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DOI: 10.1148/radiol.2283020928
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(Radiology 2003;228:753-759.)
© RSNA, 2003

Experimental Studies

CT Colonography: Protocol Optimization with Multi–Detector Row CT—Study in an Anthropomorphic Colon Phantom1

Johannes Wessling, MD, Roman Fischbach, MD, Norbert Meier, MD, Thomas Allkemper, MD, Jutta Klusmeier, MD, Karl Ludwig, MD and Walter Heindel, MD

1 From the Department of Clinical Radiology, University of Muenster, Albert-Schweitzer-Strasse 33, 48 149 Muenster, Germany. Received August 1, 2002; revision requested October 1; final revision received December 9; accepted January 2, 2003. Address correspondence to J.W. (e-mail: weslingj@uni-muenster.de).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
PURPOSE: To determine optimal detector collimation, section thickness, and tube current for multi–detector row computed tomography (CT) colonography.

MATERIALS AND METHODS: An anthropomorphic colon phantom with simulated polyps of varying size (2, 6, 8, 10, and 12 mm) was examined by using multi–detector row CT with varying combinations of detector collimation (4 x 1.0 mm and 4 x 2.5 mm), dose per section (10, 20, 40, 60, 80, 100, and 140 mAs), and section thickness/reconstruction interval (1.25/0.6, 2.0/1.0, 3.0/1.0, and 5.0/2.0 mm). Polyp depiction, longitudinal polyp distortion, and presence of rippling artifacts were assessed on reformatted three-dimensional endoluminal images by three reviewers.

RESULTS: Longitudinal distortion and rippling artifacts increased with increasing section thickness and use of broader detector collimation. Polyps 8 mm or larger were depicted with any combination of section thickness, detector collimation, and tube current. Depiction of polyps 6 mm or smaller depended on the detector collimation/reconstructed section thickness and was rated optimal for the 4 x 1.0-mm detector collimation with a section thickness of 1.25 mm. This was also observed for low-dose protocols. Polyps 6 mm or smaller that were not detected with 3-mm section thickness and 4 x 2.5-mm detector collimation were detected with 1.25-mm section thickness and 10 mAs.

CONCLUSION: A narrow detector collimation with thin-section imaging (4 x 1.0-mm detector collimation, 1.25-mm section thickness) is a prerequisite for low-dose (10-mAs) multi–detector row CT colonography.

© RSNA, 2003

Index terms: Colon, CT, 75.1211 • Computed tomography (CT), multi–detector row, 70.12119 • Computed tomography (CT), technology • Computed tomography (CT), three-dimensional • Phantoms


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Computed tomography (CT) and magnetic resonance imaging are increasingly being considered as alternative imaging modalities for colorectal screening (1,2). By using thin sections and dedicated software, both techniques allow for the generation of three-dimensional endoluminal views of the colon, simulating those obtained with conventional colonoscopy.

Compared with single–detector row CT colonography, multi–detector row CT provides high-volume coverage and thin-section imaging within a single breath hold (3). Multi–detector row CT has been shown to improve colonic distention, produce fewer respiratory artifacts (4), and increase detection rates for small polyps (5).

While various examination parameters have been suggested by different groups for single–detector row CT colonography (6–10), to our knowledge no generally accepted examination protocols have been established for multi–detector row CT colonography. Ideally, the maximum section thickness that allows for the detection of all clinically relevant polyps should be preferred. The minimal spatial resolution required for detection of colorectal lesions, however, needs to be determined.

One major concern in population-based screening is radiation exposure. The effect of radiation dose on polyp detection has received little attention to date. Hara et al (8) reported that the radiation dose may be substantially reduced because of the high contrast between the air-filled colon and the colonic wall. Obviously, the tube current should be as low as can reasonably be achieved.

