Dose Reduction and Image Quality in MDCT Colonography Using Tube Current Modulation
Abstract
OBJECTIVE. The purpose of our study was to evaluate the dose reduction potential of combined online (x- and y-axes) and topogram-based (l) X-ray tube current modulation in CT colonography in a screening population.
MATERIALS AND METHODS. Eighty asymptomatic individuals underwent CT colonography screening for colon polyps. A 16-MDCT scanner (Somatom Sensation 16) was used. Forty patients were examined at 120 kVp and 120 effective mAs (supine) and 40 effective mAs (prone) using online x- and y-axis tube current modulation. Another 40 patients were scanned using combined x-, y-, and z-axis tube current modulation. Individual patient radiation exposure was determined using the dose-length product. Image noise was determined by Hounsfield unit measurements in the colonic lumen at four anatomic levels. Image quality was rated on a 5-point confidence scale by two independent reviewers. The unpaired Student's t test (for radiation dose, image noise) and Wilcoxon's test (for image quality) were used to test for statistically significant differences between these values.
RESULTS. Radiation dose was significantly lower in the patient group scanned with x-, y-, and z-axis tube current modulation than in the group scanned with x- and y-axis tube current modulation (supine: 4.24 vs 6.50 mSv, p < 0.0001; prone: 1.61 vs 2.38 mSv, p < 0.0001). Radiation dose was reduced by 35% (supine) and 33% (prone). No statistically significant difference was seen in overall image noise (supine: 15.9 vs 16.3 H, p = 0.13; prone: 23.5 vs 24.8 H, p = 0.44) or image quality (supine: 4.6 vs 4.5, p = 0.62; prone: 3.5 vs 3.6, p = 0.54).
CONCLUSION. Combined x-, y-, and z-axis tube current modulation leads to a significant reduction of radiation exposure in CT colonography without loss of image quality.
Introduction
CT colonography, also referred to as virtual colonoscopy, shows a promising potential for colorectal cancer screening because of its high sensitivity and specificity in the detection of colorectal polyps [1, 2]. Its minimally invasive character and short examination time have led to high acceptance among patients. Little doubt exists that effective screening for polyps leads to a significant reduction in colorectal cancer incidence when subsequent therapeutic colonoscopy removes the identified precursor lesions [3, 4]. Because CT colonography is mainly a screening examination performed in asymptomatic subjects, radiation exposure is a critical issue.
Tube current is an important determinant of radiation dose and image quality in X-ray-based examinations. Recent advances in CT technology, including implementation of automatic tube current modulation techniques, allow reduction of radiation exposure [5-8]. Tube current modulation enables automatic adjustment of the tube current in the x-y plane (online angular modulation) or along the z-axis (z-axis modulation) according to the size and attenuation characteristics of the body part being scanned, and achieves constant CT image quality with less radiation exposure [9]. In this study, a combined x-, y-, and z-axis dose modulation technique was compared with online x- and y-axis modulation. We did not compare this technique with fixed tube current scanning because the dose reduction capabilities of online tube current modulation (i.e., x- and y-axis tube current modulation) as compared with fixed tube current scanning have previously been shown [10, 11].
In CT colonography, image noise markedly deteriorates image quality on endoluminal views at low-dose settings, moreso than on axial source images [12]. At fixed tube current (mAs) settings, image noise tends to be higher in the pelvis than in other parts of the abdomen because of the presence of bone structures and the larger lateral diameter [12]. The pelvis, however, is an important anatomic region in CT colonography because it contains parts of the cecum, sigmoid colon, and rectum. Constant noise levels and high image quality are desirable goals that can possibly be achieved using online attenuation-based tube current modulation software.
We performed this study because in CT colonography two important criteria must be met: On the one hand, the radiologist should try to lower the total amount of ionizing radiation applied to the individual; on the other hand, optimum image quality is mandatory for high sensitivity in the detection of polyps and for the evaluation of extracolonic findings. Results of two previous multicenter studies showed low sensitivities in polyp detection at a nominal slice thickness of 2.5 mm [2, 13]. However, a study by Pickhardt et al. [1] underlines the high sensitivity of the technique in polyp detection [1]. Because optimum image quality depends on slice collimation, tube voltage, and tube current, the narrowest available slice collimation was chosen and kept constant while the effect of tube current on image quality was evaluated in this study. Our hypothesis was that the counteracting criteria can be met by using newly developed x-, y-, and z-axis tube current modulation software.
