Coronary CT speed, resolution edge out catheter angiography

March 4, 2008
Constantino Pe?a, MD

With its 95% accuracy, practitioners increasingly choose CTA as theimaging modality for suspected or known coronary artery disease

The potential benefits of CT angiography have expanded greatly in the 30 years since its introduction. Although imaging of the coronary arteries was a goal from the early days of CT, slow speeds and poor resolution prevented expansion into that area initially. The advent of multislice CT in the last decade has increased speeds and improved resolution, allowing increasingly accurate imaging of the ever-moving small coronary arteries.

Our institution has begun to move in recent years toward less invasive methods of diagnosis and treatment planning, electing to strengthen our CT and MRI capabilities rather than add more catheterization labs. Its noninvasive nature and 3D imaging capabilities give CTA a growing number of ways to be used to study the heart and coronaries, ranging from diagnosis to procedural guidance and follow-up of interventions. In addition to helping diagnose and determine treatment for coronary artery disease, CTA is now starting to be employed to assess cardiac function, wall motion, ejection fraction/perfusion, and structural heart disease. It is also being used for guidance in procedures such as radiofrequency ablation in patients with atrial fibrillation.

For decades, catheter-based angiography used with stress testing has been the standard of care in the diagnosis of patients with suspected coronary artery disease. Although minimally invasive, angiography carries procedural risks and complications along with the high use of radiation and contrast. Its high cost stems from the significant amount of time the patient must spend in the catheterization lab, as well as the required pre- and postprocedural care.

Angiography is a 2D tool, and its limitations have become increasingly apparent. The flattening of the images results in imaging artifacts and other drawbacks, including foreshortening and overlap and lack of appreciation of vessel angulation and tortuosity. Determining true luminal diameter is difficult if not impossible, as variations in anatomy make sampling inadequate. This can result in over- as well as underdiagnosis of stenosis, including severity and extent. In attempting to overcome these shortcomings, cardiologists may obtain many views, subjecting the patient to greater amounts of radiation and contrast.

Catheter-based angiography is operator-dependent as well. This limitation has become apparent in the face of recent findings suggesting that drug-eluting coronary stents are not optimally placed in a significant number of cases. Some suggest this may be due to less-than-ideal working views chosen by the operators, even those with significant experience. Issues include poor placement proximally or distally (geographic miss), failure to stent the entire lesion, inappropriately sized stents, and incomplete expansion of the stent. In addition to suboptimal treatment and revascularization, a major concern is that higher rates of myocardial infarction may result from stent-induced thrombosis. The latter problem has been blamed on various issues, including the polymer coating and the drug’s interference with positive remodeling, but reliance on this new miracle stent could also have led to laxity in placing the device precisely.

To overcome the shortcomings of catheter-based angiography, physicians increasingly have turned to intravascular ultrasound (IVUS), which offers cross-sectional views and can measure minimum luminal area (MLA), identify remodeling, and determine plaque characterization. IVUS increases procedure time as well as cost, however, and it is invasive.

With rapid improvements in technology, including 64-slice scanners that can more accurately image the arteries of the beating heart, CTA may be moving into the area occupied by IVUS. In fact, many argue that IVUS, rather than catheter-based angiography, should be the gold standard against which CTA is measured.

Tomographic intravascular analysis via multislice coronary CT can, like IVUS, measure MLA and identify positive and negative remodeling. CTA also shows promise in identifying plaque type. It is valuable as well in more precise stent placement and decreases the chance of geographic miss, an indication for which IVUS is not typically employed.

Although catheter-based angiography is still given the nod in terms of better resolution, MSCT has improved in that regard. Currently, the spatial resolution of a 64-slice CT scan is 0.4 mm compared with 0.2 mm for cardiac catheterization.

Like CTA, noninvasive MR angiography is experiencing great growth. Choice of one modality over the other may depend on what is available at a facility, as well as the particular clinical question. MRA requires no radiation or iodinated contrast, giving it the edge there. But CTA has better spatial resolution and is usually more reproducible, particularly in the evaluation of the coronary arteries.

BENEFITS OF CTA

Physicians are finding CTA increasingly valuable at all stages of coronary artery disease management, from diagnosis to treatment planning to follow-up.

• Diagnosis. CTA is increasingly chosen as the imaging modality for suspected or known coronary artery disease, as it has been shown to be 95% accurate or greater in detecting the condition. In low- or intermediate-risk patient populations, the procedure has developed a role before stress testing as well as after indeterminate stress testing. In addition to depicting the vessel lumen and wall, CTA allows identification of vessel length, curvature, and branching angles, as well as lesion length, location, and severity. Early results suggest the modality is beneficial in determining whether plaques are calcified or lipid-rich and may possibly help identify vulnerable plaque .

