Since its original description in 1992, computed tomography angiography (CTA) has advanced from the method of using a simple single-detector to a powerful imaging multidetector system capable of acquiring 16 channels of data with rapid imaging acquisition and higher spatial resolution, while simultaneously allowing patient coverage of more than 120cm with a single scan. Computed tomography (CT) has undergone a remarkable evolution over the last two decades and now allows subsecond scan times, submillimeter scan thickness and the acquisition of more than 1,000 slices per examination with a single venous injection. Optimized contrast enhancement, improved three-dimensional (3-D) volumetric data analysis, and sophisticated computer software and workstations are but several advantages of the current technology and is why several sources are predicting that multidetector CTA (MDCTA) will "replace 80% to 90% of all conventional diagnostic angiography". MDCTA has already demonstrated comparability and even diagnostic superiority over conventional diagnostic angiography (CDA) in several vascular applications including the aorta, carotid, renal, iliac, and pulmonary vessels.
CDA with digital subtraction angiography (DSA) remains the clinical 'gold standard' for vascular imaging but has multiple limitations. Magnetic resonance angiography (MRA) has been advocated to address the limitations of DSA but MRA also possesses significant limitations. In comparing MDCTA with DSA and MRA, noteworthy MDCTA advantages include lower costs, less invasiveness (venous stick), much faster operating times, potentially less contrast use and radiation exposure, allowance for 3-D reconstruction, it does not require an angiosuite or cathode laboratory team, and has potentially fewer complications.
MDCTA can also be utilized in evaluating coronary artery disease (CAD), chronic vascular thrombus or calcification, and non-vascular soft tissues and osseous structures. Since MDCTA does not require an arterial access, patients on anticoagulation or with hyper-coagulable states need no pre- or peri-procedural preparations, and MDCTA can be utilized in patients with limited vascular access (grafts, severe peripheral vascular disease (PVD), or absent pulses). This article describes the authors' early 16-channel and current 64-channel out-patient office experience with MDCTA, and how it has changed and influenced patient care.
The three major principles of MDCTA are:
- to achieve an adequate level of arterial contrast enhancement during acquisition;
- to provide cephalocaudad coverage of the targeted anatomy during an early sustainable breath-hold interval (less than 20 seconds); and
- to time the onset of CT acquisition after contrast injection accurately, so the first circulation enhancement is obtained from the start to the end of acquisition.
An important aspect of efficient clinical MDCTA is the accurate determination of the suspected vascular pathology of the patient, and therefore the clinical information requested. This requires developing close communication between the physician and MDCTA team, particularly during the early experience.
Typically, 80-100mL of an intravenous (IV) non-ionic contrast agent is administered through a 20-gauge plastic venous cannula in an antecubital vein at an injection rate of 4-6mL per second. In clinical MDCTA, the contrast enhancement, acquisition parameters, coverage speed, and circulating time are accurately determined by protocols based on the clinical information requested, imaging modality used, and patient-specific variables (age, body size, cardiac output, and renal function, etc.). With current technology, it is reasonable to obtain images from the clavicles or abdomen down to the knees or the abdomen and pelvis to the patient's feet, all within 30 seconds. After the primary author observed his first MDCTA, he commented, "this is like shooting 16 angiograms all from different angles, simultaneously, in color, and in just a few seconds".
Abdominal Aortic Aneurysm
Conventional CT scanning and CDA have been considered the 'gold standard' for abdominal aortic aneurysm (AAA) diagnosis, treatment planning recommendations, and post-operative surveillance, particularly since the introduction of aortic stent grafts and endovascular aneurysm reconstruction (EVAR). Besides being non-invasive, MDCTA offers several advantages over traditional imaging including:
- superiority in identifying mural thrombus and evaluating peri-aortic tissues for rupture, endoleak, and inflammatory AAA identification;
- more precise determinations of size, length, angulation, and transverse dimensions of the AAA superior neck;
- more accurate characterization of post-procedural juxtarenal AAA endograft deformation, kinking, or migration with identification of branch vessels;
- 3-D reconstruction allows improved assessment of iliac tortuosity and detection of endoleaks; and
- post-EVAR AAA volume determinations may allow earlier detection of device failure, rupture or endoleak.