Materials and Methods
Patient Population and Examination Protocol
Between August 2003 and June 2004, 80 individuals (45 men, 35 women; mean age, 59 years; age range, 55-76 years) underwent MDCT colonography screening for colorectal cancer at our institution. All patients were adults older than 55 years. Exclusion criteria were recent or current hematochezia, known diverticulitis, and known colorectal cancer. Patients with previous endoscopic polypectomy were also excluded, as were patients with familial adenomatous polyposis and hereditary nonpolyposis colorectal cancer. Informed consent of the patient was obtained before the CT examination. The study was approved by the local institutional review board.
Forty patients were examined on a 16-MDCT scanner (Somatom Sensation 16, Siemens Medical Solutions) with online x- and y-axis dose modulation (CARE Dose, Siemens Medical Solutions), hereafter referred to as patient group 1. After implementation of new x-, y-, and z-axis dose modulation software (CARE Dose 4D, Siemens Medical Solutions), another 40 patients were examined on the same scanner (patient group 2). All other technical parameters of the scanner were kept unchanged. In both patient groups, patient age and weight were recorded for calculation of body mass indexes using the formulaPatient weight and height were measured by a study nurse in the patient preparation area using an electronic scale.
\[\mathrm{body\ mass\ index}=\mathrm{weight}[\mathrm{kg}]{/}(\mathrm{height}[\mathrm{m}]^{2}).\]
Each patient was informed in detail about the virtual colonoscopy procedure by one of the department's abdominal radiologists immediately before the examination. This radiologist was not a reviewer in the study. For bowel preparation, patients started ingesting 4 L of a solution of polyethylene glycol and electrolytes (KleanPrep, Norgine Germany) the day before the examination and were asked not to eat at least 12 hours before the examination. No fecal or fluid tagging was used.
Patients were positioned on the scanner table in the left decubitus position, and bowel distention was achieved after placement of a rectal tube by manual air insufflation performed by one of the staff radiologists. Air insufflation was performed to patient tolerance, with a target volume of 2,000 mL. To control the exact amount of air used, insufflation was performed using disposable 100-mL syringes. This is the routine procedure used at our institution and takes about 3-5 minutes. After completion of air insufflation, the patients were asked to turn over to the supine position, and a localizer radiograph was obtained with the rectal catheter in place. If bowel distention was insufficient, additional air was insufflated to maximum patient tolerance. We did not observe any complications of air insufflation. No IV scopolamine or glucagon was used. If distention was sufficient, the rectal catheter was removed.
The first set of images was obtained using the breath-hold technique with the patient in the supine position. Subsequently, patients were repositioned in the prone position and the second data set was obtained, including a second localizer radiograph. The total amount of air insufflated and the total number of images obtained per position were recorded. After the examination, the data set was sent to a 3D workstation (Leonardo, version VA 10, Siemens Medical Solutions). The scan was then interpreted by one of the department's abdominal radiologists using the virtual colonoscopy software installed on the workstation.
CT and Dose Modulation Technique
Forty patients (21 women, 19 men; patient group 1) were examined with an angular online x- and y-axis dose modulation software (CARE Dose). Another 40 patients (also 21 women, 19 men; patient group 2) were examined after the implementation of a newly developed software (CARE Dose 4D) that, in addition to the x- and y-axis dose modulation, enables topogram-based z-axis tube current modulation. The examination protocols were otherwise identical, with an X-ray beam collimation of 16 × 0.75 mm at a pitch factor of 1 (table feed, 12 mm per rotation) and a gantry rotation time of 0.5 seconds. One-millimeter-thick slices were reconstructed with an overlap of 0.5 mm. These slices were used as source images for the reconstruction of the 3D endoluminal fly-through viewing. Total scanning time for the entire abdomen was 14-16 seconds per acquisition, depending on patient height.
