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Comparison of patient doses in 256-slice CT and 16-slice CT scanners

¹ Department of Medical Physics, National Institute of Radiological Sciences, Chiba 263-8555, Japan, ² School of Allied Health Sciences, Faculty of Medicine, Osaka University, Osaka 565-0871, Japan and ³ Department of Medical Imaging, National Institute of Radiological Sciences, Chiba 263-8555, Japan

Correspondence: Shinichiro Mori, 4-9-1 Anagawa, Inage-ku, Chiba-shi, Chiba, 263-8555, Japan

	Abstract

Top Abstract Introduction Materials and methods Results Discussion Appendix 1. Field of... Appendix 2. Effective dose... References

 
The 256-slice CT-scanner has been developed at the National Institute of Radiological Sciences. Nominal beam width was 128 mm in the longitudinal direction. When scanning continuously at the same position to obtain four-dimensional (4D) images, the effective dose is increased in proportion to the scan time. Our purpose in this work was to measure the dose for the 256-slice CT, to compare it with that of the 16-slice CT-scanner, and to make a preliminary assessment of dose for dynamic 3D imaging (volumetric cine imaging). Our group reported previously that the phantom length and integration range for dosimetry needed to be at least 300 mm to represent more than 90% of the line integral dose with the beam width between 20 mm and 138 mm. In order to obtain good estimates of the dose, we measured the line-integral dose over a 300 mm range in PMMA (polymethylmethacrylate) phantoms of 160 mm or 320 mm diameter and 300 mm length. Doses for both CT systems were compared for a clinical protocol. The results showed that the 256-slice CT generates a smaller dose than the 16-slice CT in all examinations. For volumetric cine imaging, we found an acceptable scan time would be 6 s to 11 s, depending on examinations, if dose must be limited to the same values as routine examinations with a conventional multidetector CT. Finally, we discussed the studies necessary to make full use of volumetric cine imaging.

	Introduction

Top Abstract Introduction Materials and methods Results Discussion Appendix 1. Field of... Appendix 2. Effective dose... References

 
In 2001 the introduction of a 16-slice CT-scanner raised some new topics in CT technology development. 16-slice CT allows applications of three-dimensional (3D) images in clinical fields such as diagnosis, surgical simulation, planning of radiation therapy and monitoring of interventional therapy. However, it is still difficult to take dynamic 3D images of moving organs such as the heart or lung to enlarge the application fields. In order to take these images, we have developed a prototype 256-slice CT at NIRS (National Institute of Radiological Sciences) which employs continuous rotations of a cone-beam [1].

Clinical applications of CT techniques have continued to increase the dose to patients during recent decades, as CT examinations have come to provide higher quality X-ray imaging with substantial benefits in clinical diagnosis [2]. Notwithstanding the potential benefits to the healthcare of patients using CT, the fundamental concern in radiological protection is the optimization of radiation exposure.

The maximum nominal beam width of the 256-slice CT is 128 mm and is four times larger than the third-generation 16-slice CT-scanner (Toshiba Aquilion; Toshiba Medical Systems, Japan). A wider beam width is more efficient for imaging in a wider coverage. However, doses to patients with 256-slice CT are of considerable concern if it is to be used for obtaining dynamic 3D images (volumetric cine images). When scanning continuously at the same position, the effective dose is increased in proportion to the scan time and a wider coverage brings larger doses to patients. Therefore, it is very important to assess the dose of the 256-slice CT before volumetric cine imaging for patients.

This work was carried out to compare doses, including scattered radiation, of the 256-slice CT and 16-slice CT and to make a preliminary assessment of dose for volumetric cine imaging.

