(1)
Department of Cardiology, Scripps, San Diego, CA, USA
(2)
Cardiology, Aurora St. Luke’s Medical Center, Milwaukee, WI, USA
(3)
Cardiology, Aurora Sinai Medical Center, Milwaukee, WI, USA
(4)
Department of Cardiology, Loyola University Chicago, 2160 S. 1st Avenue, Building 110, Maywood, IL 60153, USA
Keywords
CT angiographyMulti-detector CTPeripheral artery diseaseMaximum intensity projectionMultiplanar reformatted imageCurved multiplanar imageVolume renderedCritical limb ischemiaIntroduction
Since the advent of multi-detector CT scanners , much progress has been made in the evaluation of peripheral arterial disease. Spiral CT was introduced in the early 1990s providing basic assessment of vasculature during contrast injection studies. However, it was multichannel CT that provided the resolution required to confidently elucidate vascular disease. Multi-slice CT emerged in the late 1990s and allowed for better volume coverage with shorter scan times, as multiple slices could be captured during one gantry rotation with decreased gantry rotation time. Overlapping image slices during reconstruction add to the improved longitudinal resolution, and post-processing techniques using submillimeter slices allow for superior two and three-dimensional rendering (Fig. 17.1). Table 17.1 provides a list of key terms to better understand the literature.
Fig. 17.1
A 67-year-old male who presented, with LLE ischemic pain, found to have left popliteal artery occlusion, and seen in a–f. (a) Zoomed 3D VR with transparent bone for visual landmarks. (b) MIP, posterior view, with abrupt interruption of luminal enhancement at the L popliteal artery (arrow). (c) 3D full MIP showing L popliteal occlusion (white arrow) with faint distal reconstitution of the peroneal artery (green arrow) due to collateral flow. (d) 3D VR with transparent bone to better appreciate the vascular anatomy in relation to adjacent bony structures. (e) Sagittal MIP with contrast proximal to occlusion highlighted (arrow). (f) Axial image showing R popliteal artery with normal enhancement (green arrow) as compared to diseased L popliteal artery (white arrow)
Table 17.1
Key terms
Pitch | Table feed per rotation in a spiral scan divided by the total width of the collimated beam. Pitch, p < 1 signifies data acquisition with overlap in the longitudinal direction |
Gantry rotation | The gantry is the spherical portion of the CT that houses the X-ray tube and the detector. One gantry rotation is one 360-degree rotation around the patient over a period of time |
Voxel | Each CT image has a defined thickness. The smallest distinguishable matrix in a CT image is known as voxel, which is a pixel with a defined thickness |
Attenuation | The loss of X-ray beam strength as it passes through an object. Usually highly attenuating structures are denser, making it harder for X-rays to pass through, allowing for stronger reflection and brighter appearance on CT imaging |
Dual-source CT | Makes use of two X-ray and detector sources perpendicular to each other. This increases temporal resolution, reduces radiation, and increases the speed of image acquisition |
Kernels | Post-processing filter or algorithm applied to the raw CT images to specifically select for desired structures. For example, a “sharper” kernel will increase the image contrast but will also introduce more noise |
Hounsfield Unit | A unit of measurement to quantify radiodensity. A more radiopaque structure has a higher value. A HU of 0 is calibrated to the radiodensity of distilled water at standard pressure and temperature (STP) |
Detector rows | The area across from the X-ray beam within the gantry. It receives the X-ray beam after it passes through an object. Multiple detector rows allow the acquisition of images from slightly different angles resulting in greater volume coverage and scan speed |
In a meta-analysis published in JAMA in 2009, 20 diagnostic cohort studies analyzed with 957 patients predominately presenting with intermittent claudication (68 %) showed that the overall sensitivity for CTA detecting more than 50 % stenosis or occlusion was 95 % (95 % CI, 92–97 %) and specificity was 96 % (95 % CI, 93–97 %), when compared to intra-arterial DSA [1] (Fig. 17.2). Understaging (underestimation of disease severity) occurred in 9 % of segments and overstaging (a significant stenosis was diagnosed by CTA as an occlusion) occurred in 4 % of segments. However, this meta-analysis only included one study with critical limb ischemia (CLI) patients, identifying the need for further evaluation in this population. In addition to lesion severity, CTA is also highly accurate in identifying length and number of lesions [2], as are required for correct treatment decisions per TASC guidelines, making CTA a valuable tool for therapeutic planning. More recent data with 64 slice CT scanning echoes previous studies, with 98 % accuracy for CT angiography detecting greater than 70 % stenosis, in a prospective cohort of 212 symptomatic PAD patients (acute CLI excluded) [3] (Fig. 17.3). It further suggested that results could be used to effectively guide clinical management, as therapy recommendations based on CT angiographic findings alone were identical to those based on DSA findings in all but one patient. Similar results can be found in a cohort of 41 patients with critical limb ischemia and severe claudication [4], making CTA a useful tool in treatment planning.
