Fig. 61.1
Pitch. Pitch defines the degree of stretch of the helical path of the gantry in spiral CT. A pitch of 1.0 results in contiguous raw data for image reconstruction (bottom of panel). If pitch decreases, gaps between the slices arise and spatial resolution drops (top of panel). For a pitch lower than 1.0, the data overlaps resulting in a vast amount of data for high-quality image reconstruction
A pitch of 1.0 results in contiguous raw data for image reconstruction. If pitch increases, gaps between the slices arise and spatial resolution drops. This is especially important in large scan ranges, like peripheral CTA. The relationship between scan range and spatial resolution is explained using the following example: for peripheral CTA scan, the range is approximately 100 cm. Maximal scanning time is set at 20 s. To prevent “helical stretch”, a pitch of 1.0 is selected. This results in a table feed (TF) of 1000 mm/20 s and 50 mm (!) thick slices. The introduction of multislice spiral computed tomography (MSCT) in the late 1990s dramatically improved scanning time and spatial resolution. MSCT allows the simultaneous acquisition of multiple slices during one gantry rotation. This is achieved by mounting multiple detectors on the gantry and overlap in data acquisition. Pitch was redefined.
Again, if the scan range is 100 cm and maximum scanning time is 20 s, this would result in TF of 1000/20 = 50 mm/s. For a 16-slice MSCT and a pitch of 1.0, this would result in 50/16 = 3 mm thick slices. Recent developments in MSCT allow the simultaneous acquisition of up to 256 slices during one gantry rotation, resulting in a further improvement in spatial resolution and reduced scanning time, producing high-quality peripheral CTA images in spite of large scan ranges.
The latest evolution in CT scanner technology is dual energy CT (DECT). Like in MSCT the gantry of a DECT scanner houses multiple detectors, but it also contains two source beams. The two beams produce x-rays at different voltages. Using spectral differentiation DECT allows tissue discrimination. This holds great promise for CTA as it allows the differentiation between iodine contrast enhancement and calcifications generating a CTA-luminogram [1]. Using image post-processing tools, a DSA-like image is reconstructed allowing fast and high-resolution assessment of peripheral arterial disease.
Image Post-processing Tools
Because of fast data acquisition using multi-row detector and dual beam CT scanners, current volume datasets are large and consist of several thousands of images. To allow effective image analysis, post-processing tools are used to generate three-dimensional models and reconstructions. The most useful post-processing tools for peripheral artery disease include multiplanar reconstruction (MPR), maximum intensity projections (MIP) and three-dimensional reconstructions.
Multiplanar reconstructions are generated to visualize the artery in axial, coronal, sagittal and orthogonal orientation. A commercially available workstation is used to scroll through the stack of images (cine mode). Orthogonal images, reconstructed around the central luminal line of the vessel, allow the visualization of the aorta and iliac arteries in a perpendicular plane (Fig. 61.2). These images are especially useful for diameter and length measurements [2]. Early diameter assessment using axial CT images was often inaccurate as tortuous vessels appeared elliptical, leading to an overestimation of vessel diameter. The use of orthogonal images has overcome this problem and allows true diameter measurements.
Fig. 61.2
Image post-processing tools . Panels (a) to (c) show different image post-processing tools in a patient with an external iliac stenosis. Panel (a) shows a reconstruction perpendicular to the central luminal line of the common iliac artery. The perpendicular reconstructions allow accurate diameter assessment. Panel (b) shows a three-dimensional reconstruction using volume rendering (VR). Rotating the model allows an accurate and fast assessment of the stenosis. Panel (c) shows the stretch path along the central luminal line for accurate length measurement
The second useful image post-processing tool for CTA is maximum intensity projection (MIP). This tool retains high-intensity pixels (iodine and calcifications) and discards the remaining soft tissue (Fig. 61.3). In combination with DECT (above), this results in a two-dimensional CTA-luminogram. A comparable but three-dimensional tool is volume rendering (VR). In this technique the difference in attenuation factor between calcium, iodine and the remaining soft tissue is used to identify the lumen, thrombus and calcification, a technique called segmentation . Based on these voxels, VR produces three-dimensional models of the peripheral artery lumen and wall. Rotating the model allows an accurate assessment of possible stenosis, angulation or occlusion in any direction (Fig. 61.3).