	Materials and methods

Top Abstract Introduction Materials and methods Results Discussion Appendix 1. Field of... Appendix 2. Effective dose... References

 
Acquisition systems of 256-slice CT and 16-slice CT scanners
The prototype 256-slice CT-scanner uses a wide-area 2D detector designed on the basis of the present CT technology and is mounted on the gantry frame of a state-of-the-art CT-scanner (Figure 1) [3]. The number of elements is 912 channels x 256 segments; element size is approximately 1 mm x 1 mm, corresponding to a 0.5 mm (transverse) x 0.5 mm (longitudinal) beam width at the centre of rotation. Gantry rotation time is 1.0 s. Data sampling rate is 900 views/s, and the dynamic range of the A/D converter is 16 bits. As shown in Appendix 1, the reconstructed regions are cylinders of 240 mm diameter and 102.4 mm length for the head scan and 320 mm diameter and 93.9 mm length for the body scan. The detector element consists of a scintillator and photodiode, which are the same as for the scintillator of multidetector CT (MDCT) (Toshiba Aquilion). Three wedge designs (large, small, and flat) on the 256-slice CT are intended to extend the conventional wedge designs of the third-generation 16-slice CT-scanner (Toshiba Aquilion) in the longitudinal direction. The large and small wedges are shaped to compensate for the variable path length of the patient across the scan field of view (FOV). The small wedge is used for an object under 240 mm FOV, and the large wedge is used for over 240 mm FOV (e.g. chest and abdomen). The flat wedge is thicker at the centre than the other wedges.

View larger version (63K): [in this window] [in a new window]   Figure 1. (a) Front view of 256-slice CT-scanner. (b) A wide-area 2D detector is designed on the basis of the present CT technology and mounted on the gantry frame of the state-of-the-art CT-scanner.

The 16-slice CT detector consists of 40 segments, which can be electronically grouped to provide different image slice configurations. The longitudinal FOV is 32 mm at the maximum. Other major components are the same as those of the 256-slice CT. In addition to the axial scan, the helical scan mode can be selected to cover volumes beyond the detector width.

Phantoms
The length of the IEC-recommended dosimetry phantom [5] is at least 140 mm. This conventional phantom contains holes just large enough to accept the pencil-shaped ionization chamber. For dose measurement in cone-beam CT, the length of the phantom should be longer, because of the wider scatter distribution. According to our previous results [6], the phantom length and integration range for dosimetry needed to be at least 300 mm to represent more than 90% of line integral dose with the beam width between 20 mm and 138 mm. Therefore, in the present study we used 300 mm long phantoms of PMMA (polymethylmethacrylate). The diameters of the phantoms are 160 mm for head and 320 mm for body examination. These phantoms were provided by joining unit cylinders 150 mm long. The details of the phantoms were described by Mori et al [6].

Detectors
A pencil-shaped ionization chamber (CT-30; Oyogiken, Japan) of active length 300 mm was connected to a dosemeter (AE-132; Oyogiken, Japan) and used to measure dose. The dosemeter was calibrated (National Institute of Advanced Industrial Science and Technology, Japan) for the appropriate radiation qualities.

Clinical scan conditions
We compared the doses of the 256-slice CT and the 16-slice CT for clinical scan conditions. These conditions were mainly derived from those recommended by the manufacture for the 16-slice CT. The X-ray tube current was set such that the effective mAs should be the same for both CTs, as given by (current) x (rotation time)/(helical pitch) for the 16-slice CT and by (current) x (rotation time) for the 256-slice CT. For the 256-slice CT, slice collimation was 224 mm x 0.5 mm for the head, 128 mm x 1.0 mm for the pelvis, and 256 mm x 0.5 mm for other sites. For the 16-slice CT, the slice collimation was set to 16 mm x 1.0 mm for pelvis and 16 mm x 0.5 mm for other sites, helical pitch was 0.69 for the head, and 0.94 for other sites, because the scan conditions were chosen to obtain the same spatial resolution as for the 256-slice CT.

The whole scan ranges were 93.9 mm for chest, 187.8 mm for abdomen, and 281.7 mm for pelvis. These scan ranges, except chest examination, were beyond the detector width of the 256-slice CT in the longitudinal direction, therefore they were set as multiples of 93.9 mm, the maximum longitudinal FOV of the 256-slice CT (Appendix 1). For the head examination, because the recommended value for the 16-slice CT was shorter than the maximum FOV of 256-slice CT, the FOV was adjusted to narrow the collimator width for the 256-slice CT. The clinical scan conditions thus obtained are summarized in Table 1.