To our knowledge, no study has been performed to assess optimal examination parameters for multi–detector row CT colonography, including the influence of radiation dose. The purpose of our study was to determine optimal detector collimation, section thickness, and tube current in multi–detector row CT colonography.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Colon Phantom
We constructed an anthropomorphic colon phantom by using a plastic material (Dubliplast; Dentaurum, Ispringen, Germany) that was formed to an elastic cylinder (inner diameter, 45 mm; length, 130 mm; wall thickness, 7 mm). Once cured, the liquid plastic material can assume a definite shape within a mold. The CT attenuation of the hardened plastic was approximately 45 HU and comparable to the attenuation of the normal bowel wall. This yielded a physiologic edge profile of the air–phantom wall interface. Five sessile spherical polyps of varying size (2, 6, 8, 10, and 12 mm) were distributed along the xy and the z axes (Fig 1a) and resulted in a total of nine polyps (two polyps of 2, 6, 10, and 12 mm, and one polyp of 8 mm). Because the cylinder and the sessile polyps were poured in a one-step procedure, no joints between the tube wall and the simulated polyps occurred. To prevent collapse of the flexible model by water pressure and to achieve comparable x-ray absorption by the bowel wall and the surrounding tissue, the colon phantom was inserted in a water-filled acrylic cylinder with a diameter of 150 mm and a wall thickness of 2 mm. The acrylic cylinder with the colon phantom was then submerged in a water-filled body phantom made of polymethyl methacrylate (Vink Kunststoffe, Emmerich, Germany) with a diameter of 45 cm and a wall thickness of 10 mm in scan volume. The phantom was placed in the center of the CT scanner tube with the longitudinal axis of the phantom parallel to the longitudinal axis of the gantry (Fig 1b).



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  		Figure 1a. (a) Endoluminal view shows five spherical phantom polyps along the xy and the z axis. Polyp size was 2, 4, 6, 8, and 12 mm. (b) Surface view shows colon phantom submerged in fluid-filled acrylic body phantom. (c) Transverse view of body and colon phantom. For objective assessment of image noise, a region of interest was placed at the 3-o’clock position of the water-phantom for each protocol.

 


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  		Figure 1b. (a) Endoluminal view shows five spherical phantom polyps along the xy and the z axis. Polyp size was 2, 4, 6, 8, and 12 mm. (b) Surface view shows colon phantom submerged in fluid-filled acrylic body phantom. (c) Transverse view of body and colon phantom. For objective assessment of image noise, a region of interest was placed at the 3-o’clock position of the water-phantom for each protocol.

 


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  		Figure 1c. (a) Endoluminal view shows five spherical phantom polyps along the xy and the z axis. Polyp size was 2, 4, 6, 8, and 12 mm. (b) Surface view shows colon phantom submerged in fluid-filled acrylic body phantom. (c) Transverse view of body and colon phantom. For objective assessment of image noise, a region of interest was placed at the 3-o’clock position of the water-phantom for each protocol.

 
Image Acquisition
The study was performed with a four-row CT scanner (Somatom Volume Zoom; Siemens, Forchheim, Germany). Images were acquired with different combinations of detector collimation (4 x 1.0 mm and 4 x 2.5 mm), dose per section (10, 20, 40, 60, 80, 100, and 140 mAs), tube voltage (120 and 140 kV), and section thickness/reconstruction interval (1.25/0.6, 2.0/1.0, 3.0/1.0, and 5.0/3.0 mm). Pitch values were kept constant at 1.5 with 4 x 1.0-mm detector collimation and at 1.0 with 4 x 2.5-mm detector collimation, since the pitch does not influence the section sensitivity profile in our system and since reasonable and acceptable acquisition times were to be ensured.

To compensate for image noise, we used a very smooth filter (B10) for protocols with a tube current equal to or less than 60 mAs and a medium-smooth filter (B30) for protocols with a tube current greater than 60 mAs. The acquisition matrix was 512 x 512 with a 40-cm display field of view, which resulted in a nominal pixel size of 0.78 x 0.78 mm.

Image Processing and Assessment
Image data were transferred to a three-dimensional workstation with multiplanar and volume-rendering capabilities (Vitrea 1.1; Vital Images, Plymouth, Minn). Endoluminal views of the colon were created for each of the protocols by one radiologist (J.W.) who was not involved in polyp assessment. A volume-rendering algorithm with constant rendering parameters (perspective lighting, nonlinear opacity assignment of the attenuation parameters) was used. The field of view for each protocol was 60°, and the camera was located opposite the polyps; hence, the polyp presentation at endoluminal view was identical for each protocol.