Fig. 61.3
Volume render (VR) and maximum intensity projection (MIP) . In maximum intensity projections (MIP), high-intensity pixels (iodine and calcifications) are retained resulting in a CTA-luminogram (a). Volume rendering (VR) relies on the difference in attenuation factor between calcium, contrast and soft tissue to identify lumen, thrombus and calcification. All structures are colour coded (e.g. contrast is red), a technique called segmentation. VR produces three-dimensional models for image analysis (b)
Radiation Dose
With the introduction of advanced CT imaging like MSCT and the increasing demand for high-resolution MPR, MIP and three-dimensional reconstructions, radiation exposure and the related cancer risk increased [3]. With the more widespread use of dual energy CT scanners (above), radiation dose is likely to increase even further as both tubes contribute to overall radiation exposure.
A clear trade-off exists between radiation exposure and image quality. By adjusting slice thickness, increasing table feed and reducing tube voltage, radiation exposure is decreased. These altered acquisition parameters will however increase image noise and reduce image quality. As for all radiologic imaging modalities, the ALARA (as low as reasonably achievable) principle applies to CTA imaging. By adjusting the acquisition parameters on a patient-to-patient basis and by creating specific protocols for specific regions of interest, the amount of radiation can be kept as low without compromising image quality. Traditional peripheral CTA uses a tube voltage of 120 kVp. By reducing the tube voltage to 80 kVp, radiation exposure in patients with peripheral artery disease is decreased by approximately 30% (5.5 ± 0.9 vs. 8.1 ± 1.1 mSv) without compromising visual image quality and diagnostic performance [4, 5]. This remarkable finding is related to the high anatomic number of iodine. By using lower tube voltages, contrast attenuation increases providing higher vascular enhancement at lower radiation dose. Using this protocol Utsunomiya et al. also reduced contrast dose by 30% in patients with peripheral artery disease [5]. This is of interest as a second drawback of the increased use of CTA is the need for iodinated contrast.
Iodinated Contrast Reactions
Contrast Allergy Reactions
A recent review by Rose et al. provides a comprehensive overview of three grades of allergic reactions: mild, moderate and severe. Severe allergic reactions are rare (0.02–0.04%) but potentially life-threatening [6]. The majority of allergic contrast reactions are mild to moderate. True allergic reactions are often confused with non-allergic anaphylactoid-type reactions that result from histamine release in response to contrast exposure. In general, premedication is effective in 90% of patients susceptible to contrast-induced allergic reactions and includes the use of steroids and antihistamine [6]. In patients with suspected allergic reactions, it is essential to document the symptoms and possible severity of allergic or non-allergic reaction. Only labelling patients as “allergic to iodinated contrast agent” might result in possible overtreatment of patients with no previous hypersensitivity reaction or could affect the choice of imaging modality [7].
Contrast-Induced Nephropathy (CIN)
CIN is the third most common cause of iatrogenic acute renal failure [8]. The 2015 ACR Manual on Contrast Media defined contrast-induced nephrotoxicity as “a sudden deterioration in renal function that is caused by the intravascular administration of iodinated contrast medium” [9]. In general an increase of >25% or an absolute increase of >44.2 μmol/L in serum creatinine within 24 h of contrast exposure is indicative of CIN [8]. About 2% of people receiving iodine contrast develop CIN, and many patients with peripheral artery disease are at increased risk of CIN because of advanced age, diabetes or chronic kidney disease.
To prevent kidney function deterioration, it is essential to identify patients at increased risk of CIN and to initiate preventive measures [6]. Several measures are of theoretical or proven benefit.
Low- or Iso-Osmolar Agents
The exact pathophysiologic mechanisms leading to CIN are unknown, but high osmolar agents are believed to induce an osmotic-mediated decrease in renal blood flow and even ischaemia. Today most institutions use low-osmolar agents.
Intravenous Fluid Hydration
Hydration increases urine flow rate resulting in faster contrast media excretion and shorter contrast exposure [10]. Today, hydration with standard saline is the most common preventive measure. Hydration with sodium bicarbonate has been proposed as a more effective strategy to prevent CIN. A recent meta-analysis pooled data from 20 randomized trials comparing saline to sodium bicarbonate hydration in 4280 patients at increased risk of CIN because of chronic kidney impairment. Results show that hydration with sodium bicarbonate is associated with a significant decrease in the incidence of CIN. Because of the study, heterogeneity subgroup analysis is performed showing a more pronounced beneficial effect of sodium bicarbonate with the use of low-osmolar contrast media and in patients undergoing emergency procedures [11].
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