Fig. 17.2
A 65-year-old male with known peripheral arterial disease, with a history of acute-on-chronic left lower extremity claudication, found to have stenosis of the right superficial femoral artery. (a) Magnified CTA-MIP showing severe stenosis of SFA mid-thigh with atherosclerotic plaque along its course. (b) DSA showing the same SFA stenosis
Fig. 17.3
A 65-year-old male with HTN, DM, HLD presenting with LLE claudication and L common iliac stenosis. (a) CTA-MIP showing calcified plaque causing high-grade stenosis at the origin of L common iliac artery (inferior arrow). Also, note the presence of mural thrombus with calcification (superior arrow) in the distal abdominal aorta just above the bifurcation. (b) DSA demonstrating the same severe L common iliac stenosis (arrow)
CT Angiogram: Protocol Basics
Typical CT angiography scanning protocols include a digital radiograph called the “scout image,” a non-enhanced scan, a test bolus or bolus triggering, the contrast-enhanced CT angiogram, and an optional delayed-phase acquisition to capture late opacification of distal vessels. The delayed-phase acquisition may be necessary in conditions of slow moving contrast, due to severe vascular disease, low cardiac output states, or significant aneurysms. Popliteal artery aneurysms can be well visualized by CTA (Fig. 17.4).
Fig. 17.4
A 58-year-old heavy smoker, who presents with LLE pain, found to have a popliteal aneurysm containing thrombus. (a) Full MIP showing bilateral fem-pop circulation with L popliteal aneurysm (starred). (b) MPR-left sagittal view of popliteal aneurysm with thrombus. (c) MPR-axial images of the left popliteal aneurysm. Note the normal right popliteal artery for comparison in axial view. Arrows in b and c outline the aneurysm containing thrombus
Patients are positioned supine, arms overhead to reduce contributing artifact, and legs close together with slight internal rotation of the feet. Proper positioning is important. For example, plantar extension of the foot can cause compression of the popliteal artery by the gastrocnemius muscle in popliteal artery entrapment syndrome and can be misinterpreted as atherosclerotic steno-occlusive disease. Only a single breath-hold is necessary (during abdominal acquisitions), and scan times can be as short as 15–40 s. The entire study can involve less than 15 min of room time, making CTA useful when rapid assessment of critical limb ischemia is required, as opposed to more time-consuming modalities such as ultrasound or MRI. Table 17.2 contains a CTA runoff protocol utilized at Loyola University Medical Center, adopted from D. Fleischmann at Stanford University.
Table 17.2
Scanning protocol; adapted from D. Fleischmann, Stanford University Medical Center
Lower extremity runoff, Siemens S 64 | |||||
Topogram | 1500 mm AP; feet first, arms up; feet still and relaxed; support with cushions/tape | ||||
Range 1 + 2 | Bolus tracking, ROI in the abdominal aorta at the level of the celiac artery | ||||
Range 3 | Runoff: from above the celiac trunk (D12 vertebral body) through the toes 120 kV/Care dose 4D w. 250 ref-mAs (voltage weight adjusted) | ||||
64 × 0.6 mm, 0.5 s gantry rotation | |||||
40 s scan time (will result in a pitch <1) | |||||
Set scan range first and then change scan time to 40 s | |||||
Range 4 | Runoff: Preprogrammed optional second CTA acquisition to cover popliteal and crural territories. This range is only initiated if there is no contrast medium opacification seen in the popliteal/crural vascular territories | ||||
Breath-hold | At inspiration | ||||
Scan direction | Cranio-caudal | ||||
Injection | 20–22G IV line, Isovue (iopamidol) 370
 Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
Get Clinical Tree app for offline access
| ||||