Dose assessment
The dose was assessed using the dose profile integral (DPI) over 300 mm (z=±150 mm) (Appendix 2), which was given by the output of the pencil ionization chamber of 300 mm length [6].

The weighted average of DPI at the centre and peripheries of the phantoms is given by

if we assume a linear decrease (or increase) of DPI in the radial direction, where DPI_c is the DPI at the centre and DPI_p the average DPI on the peripheries.

Clinical image quality
We imaged four healthy male volunteers (mean age 30.0 years±7.6 (standard deviation) (SD); age range 23–53 years) using the 256-slice CT. The study was approved by the Institutional Review Board, and written informed consent was obtained from all subjects before starting. A non-enhanced examination with a step-and-shoot approval was carried out as follows: (i) head, (ii) chest, (iii) abdomen, and (iv) pelvis for one subject at each anatomical site. The subjects held their breath at end-inhale for the chest examination and end-exhale for the abdomen and pelvis examinations during scanning. Scan conditions were the same as the clinical conditions (Table 1) except the scan ranges, which were 102.4 mm for head (one scan), 375.6 mm for chest (four contiguous scans), 93.9 mm for abdomen and pelvis (one scan). The matrix size was 512 x 512 x 111–512 x 512 x 205, and the convolution kernel was the standard head kernel (FC43) for the head examination and the standard body kernel (FC10) for the others.

Image quality was evaluated by three board-certified radiologists who had more than 10 years experience in clinical diagnosis. They compared quality of the images taken with the prototype scanner to their quality standard formed by experience. It took about 1.5 h to read the images obtained in multiple planes in all four cases.

	Results

Top Abstract Introduction Materials and methods Results Discussion Appendix 1. Field of... Appendix 2. Effective dose... References

 
For both CTs, DPI_c, DPI_p, and DPI_w in an axial scan are summarized in Table 2. These values are normalized to 100 mAs. For the 256-slice CT, DPI_w is 1966 mGy·mm/100 mAs for the head phantom and 1109 mGy·mm/100 mAs for the body phantom. For the 16-slice CT, DPI_w is 181.6 mGy·mm/100 mAs with 8 mm beam width for the head phantom, 88.4 mGy·mm/100 mAs with 8 mm beam width and 155.9 mGy·mm/100 mAs with 16 mm beam width for the body phantom.

View larger version (89K): [in this window] [in a new window]   Figure 2. Clinical images. (a) The 0.5 mm isotropic normal anatomy images of auditory ossicles in sagittal section. (b) 3D visualization of the chest with four contiguous scans. (c) Normal anatomy images of abdomen (0.63 mm reconstruction increment). (d) Coronal image (0.63 mm reconstruction increment) of pelvis with three contiguous scans.

	Discussion

Top Abstract Introduction Materials and methods Results Discussion Appendix 1. Field of... Appendix 2. Effective dose... References

The dose for the 256-slice CT was less than that of the 16-slice CT in all examinations for the following reason. In a MDCT-scanner the actual beam width is set as the nominal beam width (slice thickness x slice number) plus a certain margin, where the margin is added to cover penumbra and mechanical errors. X-ray photons incident on a marginal portion do not contribute to image formation, but they do contribute to increased dose. If the nominal beam width becomes large, the contribution of this portion becomes smaller. Thus, the 256-slice CT with larger beam width provides smaller DPI_w values than the 16-slice CT. For the 16-slice CT the pelvis examination with 16 mm nominal beam width is more effective than the others with 8 mm beam width. In general, helical scans with pitch less than one caused overlap regions. Therefore in the present study, we set the effective mAs value to be the same to obtain the same signal-to-noise ratio in both CT systems.

Notwithstanding the dose for the 256-slice CT being smaller than that of the 16-slice CT, the 256-slice CT provides sufficient image quality for diagnosis (Figure 2) [7]. In these clinical conditions, the 256-slice CT achieved a 0.5–0.8 mm isotropic resolution and large volumes of data were taken in a one-rotation scan [8]. Therefore coronal and sagittal images were obtained at sufficient spatial resolution without secondary reconstruction.