The endoluminal images were assessed independently with a four-quadrant monitor display by three board-certified radiologists (T.A., R.F., K.L., each with more than 5 years experience) who were blinded to the image acquisition parameters. Depiction in terms of polyp visualization and contour definition for each polyp in the xy and the z directions was judged by using a four-point grading scale (0 = not depicted, 1 = fair depiction, 2 = good depiction, 3 = excellent depiction). The reviewers graded the longitudinal distortion of each polyp and the degree of rippling artifacts on a four-point scale (0 = severe, 1 = moderate, 2 = mild, 3 = none). A sum score was calculated on the basis of the assessments of the reviewers, as previously outlined.

Determination of Radiation Dose
The effective dose was calculated by using software (WinDose; Scanditronix Wellhofer, Bartlett, Tenn) that is based on data derived from Monte Carlo calculations (11). It accommodates a multitude of scanner designs and geometries and patient geometries. The necessary scanner-specific settings, such as isocenter dose, were measured with a pencil ionization chamber (WDCT 10, Solidose 400; Wellhöfer, Schwarzenbruck, Germany), the number of pre- and postrotations, overbeaming, and beam geometry filters for our scanner were known from a separate study. The patient geometries were set to male and female from T11 to tuber ischiadicum. For each protocol, the effective dose was determined.

Image noise in the CT image was measured by placing a region of interest (2,500 voxels) within water (J.W.) and calculating the standard deviation from the mean in Hounsfield units (Fig 1c).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
DISCUSSION
 REFERENCES
 
Results of all reviewers were added and are given as mean values in the figures that follow. The results of changing collimation, section thickness, and radiation dose were considered separately in terms of their effects on subjective depiction.

Polyp Depiction, Section Thickness, and Detector Collimation
Figure 2a summarizes the polyp depiction at different section thicknesses and detector collimations. Depiction of polyps 8 mm or larger was achieved with a depiction score between 2 (good) and 3 (excellent), regardless of section thickness (1.25–5.0 mm) and detector collimation (4 x 1.0 mm or 4 x 2.5 mm).



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  		Figure 2a. (a) Size-dependent polyp depiction compared with section thickness and detector collimation. Protocol numbers are in parentheses at the top: 1-4 = 4 x 1.0-mm detector collimation, 11 = 4 x 2.5-mm detector collimation. Good to excellent depiction of polyps 8 mm or larger was achieved independently of section thickness or detector collimation. Depiction of polyps 6 mm or smaller depended mainly on the reconstructed section thickness. Depiction of small polyps was superior (protocol 3 and 11) when 4 x 1.0-mm detector collimation was used for the same reconstructed section thickness (3 mm). (b) Size-dependent polyp depiction compared with radiation dose. Protocol numbers in parentheses: 1, 5-10 = 4 x 1-mm detector collimation. Depiction of polyps 8mm or larger was not influenced by tube current reduction when 4 x 1.0-mm detector collimation and 1.25-mm section thickness were used for each protocol. Depiction of smaller polyps deteriorates with reduced tube current but remains possible, even for low-dose (10-mAs) protocols. (c) Size-dependent polyp depiction compared with radiation dose. Protocol numbers in parentheses: 11-15 = 4 x 2.5-mm detector collimation. Depiction of polyps 8 mm or larger was not influenced by tube current reduction when 2.5-mm detector collimation and 3-mm section thickness were used. The effects of tube current reduction were more evident when compared with protocols that used 4 x 1.0-mm detector collimation for depiction of smaller polyps. Polyps smaller than 8 mm were not depicted, even by using the lowest (10-mAs) setting possible. (d) Longitudinal distortion of polyps and rippling artifacts depending on section thickness. Longitudinal distortion and rippling artifacts increased with increasing section thickness and use of broader detector collimation. (e) Sum score of all protocols. Protocols that use 4 x 1.0-mm detector collimation were superior to those that use 4 x 2.5-mm collimation. Low-dose thin-section protocols (protocols 5-10) tended to be superior to high-dose protocols with 4 x 2.5-mm detector collimation (protocol 11) and were only slightly different from high-dose thin-section protocols (protocol 1).

