CT Examination Technique

10.1055/b-0034-91849

CT Examination Technique

In the years since CT colonography was first introduced, significant advances have been made in scanner technology and in postprocessing software. The first attempts at colonic examination in the mid-1990s used single-slice spiral CT scanners. Although the preliminary results of CT-supported colon studies produced very promising early results, the spatial resolution was inadequate for the detection of smaller lesions or even for computing 3D images of sufficient quality.

Multidetector CT. The introduction of four-slice multidetector CT scanners (MDCT) in 1998 provided adequate spatial resolution for detection of polyps measuring less than 1 cm. Now 16-slice (from 2001 onward) and 64-slice scanners (from 2004 onward) can obtain slice thicknesses of less than 1 mm, offering high imaging quality. For CT colonography, the high acquisition rate offered by the most recent generation of scanners is particularly advantageous. With a single-slice scanner, an abdominal scan with 5-mm-thick slices took more than a minute. Using a 64-slice multidetector scanner with 0.6 mm collimation, the same examination can be done in just 7–8 seconds.

It is important for the examiner to be familiar with the technical specifications of the scanner being used, and to be able to design the examination protocols with these specifications in mind.

Multidetector CT Protocols for CT Colonography

Ideally, the goal of any examination protocol is to achieve the highest possible spatial resolution with the lowest possible radiation dose in the shortest length of time. A fundamental requirement for CT colonography is to achieve a CT scan of the whole abdomen with narrow collimation within one breath-hold. The thinner the reconstructed slice thickness, the closer one comes to a true isotropic volume CT dataset that will allow multiplanar reformatting in any plane and high-resolution 3D images. Recent multidetector scanners can scan the abdomen and pelvis much more quickly, which shortens the breath-hold. This in turn reduces the number of respiration and movement artifacts, thereby significantly improving the image quality. In one study, movement artifacts were reported in 61% of examinations conducted with a single-slice CT, while respiration artifacts occurred in only 16% when a 4-slice scanner was used. These rates may be further decreased by using 16-slice or 64-slice scanners, and in one of our own studies, out of 100 examinations performed with a 64-slice scanner, no movement artifacts occurred. Multidetector scanners with at least 4 detector rows are recommended for CT colonography, and scanners with 16 or more detector rows are preferred. Useful scanning parameters for CT colonography are listed in Tables 2.2 and 2.3.

Collimation and Slice Thickness

Basically, the smallest usable slice thickness defines the smallest lesion that can be detected. This has a direct influence on the detection of colonic lesions. The thinner the individual reconstructed slice of the CT dataset, the smaller the partial volume effects (partial volume averaging) (Fig. 2.24). Thicker slices may also reduce specificity; that is, the number of false-positive diagnoses increases, since typical morphological features of stool such as tiny pockets of air may be less conspicuous.

Collimation. In CT colonography, detection of endoluminal lesions relies heavily on precise imaging of the surface morphology of the colonic mucosa. Partial volume effects can cause blurring between the contours of polyps and normal intestinal anatomy. In addition, stair-step artifacts can considerably compromise image quality. This is especially true for single-slice scanners, in which collimation for a colon examination typically is between 3 and 5 mm. This is why CT colonography requires a high-resolution CT dataset of the wohle abdomen acquired with narrow collimation within one breath-hold. For a 4-slice or 8-slice scanner, collimation should be 2.5 or 1.25 mm, depending on the equipment manufacturer. In 16-slice scanners the narrowest collimation is 0.75 mm; in a 64-slice scanner it is 0.5–0.625 mm. The maximum collimation should not exceed 2.5 mm. Ideally, for CT colonography the narrowest collimation to which the scanner can be set should be selected. The exception to this is 4-slice scanners, as using the narrowest collimation may result in a scanning time that is too long for breath-hold. Here the imaging quality will be better (breathing artifacts reduced) if a compromise is made between an acceptable scanning time and a somewhat wider collimation of 2.5 mm. It may be preferable to use a scanner with at least 16 slices instead.