Traditional imaging can overestimate the true AAA neck diameter and length because of obliquity, therefore influencing the treatment recommendations and outcomes. MDCTA has replaced traditional CT scanning and DSA in almost all aspects of the authors' AAA pre-therapy recommendations and post-treatment follow-up.
Carotid Artery Disease
The degree of stenosis of the internal carotid artery (ICA) has been scrutinized more than any other peripheral vessel because the degree of stenosis is associated with the risk of stroke. Landmark trials over the last two decades have demonstrated reduced stroke rates in both symptomatic and asymptomatic patients with ICA stenosis of ≥50% to 60% treated by carotid endarterectomy (CEA). Several equally land-marked interventional carotid artery stent (CAS) trials have shown at least equivalency and even improved early outcomes with ICA stenting compared with surgery challenging the 'gold standard' for ICA disease treatment. This will also challenge the authors' 'gold standard' for ICA imaging since optimal CAS will require more detailed and comprehensive information on the aortic arch and entire extra- and intracranial carotid artery system, which is currently required for surgery.
CDA with DSA remains the 'gold standard' for ICA imaging but there is a definite risk of periprocedural stroke. DSA is performed in limited projections while MDCTA provides multiple views. DSA has recently been shown to underestimate the degree of ICA stenosis when surgical specimens of cross-sectional lumens were compared with DSA. Using helical MDCTA, Elgersma et al. identified additional (16%) ICA suitable for CEA compared with DSA, therefore further underscoring the complexity of the ICA bifurcation and the need for multiple views to accurately determine the degree of stenosis.
Carotid duplex ultrasound (DU) has been known to have a sensitivity and specificity of more than 90% in diagnosing ICA disease but is still operator-dependent, moderately time-consuming, and gives limited information on the distal extracranial ICA and intrathoracic common carotid artery (CCA), and no information on the intracranial or arch vessels. Several reports have shown the diagnostic accuracy of carotid MDCTA for 70% to 99% ICA stenosis to have a sensitivity of 100% and specificity of 94% to 100%. Multiplanar reconstruction (MPR) methods will allow cross-sectional luminal and plaque morphology evaluation, and potentially provide additional pertinent clinical information.
A routine carotid MDCTA examination includes 30cms of cephalocaudad coverage from the ostium of the aortic arch vessels to the circle of Willis at the base of the skull. This allows an accurate decision-making recommendation regarding a patient's CAS candidacy with a single non-invasive examination to be given. Currently, intracranial vessels are not being routinely imaged, although this capability is possible. MDCTA neuroimaging has been found to compare favorably with DSA in evaluating acute stroke, arterial dissections, and intracranial aneurysms with reduced patient risks. Excellent proximal vertebral artery imaging is available on a routine carotid MDCTA allowing identification of additional symptomatic patients who could benefit from endovascular intervention.
Aortic arch vessel anatomy, tortuosity, and ostial arch vessel disease (AVD) along with distal ICA tortuosity are often significant limitations to carotid stenting, since placement of a distal protection device has been required in most CAS trials. Isolated ostial AVD is thought to be rare with a reported incidence of 1.8%, and is reported to be even less (0.6%) in patients undergoing CEA. This most likely underestimates the true incidence of AVD as many selective carotid angiograms are performed without arch angiography, and CEA is often recommended on DU alone. The authors' group reported a 4.3% (34/784) incidence of simultaneous significant ostial AVD in patients requiring CEA, further raising questions regarding the underestimation of AVD. MDCTA now allows accurate assessment of ICA stenosis, access vessel and distal ICA tortuosity, and the detection of ostial AVD, therefore potentially allowing a determination of CAS versus CEA candidacy non-invasively without the inherent risks of DSA. In the authors' experience, carotid MDCTA has almost replaced CDA and DSA as a diagnostic tool, and increasingly important carotid treatment and surveillance decisions are made relying on MDCTA information alone.