 


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  		Figure 2b. (a) Size-dependent polyp depiction compared with section thickness and detector collimation. Protocol numbers are in parentheses at the top: 1-4 = 4 x 1.0-mm detector collimation, 11 = 4 x 2.5-mm detector collimation. Good to excellent depiction of polyps 8 mm or larger was achieved independently of section thickness or detector collimation. Depiction of polyps 6 mm or smaller depended mainly on the reconstructed section thickness. Depiction of small polyps was superior (protocol 3 and 11) when 4 x 1.0-mm detector collimation was used for the same reconstructed section thickness (3 mm). (b) Size-dependent polyp depiction compared with radiation dose. Protocol numbers in parentheses: 1, 5-10 = 4 x 1-mm detector collimation. Depiction of polyps 8mm or larger was not influenced by tube current reduction when 4 x 1.0-mm detector collimation and 1.25-mm section thickness were used for each protocol. Depiction of smaller polyps deteriorates with reduced tube current but remains possible, even for low-dose (10-mAs) protocols. (c) Size-dependent polyp depiction compared with radiation dose. Protocol numbers in parentheses: 11-15 = 4 x 2.5-mm detector collimation. Depiction of polyps 8 mm or larger was not influenced by tube current reduction when 2.5-mm detector collimation and 3-mm section thickness were used. The effects of tube current reduction were more evident when compared with protocols that used 4 x 1.0-mm detector collimation for depiction of smaller polyps. Polyps smaller than 8 mm were not depicted, even by using the lowest (10-mAs) setting possible. (d) Longitudinal distortion of polyps and rippling artifacts depending on section thickness. Longitudinal distortion and rippling artifacts increased with increasing section thickness and use of broader detector collimation. (e) Sum score of all protocols. Protocols that use 4 x 1.0-mm detector collimation were superior to those that use 4 x 2.5-mm collimation. Low-dose thin-section protocols (protocols 5-10) tended to be superior to high-dose protocols with 4 x 2.5-mm detector collimation (protocol 11) and were only slightly different from high-dose thin-section protocols (protocol 1).

 


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  		Figure 2c. (a) Size-dependent polyp depiction compared with section thickness and detector collimation. Protocol numbers are in parentheses at the top: 1-4 = 4 x 1.0-mm detector collimation, 11 = 4 x 2.5-mm detector collimation. Good to excellent depiction of polyps 8 mm or larger was achieved independently of section thickness or detector collimation. Depiction of polyps 6 mm or smaller depended mainly on the reconstructed section thickness. Depiction of small polyps was superior (protocol 3 and 11) when 4 x 1.0-mm detector collimation was used for the same reconstructed section thickness (3 mm). (b) Size-dependent polyp depiction compared with radiation dose. Protocol numbers in parentheses: 1, 5-10 = 4 x 1-mm detector collimation. Depiction of polyps 8mm or larger was not influenced by tube current reduction when 4 x 1.0-mm detector collimation and 1.25-mm section thickness were used for each protocol. Depiction of smaller polyps deteriorates with reduced tube current but remains possible, even for low-dose (10-mAs) protocols. (c) Size-dependent polyp depiction compared with radiation dose. Protocol numbers in parentheses: 11-15 = 4 x 2.5-mm detector collimation. Depiction of polyps 8 mm or larger was not influenced by tube current reduction when 2.5-mm detector collimation and 3-mm section thickness were used. The effects of tube current reduction were more evident when compared with protocols that used 4 x 1.0-mm detector collimation for depiction of smaller polyps. Polyps smaller than 8 mm were not depicted, even by using the lowest (10-mAs) setting possible. (d) Longitudinal distortion of polyps and rippling artifacts depending on section thickness. Longitudinal distortion and rippling artifacts increased with increasing section thickness and use of broader detector collimation. (e) Sum score of all protocols. Protocols that use 4 x 1.0-mm detector collimation were superior to those that use 4 x 2.5-mm collimation. Low-dose thin-section protocols (protocols 5-10) tended to be superior to high-dose protocols with 4 x 2.5-mm detector collimation (protocol 11) and were only slightly different from high-dose thin-section protocols (protocol 1).