Regarding the diagnostic reference level, the effective dose [9] for the MDCT was approximately 15 mSv for routine chest examinations and 30 mSv for routine abdomen or pelvis examinations [10]. If these values are taken as upper limits and X-ray conditions are the same as those in Table 1, the acceptable scan time in volumetric cine imaging might be estimated in the following way. From Appendix 2, the estimated effective dose for a 1 s scan was 2.21 mSv, 2.60 mSv and 3.29 mSv for chest, abdomen and pelvis, respectively. Therefore, the acceptable scan time should be 6 s (= 15 [mSv]/2.21 [mSv]), 11 s (=30 [mSv]/2.60 [mSv]) and 9 s (= 30 [mSv]/3.20 [mSv]) for chest, abdomen, and pelvis, respectively. As these scan times may not be sufficient for a dynamic study in some cases, further efforts are necessary to develop dose reduction methods such as automatic dose control [11–13], as well as to justify increasing the dose in dynamic studies consistent with risk-benefit. Resolution of these issues will allow full use of volumetric cine images which will significantly increase the amount of diagnostic information available to radiologists. In particular, we expect new applications such as computed tomographic angiography (CTA) of coronary arteries or perfusion studies of the whole brain.

	Appendix 1. Field of view for the 256-slice CT

Top Abstract Introduction Materials and methods Results Discussion Appendix 1. Field of... Appendix 2. Effective dose... References

 
In the 256-slice CT, the reconstructed images with the Feldkamp algorithm is the region that is passed through during scanning by the tetra-angular pyramid whose apex and base are the X-ray source and the 2D detector, respectively (Figure A1). The reconstructed region is a double conical shape within a maximum FOV (R_max) in the transverse plane that is determined by the detector size in the transverse direction. Reconstruction is not made in the entire R_max except at the midplane and depends on a reconstructed FOV (R). In the case of the 256 mm x 0.5 mm (= N x T) beam collimation, the length of the reconstruction region (H) is 102.4 mm for R= 240 mm and 93.9 mm for R= 320 mm. As seen in this example, the reconstructed region is generally smaller than the nominal beam width in cone beam CT.

View larger version (18K): [in this window] [in a new window]   Figure A1. Reconstruction geometry of cone-beam CT. An X-ray source and a 2D detector rotate around the z-axis. The volume that can be reconstructed with the Feldkamp algorithm is shown by the shaded region and is a double conical region within a cylinder of radius Rmax, which is determined by the detector size in the x-direction and shows the maximum field of view in the transverse plane. R and H show diameter and height, respectively, of a cylindrical reconstructed volume as it varied with an object. N x T show the nominal beam width where N is the number of slice and T is the slice collimation.

	Appendix 2. Effective dose estimation

Top Abstract Introduction Materials and methods Results Discussion Appendix 1. Field of... Appendix 2. Effective dose... References

CTDI is given as follows.

where N is the number of slices, T (mm) is the nominal slice thickness, and d(z) is the dose profile for an axial scan, l indicates the integration range. The International Electrotechnical Comission (IEC) recommended an integration range of 100 mm. However we used the integration range of 300 mm for the reason described.

DPI is given with these notations as follows.

From Equations (A1) and (A2),

Weighted CTDI (CTDI_w) is defined with CTDIs measured at the centre and peripheries of the phantoms as follows.

CTDI_c and CTDI_p represent the CTDI measured at the centre and the average CTDIs measured on the periphery of the phantom, respectively. CTDI_w is given by DPI_w as follows.

Dose–length product (DLP) for a complete examination is given as:

where L (cm) is the scan range in the longitudinal direction.

Estimation of effective dose (E) may be derived from values of DLP for an examination using appropriately normalized coefficients:

E_DLP is the region-specific normalized effective dose (mSv mGy^–1 mm^–1) [9].

From these equations CTDI_w, DLP and E can be calculated from measured DPI_w. Table A1 gives calculated DPI_w, CTDI_w, DLP and E with one second scan of the 256-slice CT in the clinical conditions for chest, abdomen and pelvis examinations, respectively.

Received for publication August 6, 2004. Revision received April 8, 2005. Accepted for publication June 13, 2005.

	References

Top Abstract Introduction Materials and methods Results Discussion Appendix 1. Field of... Appendix 2. Effective dose... References

 

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