Renal and Mesenteric Arteries
The benefits of renal artery stenosis (RAS) diagnosis and revascularization have been proven in patients with severe hypertension, refractory CHF, angina and patients with declining renal function—therefore underscoring the importance of a safe, accurate, simple non-invasive diagnostic tool. The limitations of abdominal vessel DU are well known. Three-dimensional abdominal MDCTA has addressed many of these limitations, and Berengi et al. recently reported 100% accuracy and sensitivity comparing MDCTA with DSA in RAS. Galanski et al. reported the sensitivity and specificity of MDCTA compared with DSA in RAS as 92% and 95%, respectively.
Recent reports by AbuRahma et al. and the authors' group have identified benefits with PTA/stenting in appropriately diagnosed patients with mesenteric artery disease (MAD). There are no such reports of MDCTA evaluating MAD although the authors' initial experience indicates that MDCTA will be as accurate in diagnosing MAD as in RAS. Stents are very well imaged, allowing the potential for improved accuracy in detecting restenosis, which has a 10% to 20% incidence post-renal and mesenteric PTA/stenting.
Abdominal MDCTA is rapidly replacing DU as a primary diagnostic and surveillance tool in evaluating RAS and MAD. In patients with RAS, the MDCTA scanner can 'pinpoint' the juxtarenal aorta and shorten acquisition times to less than 10 seconds and reduce contrast load to less than 40mL.
Asymptomatic and symptomatic critical MAD disease (celiac and superior-inferior MAD) has been difficult to diagnose both clinically and by traditional imaging techniques—it is therefore underappreciated, underdiagnosed, and undertreated. This is likely to be very analogous to the understanding and treatment of RAS two decades ago. An abdominal aorta with distal run-off CTA will begin acquiring 3-D images at the level of the diaphragm, and will therefore start to identify a significant amount of MAD. It is likely that MAD is as common as RAS—this will therefore be a new vascular territory available for percutaneous revascularization, because the endovascular treatment techniques are very similar to treating RAS. Traditionally, the treatment of MAD has required difficult major vascular surgical reconstructions with high mortalities and morbidities. It is possible that with MDCTA and endovascular treatments, the incidence of treating MAD will greatly increase in the near future.
Aortoiliac Occlusive Disease
An 'endovascular first' approach toward aortoiliac occlusive disease (AIOD) revascularization was adopted in the authors' practice well over a decade ago, and acceptable long-term results in iliac PTA/stenting have substantiated this policy. DSA can fail to detect aneurysmal disease and the diagnostic accuracy is adversely affected by vascular calcification. Single planar DSA frequently 'misses' eccentric lesions, which are so posteriorly prevalent at the distal aortic bifurcation and entire iliac artery segment. DSA often poorly visualizes distal vessels to a more proximal stenosis or occlusion.
Native common femoral artery (CFA) disease is not uncommon and patients frequently present with 'stick site injuries' from multiple prior procedures. Avoiding a 'diagnostic stick' with DSA has significant clinical implications. The authors have found MDCTA to be particularly useful in avoiding 'sticking into disease' and planning endovascular treatments.
Several recent studies have shown the sensitivity and specificity of MDCTA in detecting significant AIOD to be more than 96% and Rubin et al. recently reported 100% concordance between MDCTA and DSA in AIOD using 2.5mm slices covering 120mm in 60 seconds acquisition time. MDCTA has been shown to be superior to DSA in evaluating vascular trauma, dissections, and popliteal aneurysms. Several asymptomatic AAA and significant iliac and popliteal aneurysms bigger than 5cm are diagnosed each month on patients imaged for suspected AIOD. Recent reports have shown a reduction in contrast use and a four-fold reduction in radiation exposure comparing MDCTA and DSA in diagnosing AIOD.
The authors' routine MDCTA for AIOD includes cephalocaudad coverage from the supraceliac aorta to the proximal thigh (approximately 30-40cm). Scanning parameters can be adjusted to achieve a reduced contrast load (less than 75cc) with scan times ≤30 seconds. It is always recommended to scan both CFAs, since most endovascular procedures are performed via a CFA approach. MDCTA is now the authors' diagnostic mode of choice for diagnosing and following AIOD, and more than 80% of their surgical and endovascular treatment recommendations are influenced by MDCTA.