Tube voltage was set to 120 kVp, and with x- and y-axis tube current modulation the tube current-time product was 120 effective mAs (supine) and 40 effective mAs (prone). These are the routine tube current settings used in CT colonography at our institution; acquisition of images in the prone position is performed in a low-dose technique to keep the radiation dose as low as possible. With these settings, the assessment of extracolonic findings is done exclusively on supine images. With x-, y-, and z-axis tube current modulation, the same values were set as “reference mAs.” The x-, y-, and z-axis tube current modulation technique effects a change in tube current in different anatomic regions. It adapts the tube current to the patient's individual anatomy and modulates the tube current in the section and in the patient's long axis to obtain the desired image quality for all images at the lowest dose levels using the topogram scan for attenuation measurement. The reference mAs value can be adjusted manually. To achieve an image noise level comparable to the noise level of the scans without z-axis dose modulation, image quality reference mAs were set to the same values, 120 mAs for supine and 40 mAs for prone scanning. By definition, the image quality reference mAs correspond to the effective mAs value that would be applied without x-, y-, and z-axis tube current modulation. The CARE Dose 4D software also allows subsettings that lower the radiation dose in slim patients and elevate it in obese patients. These subsettings were not used in this study to prevent an elevation of radiation exposure in obese patients.
Radiation Dose Evaluation
For calculation of effective radiation exposure, the applied dose-length product values were used as dose estimates for individual patient examinations. With z-axis tube current modulation, the CT dose index (CTDIvol) is calculated for every slice position. Every axial CT image has a different effective mAs value and therefore a different CTDIvol, which is calculated during image reconstruction by the scanner. The resulting dose-length product is calculated automatically by the scanner as the product of the CTDIvol on the basis of the average effective mAs for the entire scan and the complete length of the exposed volume.
Estimates of effective dose (E) may be derived from values of dose-length product for an examination using appropriately normalized coefficients [14]. The effective dose E is related to the dose-length product (DLP) as follows:where DLP [mGy · cm] is the dose-length product and EDLP is the region-specific normalized effective dose conversion factor (mSv · mGy-1 · cm-1) [14].
\[E=E_{DLP}{\times}DLP(\mathrm{mSv}),\]
For prone and supine positions, the overall radiation exposure was calculated for both patient groups. The overall dose reduction in millisieverts and the percentage of dose reduction were also calculated.
Image Noise Measurements
For measurement of image noise, circular regions of interest (ROIs) with a size of 1.5-4.0 cm2, depending on the diameter of the distended colon, were placed in the colonic lumen at four anatomic levels: level I, at the portal vein (Fig. 1A); level II, at the renal hilum (Fig. 1B); level III, cephalad to the iliac crest (Fig. 1C); and level IV, in the pelvis cephalad to the acetabulum (Fig. 1D).
In the next step, SDs of attenuation in these ROIs were recorded. The SD in Hounsfield units of the attenuation in a particular region of interest can be accepted as noise measurement [9]. Mean image noise was calculated for each level and each patient position. These mean noise levels were tested for statistically significant differences using the unpaired Student's t test and MedCalc statistical software, version 7.3.0.1 (MedCalc Software). Overall, eight measurements were collected in each patient.
Image Quality Assessment
Image quality was assessed by two independent reviewers who had 1.5 and 5 years of experience in interpreting abdominal CT scans and who had interpreted more than 100 CT colonographic scans each. A 5-point confidence scale was used with 1 = nondiagnostic, 2 = poor, 3 = fair, 4 = good, and 5 = excellent image quality. Criteria for good image quality on endoluminal views included overall delineation and quality of visualization of the colonic wall. A smooth surface of the colonic wall and the sharp delineation of colonic folds were rated as excellent image quality. A characteristic cobblestonelike appearance of the colonic wall caused by the presence of image noise was considered inferior image quality.
The presence of beam hardening artifacts was not judged in image quality assessments because they were seen in only three patients, two from patient group 1 and one from patient group 2. The presence of these artifacts markedly deteriorated image quality at the respective anatomic sites.
