Tomographic Imaging Techniques

Tomographic imaging techniques such as CTA and MRA are excellent noninvasive, high-resolution imaging techniques that are useful tools for establishing the diagnosis of proximal arch and upper extremity vascular disease. They are capable of defining complex anatomy (as in the case of a congenital abnormality or prior intervention), allow visualization of the vasculature in multiple planes without the problem of overlap, and also show extravascular structures.

MRA has the advantage over CTA (and catheter-based angiography) in that it does not use ionizing radiation or nephrotoxic contrast dye. Gadolinium-enhanced magnetic resonance angiography (GEMRA) is the most useful MRI technique for assessing PAD. Its sensitivity and specificity for detecting occlusive arterial disease is greater than 95% (compared with catheter-based DSA). It is also highly sensitive and specific for assessing vascular wall inflammation. Phase-contrast MRA is a separate technique that provides simultaneous anatomic and functional data, similar to duplex ultrasonography. Phase-contrast MRA can quantify flow across a stenosis or a shunt and can measure turbulence. It is not typically included in MRI protocols at most institutions, so it must be specifically ordered.

There are a number of disadvantages to MRI. It is time-consuming, and imaging of the vasculature within the thorax requires prolonged breath-holding. Patients who are susceptible to claustrophobia cannot tolerate MRI or require prior treatment with anxiolyt-ics. It is highly operator dependent and susceptible to artifact. The presence of permanent pacemakers, implantable cardiac defibrillators, Swan-Ganz catheters, and shrapnel are contraindications to MRI. However, contrary to the conventional wisdom, endovascular stents and most prosthetic cardiac valves are not contraindicated in magnetic resonance scanners.

CTA provides high-resolution multiplanar images that include perivascular structures, similar to MRI,

Irregular Vertebral Arrtery Waveform
Figure 40-8. A, Normal vertebral artery duplex waveform. B, Intermittent vertebral artery flow reversal, as seen with subclavian steal. C, Permanent vertebral artery flow reversal, as seen with subclavian steal.

but is more widely available, less operator dependent, and less susceptible to artifact. Less patient cooperation is necessary compared with MRI, and claustrophobia and prolonged breath-holding are less problematic. Ferric metal is not a contraindication to CTA, but its presence may impair image gathering and cause artifacts. Unfortunately, multidetector CTA requires relatively high doses of ionizing radiation compared to catheter-based angiography, and, like catheter-based angiography, CTA requires the use of iodinated contrast dye that has the potential to cause nephrotoxicity and/or an allergic reaction.

Angiography/Digital Subtraction Angiography

Catheter-based angiography, in particular DSA, remains the gold standard for imaging the peripheral vasculature, including the aortic arch, the great vessels, and the arteries of the upper extremities. It is the most definitive method of delineating the vasculature, and it allows one to measure intra-arterial blood pressures, gradients, and waveforms. It can facilitate the use of intravascular ultrasound (IVUS, discussed later) or a pressure-sensing wire.21 If a lesion is significant and amenable to intervention, it is accessible at the time of the diagnostic catheter-ization. The disadvantages of catheter-based angiography include its invasiveness, exposure to ionizing radiation, and potential for complications (vascular trauma, bleeding, infection, contrast-induced nephropathy, and contrast-induced allergic reaction).

It is usually feasible and preferable to obtain vascular access through the femoral artery for percutaneous catheter-based angiography and angioplasty of the aortic arch and upper extremity vessels. In the case of a tight subclavian or axillary artery lesion, it may be necessary to obtain brachial artery access on the ipsilateral side.

An array of diagnostic catheters is available for accessing and imaging the great arch vessels and upper extremities. The pigtail catheter in conjunction with an Autolnjector is necessary to obtain good images of the aortic arch. Common projections are left anterior oblique (LAO) 30 degrees for origin of great vessels and right anterior oblique (RAO) 20 degrees for the brachiocephalic bifurcation. It is usually necessary to modify these angles to open up the arch. The arch vessels may be imaged with a variety of catheters, such as the Judkins right no. 4 (JR4), internal mammary (IMA), Vitek (VTK), Simmons, or multipurpose catheter. Injection rates for the arch are usually 30 to 40 mL over 2 seconds. The upper extremities may be selectively taken with hand injections.

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