 


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  		Figure 2d. (a) Size-dependent polyp depiction compared with section thickness and detector collimation. Protocol numbers are in parentheses at the top: 1-4 = 4 x 1.0-mm detector collimation, 11 = 4 x 2.5-mm detector collimation. Good to excellent depiction of polyps 8 mm or larger was achieved independently of section thickness or detector collimation. Depiction of polyps 6 mm or smaller depended mainly on the reconstructed section thickness. Depiction of small polyps was superior (protocol 3 and 11) when 4 x 1.0-mm detector collimation was used for the same reconstructed section thickness (3 mm). (b) Size-dependent polyp depiction compared with radiation dose. Protocol numbers in parentheses: 1, 5-10 = 4 x 1-mm detector collimation. Depiction of polyps 8mm or larger was not influenced by tube current reduction when 4 x 1.0-mm detector collimation and 1.25-mm section thickness were used for each protocol. Depiction of smaller polyps deteriorates with reduced tube current but remains possible, even for low-dose (10-mAs) protocols. (c) Size-dependent polyp depiction compared with radiation dose. Protocol numbers in parentheses: 11-15 = 4 x 2.5-mm detector collimation. Depiction of polyps 8 mm or larger was not influenced by tube current reduction when 2.5-mm detector collimation and 3-mm section thickness were used. The effects of tube current reduction were more evident when compared with protocols that used 4 x 1.0-mm detector collimation for depiction of smaller polyps. Polyps smaller than 8 mm were not depicted, even by using the lowest (10-mAs) setting possible. (d) Longitudinal distortion of polyps and rippling artifacts depending on section thickness. Longitudinal distortion and rippling artifacts increased with increasing section thickness and use of broader detector collimation. (e) Sum score of all protocols. Protocols that use 4 x 1.0-mm detector collimation were superior to those that use 4 x 2.5-mm collimation. Low-dose thin-section protocols (protocols 5-10) tended to be superior to high-dose protocols with 4 x 2.5-mm detector collimation (protocol 11) and were only slightly different from high-dose thin-section protocols (protocol 1).

 


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  		Figure 2e. (a) Size-dependent polyp depiction compared with section thickness and detector collimation. Protocol numbers are in parentheses at the top: 1-4 = 4 x 1.0-mm detector collimation, 11 = 4 x 2.5-mm detector collimation. Good to excellent depiction of polyps 8 mm or larger was achieved independently of section thickness or detector collimation. Depiction of polyps 6 mm or smaller depended mainly on the reconstructed section thickness. Depiction of small polyps was superior (protocol 3 and 11) when 4 x 1.0-mm detector collimation was used for the same reconstructed section thickness (3 mm). (b) Size-dependent polyp depiction compared with radiation dose. Protocol numbers in parentheses: 1, 5-10 = 4 x 1-mm detector collimation. Depiction of polyps 8mm or larger was not influenced by tube current reduction when 4 x 1.0-mm detector collimation and 1.25-mm section thickness were used for each protocol. Depiction of smaller polyps deteriorates with reduced tube current but remains possible, even for low-dose (10-mAs) protocols. (c) Size-dependent polyp depiction compared with radiation dose. Protocol numbers in parentheses: 11-15 = 4 x 2.5-mm detector collimation. Depiction of polyps 8 mm or larger was not influenced by tube current reduction when 2.5-mm detector collimation and 3-mm section thickness were used. The effects of tube current reduction were more evident when compared with protocols that used 4 x 1.0-mm detector collimation for depiction of smaller polyps. Polyps smaller than 8 mm were not depicted, even by using the lowest (10-mAs) setting possible. (d) Longitudinal distortion of polyps and rippling artifacts depending on section thickness. Longitudinal distortion and rippling artifacts increased with increasing section thickness and use of broader detector collimation. (e) Sum score of all protocols. Protocols that use 4 x 1.0-mm detector collimation were superior to those that use 4 x 2.5-mm collimation. Low-dose thin-section protocols (protocols 5-10) tended to be superior to high-dose protocols with 4 x 2.5-mm detector collimation (protocol 11) and were only slightly different from high-dose thin-section protocols (protocol 1).

 
The depiction of polyps 6 mm or smaller depended mainly on the reconstructed section thickness. Endoluminal views (Fig 3a) with 1.25-mm section thickness resulted in much better depiction of 2- and 6-mm polyps (score, 2.0 and 2.6, respectively) compared with endoluminal views based on 2- or 3-mm-thick images (score, 1.0 and 1.3, respectively).