This 5-point scale was applied to endoluminal views at four anatomic sites corresponding to the anatomic levels I-IV used for noise measurements. Furthermore, image quality of axial 1-mm-thick slices was also rated at the same levels. For image quality assessment, examinations from both patient groups were displayed in random order without image annotation. Reviewers were not informed whether the scan had been obtained with or without z-axis dose modulation.
Statistical significance was tested using the Wilcoxon's signed rank test and MedCalc software. For all statistical tests, a p value of less than 0.05 was considered statistically significant.
Results
The mean age for patient group 1 was 58 years and for group 2 was 59 years (no statistically significant difference, p = 0.35). The mean volume of air used for bowel distention was 1,990 mL in group 1 and 1,970 mL in group 2. The mean numbers of images were 578 (supine) and 564 (prone) in group 1, and 559 and 557 in group 2, respectively (no statistically significant difference between all values, p = 0.40-0.71). Mean patient body mass indexes for the respective groups were 25.6 and 24.8 (p = 0.56) (median, 24.6 and 24.1). Body mass indexes ranged from 19.4 to 34.0 in patient group 1 and from 19.9 to 35.0 in group 2. Because of the short scanning time of 14-16 seconds per acquisition, we did not observe any breathing artifacts. Furthermore, no motion artifacts caused by colonic peristalsis were seen.
Radiation Dose Evaluation
For the supine position, the mean effective radiation doses were 6.50 ± 0.91 (SD) mSv (range, 5.07-8.04 mSv) without and 4.24 ± 0.98 mSv (range, 2.55-6.83 mSv) with z-axis tube current modulation (p < 0.0001, unpaired t test). For the prone position, mean effective patient doses were 2.38 ± 0.65 mSv (1.71-4.24 mSv) without and 1.61 ± 0.34 mSv (1.12-2.34 mSv) with z-axis tube current modulation (p < 0.0001, unpaired t test). Overall radiation exposures for both virtual colonoscopy scans and two localizer radiographs were 8.81 ± 1.13 mSv with x- and y-axis tube current modulation and 6.36 ± 1.03 mSv with x, y, and z-axis tube current modulation. Overall, we observed a 34.86% dose reduction in the supine position at 120 effective mAs and a 32.70% dose reduction in the prone position at 40 effective mAs.
Image Noise Measurements
Mean image noise at anatomic levels I-IV is shown in Table 1 (supine position) and Table 2 (prone position). Overall, no statistically significant difference was seen in image noise between the two patient groups for supine and prone positions.
| Image Noise (H) | |||||||
|---|---|---|---|---|---|---|---|
| Patient Group | Modulation | Tube Current (mAs) | Mean | Level I | Level II | Level III | Level IV |
| 1 | x- and y-axis | 120 | 15.89 ± 3.88 | 15.17 ± 4.18 | 14.38 ± 3.38 | 14.08 ± 3.80 | 19.93 ± 4.06 |
| 2 | x-, y-, and z-axis | 120 referencea | 16.31 ± 2.61 | 16.62 ± 2.93 | 15.85 ± 2.57 | 15.64 ± 1.99 | 18.57 ± 3.38 |
| p | 0.13 | 0.14 | 0.08 | 0.06 | 0.24 | ||
Note–Values are mean ± SD
a
Reference mAs correspond to effective mAs value that would be applied without x-, y-, and z-axis tube current modulation
| Image Noise (H) | |||||||
|---|---|---|---|---|---|---|---|
| Patient Group | Modulation | Tube Current (mAs) | Mean | Level I | Level II | Level III | Level IV |
| 1 | x- and y-axis | 40 | 23.51 ± 5.42 | 22.97 ± 6.11 | 21.82 ± 5.56 | 20.38 ± 4.16 | 28.86 ± 6.42 |
| 2 | x-, y-, and z-axis | 40 referencea | 24.77 ± 3.98 | 24.29 ± 3.50 | 23.17 ± 2.96 | 22.49 ± 3.91 | 29.74 ± 5.29 |
| p | 0.44 | 0.34 | 0.27 | 0.06 | 0.59 | ||
Note–Values are mean ± SD
a
Reference mAs correspond to effective mAs value that would be applied without x-, y-, and z-axis tube current modulation
For supine patient position at tube current-time products of 120 mAs and 120 reference mAs, respectively, mean image noise was 15.9 H without and 16.3 H with z-axis tube current modulation (no statistically significant difference, p = 0.13). Regarding image noise at the single anatomic levels, no statistically significant difference was seen (Table 1; compare Figs. 2A and 2B). In three single patients much higher noise levels occurred without z-axis tube current modulation in body regions with high attenuation (anatomic levels I and IV; see Figs. 2A, 2B, 2C, and 2D).