If CTA depicts lesion stenosis of less than 50% and the patient is asymptomatic, medical treatment is usually recommended. Areas of stenosis greater than 50% are typically followed by selective catheter-based angiography. If significant stenosis is identified, percutaneous coronary intervention (PCI) or bypass grafting may be warranted. If no significant stenosis is identified, IVUS is recommended, followed by medical management or treatment-PCI or surgery-depending on the findings and clinical situation .

Chronic total occlusions are challenging to analyze with catheter-based angiography and difficult to successfully treat with PCI. CTA often is beneficial in determining when a chronic total occlusion may best be treated endovascularly. For instance, significant transluminal calcification, excessive length of the occlusion, and tortuosity of the vessel suggest that the occlusion is less likely to be treated successfully with PCI.

• Preprocedure planning. CTA can serve as a preprocedural map and guide in planning coronary interventions. The 3D coronary tree image acquired from CTA can be manipulated in a multitude of angles to allow determination of the ideal angles prior to beginning an intervention. Software is available that can guide the physician in choosing the best angles to maximize the evaluation of a certain stenosis. Lesion length and reference vessel diameter are accurate and clearly visualized on CTA, allowing objective decisions to be made in determining the appropriate size of the catheter, guidewire, balloon, and stent.

Many of those decisions will vary according to calcification of the lesion, as well as orientation and tortuosity of the vessel. A preprocedural map also prepares the operator for anatomical challenges (coronary origin anatomy, bifurcated lesions and occlusions) prior to beginning the intervention.

• During procedure.  Recently, CTA images have proven of benefit in the catheterization lab. CTA vessel maps can be projected in the cardiac cath lab and moved along with the C-arm to highlight the best views for treatment. Software such as the CT TrueView (Philips Healthcare, the Netherlands) determines the amount of foreshortening and overlap of a particular stenosis on a certain view. By coupling the 3D CTA vessel map with the C-arm, an operator can determine the best angle for treatment. This leads to decreases in treatment time, radiation, and contrast.

The more complicated the procedure, the greater the potential benefit of having a preprocedure CTA. In the case of revascularization of a chronic total occlusion, for instance, a 3D CTA vessel map linked to the C-arm projected alongside or superimposed upon the fluoroscopic images can help the operator determine the best wire path and direction. The operator can confirm that the guidewire is being placed correctly and is going in the right direction. A CTA map also can help aid the interventional physician in crossing difficult lesions. Because the extent of the lesion is better visualized with CTA, it can guide placement of the stent to ensure that it covers the entire lesion.

CTA also provides the best imaging to help the operator determine that the stent is being placed at the correct angle, with no foreshortening. The use of coronary CTA data in the catheterization lab should translate into safer and more effective therapies. Studies may demonstrate better placement of stents (properly expanded and extending the entire lesion) with less radiation and contrast exposure and fewer complications.

• Follow-up. CTA can also be used to assess the success of coronary stenting and bypass grafts. It is a direct but noninvasive manner to evaluate stent and graft patency in the correct patient group.

DRAWBACKS OF CTA

Despite its many virtues and benefits, CTA is not all things to all people-just yet. Not all patients benefit from CTA before undergoing percutaneous treatment. Symptomatic patients may be served best by having a catheterization directly before or after a nuclear scan. Some patients can tolerate a limited amount of iodinated contrast, in which case the CTA is deferred for a procedure that may diagnose and treat the patient. Although CTA uses far less contrast and radiation than catheter-based angiography, it nonetheless employs high doses of both.

Like any modality, especially one still in its relative infancy, CTA is not perfect. It is most effective in patients with low and minimally variable heart rates, and it works best when the time of acquisition is short in order to avoid respiratory motion. Obese patients typically are not well served by CTA, as image quality suffers. Extensive coronary artery calcification can lessen the accuracy of CTA.

An additional downside is that presently, the reimbursement of the technology is being challenged. The lack of coverage for the technology is limiting its utilization and benefit.

FUTURE CHALLENGES

Numerous early studies have found CTA to be highly useful as a means of triaging patients presenting to the emergency room with chest pain. In the near future, CTA may well determine which patients are suffering from acute coronary syndrome and in need of treatment versus those who can safely be discharged without intervention.

While vascular centers of excellence have caught on to the benefits of CTA, there is still significant room for growth and studies of the modality’s benefits. As the software is continually improved, use of this modality will unquestionably expand, including into fusion and the cardiac catheterization lab. Undoubtedly, a day will come in the not-too-distant future when patients are initially diagnosed with CTA, then transferred to the cath lab, electrophysiology lab, or surgical operating room based on the information gleaned noninvasively.