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  		Figure 3a. (a) Endoluminal views of spherical phantom polyps with different section thicknesses: A, 1.25 mm; B, 2 mm; C, 3 mm; and D, 5 mm (4 x 1.0-mm detector collimation). Longitudinal distortion and blurring increase with increasing section thickness. (b) Endoluminal views of polyps at different radiation doses: A, 120 kV and 140 mAs; B, 120 kV and 80 mAs; C, 140 kV and 10 mAs, 4 x 1.0-mm detector collimation; D, 140 kV and 10 mAs, 4 x 2.5-mm detector collimation. Despite substantial dose reduction and increasing image noise, delineation of especially small polyps remains possible for protocols that use 4 x 1.0-mm detector collimation.

 


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  		Figure 3b. (a) Endoluminal views of spherical phantom polyps with different section thicknesses: A, 1.25 mm; B, 2 mm; C, 3 mm; and D, 5 mm (4 x 1.0-mm detector collimation). Longitudinal distortion and blurring increase with increasing section thickness. (b) Endoluminal views of polyps at different radiation doses: A, 120 kV and 140 mAs; B, 120 kV and 80 mAs; C, 140 kV and 10 mAs, 4 x 1.0-mm detector collimation; D, 140 kV and 10 mAs, 4 x 2.5-mm detector collimation. Despite substantial dose reduction and increasing image noise, delineation of especially small polyps remains possible for protocols that use 4 x 1.0-mm detector collimation.

 
For images with 3-mm section thickness, the choice of detector collimation also seems important. Depiction of small polyps was superior when the narrow detector collimation was used for the same reconstructed section thickness. For example, endoluminal views created with 3-mm section thickness failed to depict 2-mm lesions (score, 0) with 4 x 2.5-mm detector collimation, while the depiction score with 4 x 1.0-mm detector collimation was 1.0.

Polyp Depiction and Radiation Dose
The Table summarizes the different examination parameters with respect to relative dose. The effective dose in our study ranged from 0.7 to 11.6 mSv calculated for female and male patients. Depiction of polyps 8 mm or larger was not influenced by tube current reduction (Fig 2b, 2c); however, depiction of smaller polyps deteriorated with reduced tube current. Image degradation was partly compensated for by using a softer filter in images obtained with 60 mAs or less. The effects of tube current reduction were more evident on images obtained with 3-mm section thickness and 4 x 2.5-mm detector collimation than on images obtained with 1.25-mm section thickness and 4 x 1.0-mm detector collimation (Fig 2b, 2c). By using the lowest setting possible (10 mAs), polyps smaller than 8 mm were not depicted by protocols with 3-mm section thickness. On the other hand, with a detector collimation of 4 x 1.0 mm and a section thickness of 1.25 mm, the depiction score was 2 for 6-mm polyps and 1 for 2-mm polyps (Fig 3b).


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  		CT Examination Protocols, Effective Radiation Dose, and Image Noise

 
Image Noise
The protocols with reduced tube current and thin-section protocols (1.25–2.0 mm) resulted in increased image noise. The image noise of the 4 x 1.0-mm collimation was 24.8 HU at 80 mAs, compared with 17.8 HU at 140 mAs (Table). The image noise decreased to 19.3 HU by using a smooth filter at 60 mAs and, thus, was below the image noise with a normal filter at 80 mAs (24.8 HU). Although an increase in image noise was measured for low-dose thin-section protocols, polyp delineation was not substantially impaired at subjective evaluation.

Rippling Artifacts and Longitudinal Distortion
Rippling artifacts were rarely notable in our study. As seen in Figure 2d, a narrow detector collimation and thin-section imaging prevent image degradation caused by rippling artifacts. Longitudinal distortion resulted in an artificial enlargement of polyps. Figure 2d shows that distortion depended on section thickness and detector collimation. Consequently, distortion is more pronounced when using thicker sections and a broader detector collimation. Endoluminal views from 5-mm sections showed pronounced distortion in the direction of the xy and the z axis (Fig 3a).

Sum Scores
The calculated sum scores of the different examination protocols reflect the tendencies just outlined (Fig 2e). For example, the sum score obtained with 4 x 1.0-mm detector collimation is superior to the sum score obtained with 4 x 2.5-mm collimation. More interestingly, the low-dose thin-section protocols (section thickness, 1.25 mm) tended to be superior to high-dose protocols with a 4 x 2.5-mm detector collimation and are only slightly different from high-dose thin-section protocols. This trend was also found with the low-dose protocols with a 4 x 2.5-mm detector collimation.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
REFERENCES
 
CT colonography is currently being assessed as an attractive alternative to existing screening tests for colorectal cancer. Reported sensitivities for the detection of polyps 10 mm or larger range from 51% to 100% (12–16). No clear consensus exists, however, on numerous issues regarding multi–detector row CT colonography acquisition parameters.