For the prone patient position at tube current-time products of 40 mAs and 40 reference mAs, respectively, mean image noise over all four anatomic levels was 23.5 H without and 24.8 H with z-axis tube current modulation (no statistically significant difference, p = 0.44). As expected, mean image noise was significantly higher at 40 mAs and 40 reference mAs than at 120 mAs and 120 reference mAs (p < 0.0001). At all four anatomic levels, again no statistically significant difference was seen (Table 1) between the two patient groups. For prone positions, overall noise levels are shown in Figures 2C and 2D.
Image Quality Assessment
Results from image quality assessment showed no statistically significant difference between patient groups 1 and 2. Overall image quality scores on endoluminal views for the supine patient position were 4.6 for x, y, and z-axis tube current modulation and 4.5 for x- and y-axis tube current modulation (p =0.62); on axial multiplanar reconstruction views, scores were 4.5 and 4.4 (p = 0.54). For the prone patient position, mean image quality scores were 3.7 and 3.6, respectively (p = 0.57). Although image quality on endoluminal visualization tended to be rated higher at level IV on scans obtained with x, y, and z-axis tube current modulation (4.6 vs 4.5 for supine and 3.7 vs 3.6 for prone), this difference was not statistically significant (p = 0.47).
Between the prone and supine positions, endoluminal image quality differed significantly as stated. In patient group 1, the scores were 4.5 at 120 mAs and 3.6 at 40 mAs (p < 0.001). In patient group 2, image quality scores were 4.6 at 120 mAs and 3.7 at 40 mAs (p < 0.001).
Discussion
The increasing availability of MDCT scanners and the growing demand for CT examinations has led to an increase of radiation exposure to the public [15]. In MDCT colonography, patient radiation exposure is an important issue because the most frequent clinical indication for CT colonography is colorectal cancer screening [4]. Especially in healthy subjects, radiation exposure must be kept to a minimum.
In CT colonography, recent studies have shown that a substantial reduction of radiation dose is feasible by lowering the tube current [16, 17]. The study by Iannaccone et al. [17] used a 4-MDCT scanner at a collimation of 4 × 2.5 mm at 140 kV and 10 mAs. At these settings, the sensitivity for polyps smaller than 5 mm was 51%. In our study, examinations were performed on a 16-MDCT scanner with the narrowest collimation to guarantee the highest resolution. In our clinical experience, effective tube current settings of less than 40 mAs cause substantial noise at 0.75-mm collimation, as used in this study, which deteriorates the quality of endoluminal 3D reconstructions and may therefore reduce the sensitivity in the detection of colorectal polyps.
In a study by Taylor et al. [18], the effects of pitch, slice collimation, and tube current on the detection of polyps in a colectomy specimen of a patient with familial adenomatous polyposis were investigated. The results showed that the identification of polyps smaller than 5 mm requires narrow collimation and a slow table feed. In addition, even the detection of polyps of 6-9 mm increased with a decrease in slice collimation from 2.5 to 1.25 mm, which are the two settings investigated in this study using a 4-MDCT scanner.
To reach sufficient image quality for the detection of small lesions, the reported dose in this study reached peak levels of 20 mSv at 1.25-mm collimation, 3.75-mm table feed per rotation, and 150 mA. As reported in our study, with sophisticated dose modulation techniques, optimum image quality has been reached at much lower radiation doses.