**Improving 3D image quality by reducing slice thickness. a** 5 mm, b 3 mm, c 1 mm. The greatest level of detail is with a 1-mm slice thickness (c). At 3 mm (b) there is already a noticeable reduction in quality, while 5-mm slices (a) can cause significant artifacts and should therefore not be used for 3D imaging of the colon.

Scanning parameters for multidetector CT colonography
Parameters	4-slice	6-slice	8-slice	10-slice	16-slice	40-slice	64-slice
Collimation, mm	1.25–2.5	1–1.25	1–1.25	0.75	0.75–1.5	0.6–0.625	0.5–0.625
Pitch	1.5	1.5	1–1.5	1.25	1.1–1.25	0.9–1.2	0.9–1.2
mAs
Prone	50	50	50	50	50	50	50
Supine	50–100	50–100	50–100	50–100	50–100	50–100	50–100
Supine, intravenous contrast	100–200	100–200	100–200	100–200	100–200	100–200	100–200
kVp a	120	120	120	120	120	120	120
Rotation time, s	0.5–0.8	0.5–0.8	0.8	0.5–0.75	0.5–0.8	0.5–0.75	0.5–0.8
Reconstructed slice thickness, mm	1.25–2.5	1.25	1–1.25	1	1–1.25	0.7–1	0.5–1
Reconstruction increment, mm	0.7–1.25	0.7	0.7	0.7	0.7	0.7	0.5–0.7
^a Increase in obese patients.

Scan direction. To minimize breathing artifacts, CT colonographic scans should be performed in the craniocaudal direction.

Slice thickness and data volume. For high-quality images, individual slices must be overlapped and reconstructed. For 16-slice and 64-slice scanners, for instance, 1-mm-thick slices are reconstructed with a reconstruction increment of 0.7 (= 30% overlap) (Table 2.2). This results in a large number of single images, typically as many as 1200, which can only be properly interpreted with the aid of a special off-line workstation. It is important that the internal data transmission system or PACS (picture archiving and communication system) is suitable for supporting the volume of data required for CT colonography. Some manufacturers also offer installation of the interpretation software on the CT scanner console using “shared memory,” eliminating the need for image transmission.

In addition to the thin slices needed for 3D reconstructions, it is also advisable to reconstruct thicker slices (3–5 mm). Due to the reduced image noise, these slices are better suited to the evaluation of extracolonic structures.

Examination protocol for CT colonography using a 64-slice MDCT scanner
	Screening CT colonography		Diagnostic CT colonography
General information
Preparation	Laxatives + diet a		Laxatives + diet a
Fecal tagging	Barium and/or iodinated contrast agent		(Barium and/or) b iodinated contrast agent c
Scanned area	Left dome of the diaphragm to the ischial tuberosity
Breath-hold phase	Inspiration		Inspiration
	Prone position	Supine position	Prone position	Supine position
Scan parameters
Scout images	PA	AP	PA	AP
Collimation, mm	0.5–0.625	0.5–0.625	0.5–0.625	0.5–0.625
Pitch	1.4	1.4	1.4	1.4
mAs	≤50	50–100	≤50	100–200
kVp	120	120	120	120
Rotation time, s	0.5	0.5	0.5	0.5
Injection of contrast	–	–	–	140/40/4/70 d
Reconstruction parameters
Colon
Reconstructed slice thickness, mm	1	1	1	1
Reconstruction increment, mm	0.7	0.7	0.7	0.7
Window setting	1200/–150	1200/–150	1200/–150	1200/–150
Extracolonic structures
Reconstructed slice thickness, mm	3	3	3	3
Reconstruction increment, mm	2	2	2	2
Window setting	400/10	400/10	400/10	400/10
^a Complete bowel preparation with a 24-hour liquid diet and laxatives (Phospho-soda/PEG/magnesium citrate); see “Patient Preparation.” ^b Fecal tagging with barium can impede follow-up colonoscopy. ^c If fecal tagging is done immediately following colonoscopy, an iodinated contrast medium must be used. An interval of at least 2 hours must be ensured between administering the oral contrast agent and performing CT colonography. ^d Contrast volume, mL/NaCl bolus, mL/flow rate, mL/delay, s. The amount of contrast agent to be administered is determined by patient weight. AP, anteroposterior; PA, posteroanterior.