It is widely accepted that the target lesion size should be 10 mm or larger for screening purposes. Although the risk of neoplastic transformation of preexisting adenomas is thought to be very small (<=0.1%) in polyps less than 5 mm in diameter (17,18), we believe that virtual colonography should be able to depict these small lesions with sensitivity that is acceptable and comparable to that of conventional colonoscopy to gain acceptance for the targeted screening population.

Single–detector row CT is characterized by a tradeoff between the amount of coverage to be obtained with a single breath hold, spatial resolution, and patient dose. Multi–detector row CT facilitates single breath-hold imaging at a high spatial resolution with nearly isotropic voxels. Indeed, multi–detector row CT colonography has been shown to increase sensitivity and specificity of detection of small polyps in particular (5).

In regards to single–detector row CT, several reports suggested use of a collimation of 5 mm and a pitch of 1–2 for detection of polyps larger than 4 mm (7–10). Springer et al (19) found the image quality to be less affected by pitch values than by beam collimation. Power et al (20) proposed a 3-mm section thickness with a pitch of 2, thus providing optimal polyp conspicuity with a relatively low radiation dose in their phantom study. This proposal matches our findings that detection of small (2- and 6-mm) lesions requires thin-section imaging with section thickness between 1.25 and 3.0 mm. Interestingly, 3-mm sections from 4 x 2.5-mm detector collimation tended to be insufficient for this purpose in our study. This might be due to the better section sensitivity profile being accompanied by a narrow detector collimation. To ensure adequate scanning times, a pitch of 1.25–1.50 is required for 4 x 1.0-mm detector collimation (scanning time, 30–40 seconds) and a pitch value of 1.0–1.5 is required for 4 x 2.5-mm detector collimation (scanning time, 13–20 seconds). Since the pitch does not influence the section sensitivity profile in our system, we kept the pitch constant within a chosen detector collimation.

The need for ionizing radiation is a disadvantage of CT colonography and might hamper its application to colorectal cancer screening; therefore, optimization of imaging protocols with regard to polyp detection rate and radiation exposure is necessary. As is known from lung cancer screening, low-dose protocols are feasible in high-contrast situations, without substantial effect on nodule detection. A similar high-contrast situation is found between the colonic wall and the air-filled lumen. Thus far, little is known about optimization of imaging parameters for this purpose. The effective dose is influenced by the tube voltage and current and scanner characteristics (eg, detector collimation). Generally, the choice of examination parameters is a compromise between maximum spatial resolution, signal-to-noise ratio, and coverage of the entire colon with a minimum dose. Decreasing the tube current or using a broader detector collimation can, therefore, reduce the effective radiation dose. Hara et al (8) reported no change in the diagnostic efficacy between CT settings of 140 mA and 70 mA for single–detector row CT, which resulted in an effective dose equivalent for the acquisition of a single supine study of 1.87 mGy for men and 2.85 mGy for women. Van Gelder et al (21) found comparable sensitivity and specificity for polyp detection at 30 mAs (3.6 mSv) and 100 mAs (5.9 mSv), although the image quality decreased. Our results show that low-dose thin-section imaging is feasible. The effective minimum dose equivalent for the acquisition of a single supine study was 0.9 mSv for men and 1.2 mSv for women. This dose would double if prone and supine examinations were both performed, as is suggested (22).