Constant image noise at different anatomic levels is difficult to achieve at a constant tube current. This has led to the development of various means of dose modulation. Most up-to-date scanners have a routine x- and y-axis online tube current modulation, as used in patient group 1, which adapts radiation dose to anteroposterior and lateral diameters of the trunk and leads to dose reductions of 16.9% in the thorax and 20% in the abdomen [10]. In addition, angular dose modulation software has recently been shown to substantially reduce radiation exposure in pediatric patients, with a mean effective tube current-time product reduction of more than 20% [11]. It is expected that z-axis dose modulation will lead to greater reduction in patient radiation exposure because it automatically adapts X-ray tube current to patient size and body region [7, 8], and initial results show a dose reduction of 20-60% [19].
As shown recently, the high sensitivity and specificity of CT colonography in the detection of colorectal polyps is based on the high resolution of modern MDCT scanners [20-23]. Accordingly, some studies that were performed with older scanners and wider collimations show disappointing results, whereas others present promising results [1, 2, 13]. In the meantime, endoluminal 3D views have been widely accepted and are used as the primary approach by different research groups [1, 23], although controversy still exists as to whether 2D or 3D interpretation should be used primarily. For evaluation of endoluminal 3D reconstructions in CT colonography, image noise is an important factor, especially in the detection of small lesions. Furthermore, the assessment of extracolonic findings is impossible if image quality is deteriorated by too much image noise, as seen in very low-dose settings [12].
In our study, we achieved a substantial dose reduction using x, y, and z-axis dose modulation software as compared with x- and y-axis modulation alone. This finding underlines the potential of an adaptation of the tube current to patient attenuation. Constant results over different patient body mass indexes confirm that there is no need for manual adaptation of tube current settings to patient weight. Information as to the patient's body weight is not required to obtain optimum dose reduction because the scanner derives the required attenuation profile from the topogram. Therefore, that the topogram cover the whole scanning range is important to guarantee optimum dose modulation.
Images acquired with x, y, and z-axis tube current modulation did not show an increased noise level. Image quality was also comparable and good to excellent in all cases. A substantial dose reduction of about 33% was seen in the supine and prone positions.
In three obese subjects scanned with x- and y-axis tube current modulation, we observed high noise levels in the pelvis (Figs. 2A, 2B, 2C, and 2D). These subjects had body mass indexes of 31.1, 32.0, and 34.0. In the population scanned with x, y, and z-axis tube current modulation, we did not witness this effect, which indicates that the x, y, and z-axis tube current modulation correctly raises the tube current in areas with high attenuation. Three obese patients with body mass indexes of 30-35 showed noise levels that were equal to those recorded in normal-weight patients. Respective axial source images showed that the scanner had been raising the tube current-time product to as much as 210 mAs in the pelvis in these patients. Nevertheless, because of the reduced dose delivered to the rest of the abdomen, overall radiation exposure remained reasonably low at 8.0-9.0 mSv in these three patients.
Mathematic estimations of effective doses for CT protocols may underestimate or overestimate radiation exposure to the patient [24]. Nevertheless, dose estimates based on the dose-length product from the CT protocol yield results that are close to measured radiation exposure values, as shown previously [25]. Therefore, the calculated effective radiation dose in our study should be nearly as exact as values that have been achieved by means of phantom measurements.
The scanning parameters evaluated in this study were chosen to achieve the highest possible resolution. Currently, controversy continues about the significance of polyps smaller than 5 mm. Until their significance is fully clarified, we believe that in CT colonography the highest sensitivity for the detection of all polyps must be the ultimate goal. Therefore, we used a collimation of 0.75 mm and reconstructed 0.5-mm overlapping 1-mm-thick slices. A previous study has shown that dose reduction in CT colonography can also be achieved with higher X-ray beam collimations and faster table feed [18], but these changes will result in impaired image quality and lower detection rates for small polyps. In addition, extracolonic findings, especially in parenchymatous organs, cannot be assessed if there is too much image noise.
