very high operator dependency for this procedure as well as the extremely long and intensive training required for high diagnostic accuracy.5,6 Results also depend on the care taken during patient selection.

OPTIMAL METHODOLOGY

Angina following CABG is caused by graft descent and failure (occlusion), often in combination with progressive atherosclerosis in the native coronaries. Chest pain can arise in the early postoperative phase (within one month of surgery), owing to technical factors and superimposed thrombosis, and in the intermediate phase (within a year), due to intimal hyperplasia and luminal narrowing. Chest pain occurring in the late phase (over a year after surgery) can be caused by lipid-rich atherosclerosis similar to that which affects native coronary vessels.

Progression of atherosclerosis in native coronary arteries is responsible for acute coronary syndrome in up to 47% of patients within five years of surgery and up to 24% of patients within 10 years of surgery.7 CTA evaluation should therefore cover native coronary arteries as well as bypass conduits (Figure 2). Postsurgical patients will have diffuse and advanced atherosclerosis, causing vessels to be diffusely narrowed and extensively calcified. They will often have undergone percutaneous coronary intervention prior to bypass grafting. Detection of significant luminal narrowing can be challenging with such diseased vessels.8

Evaluation of the entire thorax is unnecessary given the rarity of significant stenosis at the ostium of mammary arteries.9 Lengthening the scan

volume means decreasing the tube current intensity. This hampers evaluation of native coronaries and impairs image quality.

We use saphenous vein bypass grafts. It has been demonstrated, however, that atherosclerosis affects all grafts after the early postoperative phase. Occlusion rates are generally 15% to 20% in the first year. A further 2% of grafts then occlude annually for the next five years. The annual occlusion rate rises to 5% six years after surgery and remains at this rate until 10 years postsurgery. By this point, only 35% to 45% of the venous bypass grafts will still be open (Figure 3).10

The left internal mammary artery (LIMA) is used most frequently for CABG. Surgical options include end-to-side, free, sequential, and T or Y grafts. The LIMA graft is the most important graft performed in ischemic patients. The rate of occlusion during the first year after surgery is 5%, and it rises to 10% to 20% at 10 years.11 Numerous studies have shown that this procedure should not be withheld from any group of patients. Contraindications include radiation damage and documented subclavian stenosis.

The radial artery is considered the next best choice for CABG. Its length (greater than or equal to 20 cm) makes it suitable for extensive grafting. Disadvantages include the need to assess ulnar collateral circulation to avoid paresthesia and ischemia. A large number of metallic clips must be used along the vessel, and these can cause streak artifacts. The radial artery is also reportedly liable to spasms after surgical removal.

Assessing arterial conduits can be more difficult than examinations of venous conduits. Visualization is hindered by the smaller size of vessels and the clips placed to seal collateral arteries and distal anastomosis with the native coronary vessels. Visualization of the graft lumen can be limited by beam-hardening artifacts from metallic sternal wires if the LIMA runs close to the anterior chest wall. The lack of dynamic representation and quantification of blood flow abnormalities on CTA also prevents evaluation of flow competition between grafts and native vessels. This mechanism accounts for a significant number of venous and arterial occlusions.

Evidence from several studies indicates that CT-based CABG evaluation is more accurate when the number of detector rows is increased (see table). Sensitivity and specificity change from 83% to near 100% with the move from four- to 64-slice CT. Faster gantry rotation and decreased individual detector widths have also contributed to this improvement in performance. Very few groups have used CT to evaluate native coronaries in patients who have undergone CABG. Yet 64-slice technology provides sufficient spatial and temporal resolution to evaluate even native vessels reliably.

CLINICAL APPLICATION

Noninvasive stress tests, such as exercise electrocardiography, echocardiography, and nuclear imaging, have been used to detect myocardial ischemia during postsurgical coronary evaluation. These tests have shown low accuracy, however, and they have been unable to determine the exact site of stenosis or occlusion. Coronary CTA is the only technique that permits noninvasive visualization of the coronary tree. It is considered the last diagnostic step prior to invasive procedures, though it is possible to consider a future role for this technique both before and after surgery.

Patients with valvular disease should undergo conventional coronary angiography. This will enable performance of coronary stenosis detection, valvular replacement, and bypass grafting (or percutanous interventions) during the same procedure. Patients without significant stenoses could avoid conventional angiography.

The main role of CTA in patients with chronic chest pain (stable angina) is to provide information that angiography cannot. This could include the wall characteristics of the ascending aorta and degree of calcification in the aorta, LIMA, and native vessels. Information on vessels and plaque will influence whether the patient undergoes an endovascular procedure or surgical treatment.

Preoperative planning of mild invasive direct and totally endoscopic CABG can also be carried out with CTA.17 These surgical techniques avoid the need for thoracotomy, cardiopulmonary bypass, or other procedures associated with high morbidity and mortality in older patients. The availability of more accurate anatomic information not provided by traditional angiographic modalities may benefit surgeons. One example is the presence of intramyocardial course and severe calcification of the native coronaries that can affect the endoscopic approach. Another example of this information involves the anatomic relationship between the LIMA and the left anterior descending coronary artery and its branches, which must be clearly shown when CT is performed for the planning of minimally invasive surgery.

Ostial lesions of the left main (LM) artery can be difficult to evaluate with conventional coronary angiography, owing to catheter placement. Intravascular ultrasound and/or wire pressure measurements are often required to better define the degree of stenosis. CTA offers an alternative noninvasive option of evaluating ostial LM disease. CT additionally provides reliable measurements of ventricular wall thickness and volumes prior to ventriculoplasty for postinfarction aneurysmatic left ventricle dilatation. Bypass surgery may be planned together with a ventricular patch.

CT has been proposed as a method of assessing myocardial viability, showing comparable results with MRI.18 MRI and CT can both identify the presence, location, and extent of irreversible myocardial injury correctly, whether acute or chronic. Preoperative CT could assess left myocardial viability that may benefit from revascularization. Follow-up CT at six months could demonstrate midterm response to surgery, especially for patients who suffered severe preoperative severe hypokinesia or akinesia.19

Coronary CTA could have a role in the follow-up of symptomatic patients. It can detect stenoses in CABGs and native vessels (lumen reduction > 50%) and indicate which patients should undergo repeat surgery or percutaneous intervention. Follow-up could be extended to all patients four to six weeks postsurgery. Prompt percutaneous treatment of thrombotic occlusions leads to remarkable outcomes. Efficacy of venous grafts is also improved.

Will noninvasive CT coronary angiography replace conventional coronary angiography? This remains a point for debate.20 Sixty-four-slice CT, and perhaps dual-energy x-ray source technology, will surely have a role in the assessment of CABG patients. This may improve the standing of CT as the cornerstone of clinical coronary imaging in the near future.

DR. PALUMBO and DR. CADEMARTIRI are radiologists at the Erasmus Medical Center in Rotterdam, the Netherlands, and at Azienda Ospedaliero-Universitaria in Parma, Italy. PROF. KRESTIN is chair of radiology at Erasmus Medical Center.

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