The increase in image noise can cause image degradation, mainly in thin-section protocols. To compensate for the increase in image noise, we used a smoothing filter with a tube current of 60 mAs or less. Use of a broader detector collimation of 4 x 2.5 mm results in a dose reduction of 13%–15%. When using thin-section low-dose protocols, however, this would mean a difference of 0.2 mSv for a 10-mAs protocol only. van Gelder et al (22) used a 4 x 2.5-mm detector collimation and a section thickness of 3 mm. They described a substantial decrease of image quality when they reduced the tube current to 30 mAs. We found the effects of tube current reduction more evident for protocols with a 4 x 2.5-mm detector collimation than for those with a 4 x 1.0-mm detector collimation. Polyps of 6 mm or smaller were not depicted by protocols with a 4 x 2.5-mm detector collimation at the lowest (10 mAs) setting. Luboldt et al (24) showed that collimation, and thus section thickness, has the strongest influence on spatial resolution. This has been confirmed in our study. We found the reconstructed section thickness and, to a lesser extent, the detector collimation to be more influential than tube current on image quality. Indeed, low-dose thin-section protocols (4 x 1.0 mm-detector collimation) tended to be superior to normal-dose protocols when a section thickness of 3 mm was used. The thin-section protocols realized with a narrow detector collimation improved the detection of small lesions as a result of better surface feature delineation and junction depiction between the lesion and the colon wall. This might be helpful in lesion characterization (eg, stool vs polyp). Furthermore, with regard to computer-aided diagnosis, it may be reasonable to further improve spatial resolution to reduce partial volume effects. In our opinion, these advantages outweigh the relative increase of the effective dose by using the 4 x 1.0-mm detector collimation.

Although the CT attenuation of the phantom polyps was comparable to the attenuation of the normal bowel wall, the model is restricted by several factors. There was neither bowel or patient movement nor fecal or fluid residue, any of which normally affect polyp depiction in vivo. Furthermore, placement of the colon model in a homogeneous fluid-filled body phantom will exhibit a reduced scattering compared with in vivo situations (eg, the pelvic region). Our study results do not allow us to draw any conclusion on the feasibility of low-dose protocols in large or obese patients, since our phantom corresponded to a 75-kg man. Image assessment included only volume-rendering images. The increase of image noise caused by substantial dose reduction and thin-section imaging might be more influential for two-dimensional reformatted images; however, thin-section imaging with a narrow detector collimation is regarded as necessary (6), as rippling artifacts in three-dimensional displays can develop. In addition, images obtained with 4 x 1.0-mm detector collimation can be fused to thick-section images to compensate for image noise by using two-dimensional reformatted images. We acknowledge discrepancies between our in vitro study and the in vivo situation as a drawback; however, our data may help limit the range of scanning parameters to be tested in vivo.

Of course, it must be stressed that display of extracolonic findings of medical importance is impaired by dose reduction. More extracolonic than colonic abnormalities are expected when persons over 50 years of age are examined with CT colonography (24). While controversy still exists over the need for displaying extracolonic findings, we suggest that it should not be included in a screening regimen so that CT colonography will remain a quick and cost-effective procedure. Low-dose thin-section CT colonography may, therefore, be recommended for screening purposes.

For patients in whom colorectal carcinoma is known or highly suspected to exist, the search for extracolonic abnormalities is justified. Since it is necessary to examine patients in both the prone and supine position to redistribute stool, fluid, and air, in our opinion it might be reasonable to examine at-risk patients with a normal-dose thin-section contrast material–enhanced protocol first, followed by a low-dose thin-section protocol with the patient in the supine position. This procedure might help reduce the radiation dose substantially, even in symptomatic patients.

In summary, a narrow detector collimation with thin-section imaging (detector collimation, 4 x 1.0 mm; section thickness, 1.25 mm; pitch, 1.5) is a prerequisite for low-dose (10-mAs) multi–detector row CT colonography. Depiction of small polyps with this protocol is superior to depiction with normal-dose protocols, which use a wider detector collimation with a section thickness of 3 mm.

Practical application: Low-dose multi–detector row CT colonography with thin-section imaging might be applicable for screening purposes in asymptomatic patients and as an additional examination after an examination of symptomatic patients in the prone position; however, further in vivo investigation of low-dose protocols for CT colonography is warranted.


   FOOTNOTES
 
Author contributions: Guarantors of integrity of entire study, J.W., R.F.; study concepts and design, J.W., R.F., N.M., W.H.; literature research, J.W., J.K.; experimental studies, J.W., J.K., N.M.; data acquisition, J.W., J.K., N.M.; data analysis/interpretation, J.W., J.K., R.F., K.L., T.A.; manuscript preparation, J.W., R.F., N.M., K.L., W.H., T.A.; manuscript definition of intellectual content, all authors; manuscript editing, J.W., R.F.; manuscript revision/review, J.W., R.F., N.M., K.L., W.H., T.A.; manuscript final version approval, all authors


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 

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