The results of our study show a significant dose reduction at 120 effective mAs and 40 effective mAs, which suggests that x, y, and z-axis tube current modulation enables dose reduction at tube current values in this range. Further studies are needed to show the potential of this technique in combination with other tube current, pitch, and X-ray beam collimation settings.
Our study has several limitations. Because only 37 of 80 patients underwent optical colonoscopy for removal of polyps, sensitivities and specificities for polyp detection were not evaluated. It remains to be proven that sensitivity for polyp detection is higher using 16-MDCT scanners at the settings discussed in this study.
Furthermore, because of ethical concerns based on radiation issues, the study was not designed as an intraindividual comparison. Because patient body mass indexes did not differ significantly, we assumed that both patient populations can be compared. Another limitation might be the use of a tube current value of 120 mAs for supine scanning. This value was chosen to get the best possible image quality at 0.75-mm collimation. Disappointing results from recent multicenter trials [2, 13] may partially be based on the scanning parameters and imaging techniques used in those studies.
Another limitation of our study is that the dose reduction software described is not currently available on scanners of other vendors. In addition, we did not compare x-, y-, and z-axis tube current modulation with fixed tube current scanning. This comparison was not done because results of previous studies show a substantial dose reduction achieved with online x-and y-axis tube current modulation as compared with fixed tube current scanning [10, 26]. In our department, all abdominal CT examinations are routinely performed with x-, y-, and z-axis dose modulation, and before the implementation of this technique they were routinely performed with x- and y-axis tube current modulation. Because the latter technique has a scientifically proven benefit, we did not go back to fixed tube current scanning for this study. Our goal was to show the additional dose reduction achieved by x-, y-, and z-axis tube current modulation as compared with x- and y-axis tube current modulation.
In conclusion, our results underscore that substantial dose reduction in CT colonography can be achieved by combined x-, y-, and z-axis tube current modulation as compared with x- and y-axis tube current modulation alone, which is most important in examinations performed for screening. When comparing patients scanned with x- and y-axis tube current modulation and those scanned with x-, y-, and z-axis tube current modulation, image quality and image noise do not differ significantly. Because the patient population undergoing CT colonography may consist predominantly of healthy adults, dose reduction is an issue of importance. The x-, y-, and z-axis dose modulation showed constant radiation dose reduction of more than 30% at 40 and 120 mAs and can therefore be recommended for routine use in CT colonography.
Footnote
Address correspondence to A. Graser ([email protected]).
References
1.
Pickhardt PJ, Choi JR, Hwang I, et al. Computed tomographic virtual colonoscopy to screen for colorectal neoplasia in asymptomatic adults. N Engl J Med 2003; 349:2191-2200
2.
Cotton PB, Durkalski VL, Pineau BC, et al. Computed tomographic colonography (virtual colonoscopy): a multicenter comparison with standard colonoscopy for detection of colorectal neoplasia. JAMA 2004; 291:1713-1719
3.
Podolsky DK. The AGA and future trends in gastroenterology: CT colonography. Gastroenterology 2004; 127:985-986
4.
van Dam J, Cotton P, Johnson CD, et al. AGA future trends report: CT colonography. Gastroenterology 2004; 127:970-984
5.
Kalender WA, Wolf H, Suess C, Gies M, Greess H, Bautz WA. Dose reduction in CT by on-line tube current control: principles and validation on phantoms and cadavers. Eur Radiol 1999; 9:323-328
6.
Greess H, Wolf H, Baum U, Kalender WA, Bautz W. Dosage reduction in computed tomography by anatomy-oriented attenuation-based tube-current modulation: the first clinical results [in German]. Rofo 1999; 170:246-250
7.
Kalra MK, Maher MM, Toth TL, Kamath RS, Halpern EF, Saini S. Comparison of z-axis automatic tube current modulation technique with fixed tube current CT scanning of abdomen and pelvis. Radiology 2004; 232:347-353
8.
Kalra MK, Maher MM, Kamath RS, et al. Sixteen-detector row CT of abdomen and pelvis: study for optimization of z-axis modulation technique performed in 153 patients. Radiology 2004; 233:241-249
9.