The SFA and crural vessels are among the most calcified vessels in the body, therefore are a challenge to accurate assessment of the degree of stenosis. The high incidence of vessel occlusions, asymmetric disease, and vascular calcifications are highly characteristic of infrainguinal disease and known limitations of DSA and DU. Known advantages of MDCTA include improved accuracy in vascular occlusions, calcifications, and patients with asymmetrical disease.
There is no published data comparing DSA or DU with MDCTA in infrainguinal vessel, but several promising image-enhancement processing techniques are available, especially in infrapopliteal vessels. Curved planar reformation (CPR) and semi-transparent volume rendering (STVR) with automated measurements are new 3-D imaging modalities that improve the accuracy of MDCTA in highly calcified vessels. The editing of bony structures (osseous segmentation) is now available at the workstation using automatic region growing imaging techniques. Segmentation of the tibia, fibula, and tarsal osseous structures can have significant clinical implications in achieving limb salvage. Maximum intensity projection (MIP) is a 3-D workstation that allows maximal contrast opacification and vessel interrogation.
Rubin et al. reported the identification of 26 additional infrainguinal arterial segments that were not identified with DSA utilizing MDCTA due to the improved arterial opacification distal to an occluded segment. Identification of 'distal targets' could identify patients for tibial bypass or endovascular revascularization who otherwise may only be offered amputation. This becomes particularly important with the excellent six-to 12-month limb salvage rates reported by Laird et al. (more than 90%) utilizing the excimer laser (Spectranetics Corporation, Colorado Springs, CO) in the multicentered Laser Angioplasty for Critical Limb Ischemia (LACI) trial. Similar results were reproduced by the authors' group in a 'LACI-equivalent' report. MRA is considered the 'gold standard' in imaging pedal vessels but MRA is not reliable in calcified vessels, is time-consuming, not widely available, and affected by retrograde flow artifacts, existing stents, and pacemakers. The authors have found infrainguinal MDCTA invaluable in the diagnosis, treatment, and planning and follow-up of their infrainguinal interventions, particularly in infrapopliteal vessels where DU is rarely helpful.
Currently, the authors' most frequently utilized imaging is an abdominal MDCTA with bilateral run-off to the feet.This can usually be accomplished with 80-100mL of contrast injected over approximately 120cm with a scanning duration of less than 30 seconds.
Bypass Graft Surveillance
Both infrainguinal bypass grafts and endovascular treated vascular segments have been shown to require a 20% to 30% secondary intervention rate to achieve an acceptable one- to two-year patency, underscoring the need for non-invasive post-procedural surveillance. DU has been the 'gold standard' in the post-surgical patient, but DU limitations not found with MDCTA include difficulty in following non-anatomically placed grafts, inaccuracy in cases with inflow lesions, multiple graft lesions, A-V fistulas, inability to 'pinpoint' stenosis, and failure to display tibial or pedal vessels distal to the anastomosis. Willmann et al. reported 98% sensitivity and specificity in comparing DSA with four-channel MDCTA in 85 bypass grafts. In the authors' experience, MDCTA has allowed accurate diagnosis and planning for graft reintervention, while DU has not.
Cardiac MDCTA differs from routine MDCTA for PVD primarily due to the dynamic motion of the heart and static nature of PVD. Detector, table, and imaging parameters vary greatly from these used in PVD.The scan times are much longer and oral and IV beta-blockers are required to keep the heart rate at less than 60bpm. Current MDCTA is unable to accurately grade coronary artery stenosis, particularly in small vessels. Cardiac MDCTA has shown a high negative-predictive value for significant disease and the sensitivity, specificity, and reproducibility of MDCTA for detection of coronary calcification has been found to be higher than electron beam CT (EBCT).
MDCTA has been found to identify intracoronary thrombus much like in PE, and may have a promising role in diagnosing acute coronary syndromes. Other potential future applications include post-PCI stent evaluation and vulnerable plaque identification. The authors are not currently performing cardiac MDCTA for CAD screening or patients with stable angina but they are increasingly using MDCTA to evaluate coronary artery bypass grafts (CABGs) and pulmonary venous anatomy in planning electrophysiological procedures.
The authors would like to thank Mrs Kelly Tilbe for her technical help with manuscript preparation.