Kalra MK, Maher MM, Toth TL, et al. Techniques and applications of automatic tube current modulation for CT. Radiology 2004; 233:649-657
10.
Tack D, De Maertelaer V, Gevenois PA. Dose reduction in multidetector CT using attenuation-based online tube current modulation. AJR 2003; 181:331-334
11.
Greess H, Nomayr A, Wolf H, et al. Dose reduction in CT examination of children by an attenuation-based on-line modulation of tube current (CARE Dose). Eur Radiol 2002; 12:1571-1576
12.
van Gelder RE, Venema HW, Florie J, et al. CT colonography: feasibility of substantial dose reduction—comparison of medium to very low doses in identical patients. Radiology 2004; 232:611-620
13.
Rockey DC, Paulson E, Niedzwiecki D, et al. Analysis of air contrast barium enema, computed tomographic colonography, and colonoscopy: prospective comparison. Lancet 2005; 365:305-311
14.
Bongartz G, Golding SJ, Jurik AG, et al. European guidelines on quality criteria for computed tomography. Report EUR 16262EN Luxembourg: Office for Official Publications of the European Communities, 1999
15.
Brix G, Nagel HD, Stamm G, et al. Radiation exposure in multi-slice versus single-slice spiral CT: results of a nationwide survey. Eur Radiol 2003; 13:1979-1991
16.
Cohnen M, Vogt C, Aurich V, Beck A, Haussinger D, Modder U. Multi-slice CT-colonography in low-dose technique: preliminary results. Rofo 2002; 174:835-838
17.
Iannaccone R, Laghi A, Catalano C, et al. Detection of colorectal lesions: lower-dose multi-detector row helical CT colonography compared with conventional colonoscopy. Radiology 2003; 229:775-781
18.
Taylor SA, Halligan S, Bartram CI, et al. Multi-detector row CT colonography: effect of collimation, pitch, and orientation on polyp detection in a human colectomy specimen. Radiology 2003; 229:109-118
19.
Suess C, Chen X. Dose optimization in pediatric CT: current technology and future innovations. Pediatr Radiol 2002; 32:729-734
20.
Luz O, Schafer J, Dammann F, Vonthein R, Heuschmid M, Claussen CD. Evaluation of different 16-row CT colonography protocols using a porcine model [in German]. Rofo 2004; 176:1493-1500
21.
Laghi A, Catalano C, Panebianco V, Iannaccone R, Iori S, Passariello R. Optimization of the technique of virtual colonoscopy using a multislice spiral computerized tomography [in Italian]. Radiol Med (Torino) 2000; 100:459-464
22.
Laghi A, Iannaccone R, Mangiapane F, Piacentini F, Iori S, Passariello R. Experimental colonic phantom for the evaluation of the optimal scanning technique for CT colonography using a multidetector spiral CT equipment. Eur Radiol 2003; 13:459-466
23.
Rottgen R, Schroder RJ, Lorenz M, et al. CT-colonography with the 16-slice CT for the diagnostic evaluation of colorectal neoplasms and inflammatory colon diseases [in German]. Rofo 2003; 175:1384-1391
24.
Becker C, Schatzl M, Feist H, et al. Assessment of the effective dose for routine protocols in conventional CT, electron beam CT and coronary angiography [in German]. Rofo 1999; 170:99-104
25.
Jakobs TF, Wintersperger BJ, Herzog P, et al. Ultra-low-dose coronary artery calcium screening using multislice CT with retrospective ECG gating. Eur Radiol 2003; 13:1923-1930
26.
Greess H, Wolf H, Baum U, et al. Dose reduction in computed tomography by attenuation-based online modulation of tube current: evaluation of six anatomical regions. Eur Radiol 2000; 10:391-394
Information & Authors
Information
Published In
Copyright
© American Roentgen Ray Society.
History
Submitted: April 17, 2005
Accepted: July 21, 2005
First published: November 23, 2012
Keywords
Authors
Metrics & Citations
Metrics
Citations
Export Citations
To download the citation to this article, select your reference manager software.