One-year analysis of Elekta CBCT image quality using NPS and MTF.

The image quality (IQ) of imaging systems must be sufficiently high for image-guided radiation therapy (IGRT). Hence, users should implement a quality assurance program to maintain IQ. In our routine IQ tests of the kV cone-beam CT system (Elekta XVI), image noise was quantified by noise standard deviation (NSD), which was the standard deviation of CT numbers measured in a small area in an image of an IQ test phantom (Catphan), and the high spatial resolution (HSR) was evaluated by the number of line-pairs (LPN) visually recognizable in the image. We also measured the image uniformity, the low contrast resolution, and the distances of two points for geometrical accuracy. For this study, we did an additional evaluation of the XVI data for 12 monthly IQ tests by using noise power spectrum (NPS) for noise, modulation transfer function (MTF) for HSR, and CT number-to-density relationship. NPS was obtained by applying Fourier analysis in a small area on the uniformity test section of Catphan. The MTF analysis was performed by applying the Droege-Morin (D-M) method to the line-pair bar regions in the phantom. The CT number-to-density relationship was obtained for insert materials in the low-contrast test section of the phantom. All the quantities showed a noticeable change over the one-year period. Especially the noise level improved significantly after a repair of the imager. NPS was more sensitive to the IQ change than NSD. MTF could provide more quantitative and objective evaluation of HSR. The CT number was very different from the expected CT number, but the CT number-to-density curves were constant within 5% except for two months. Since the D-M method is easy to implement, we recommend using MTF instead of LPN even for routine QA. The IQ of the imaging systems was constantly changing; hence, IQ tests should be periodically performed. Additionally, we found the importance of IQ tests after every service work, including detector calibration as well as preventive maintenance.

[Influence on dose calculation by difference of dose calculation algorithms in stereotactic lung irradiation: comparison of pencil beam convolution (inhomogeneity correction: batho power law) and analytical anisotropic algorithm].

The monitor unit (MU) was calculated by pencil beam convolution (inhomogeneity correction algorithm: batho power law) [PBC (BPL)] which is the dose calculation algorithm based on measurement in the past in the stereotactic lung irradiation study. The recalculation was done by analytical anisotropic algorithm (AAA), which is the dose calculation algorithm based on theory data. The MU calculated by PBC (BPL) and AAA was compared for each field. In the result of the comparison of 1031 fields in 136 cases, the MU calculated by PBC (BPL) was about 2% smaller than that calculated by AAA. This depends on whether one does the calculation concerning the extension of the second electrons. In particular, the difference in the MU is influenced by the X-ray energy. With the same X-ray energy, when the irradiation field size is small, the lung pass length is long, the lung pass length percentage is large, and the CT value of the lung is low, and the difference of MU is increased.

[Comparison of dose evaluation index by pencil beam convolution and anisotropic analytical algorithm in stereotactic radiotherapy for lung cancer].

We previously studied dose distributions of stereotactic radiotherapy (SRT) for lung cancer. Our aim is to compare in combination pencil beam convolution with the inhomogeneity correction algorithm of Batho power low [PBC (BPL)] to the anisotropic analytical algorithm (AAA) by using the dose evaluation indexes. There were significant differences in D95, PTV mean dose, homogeneity index, and conformity index, V10, and V5. The dose distributions inside the PTV calculated by PBC (BPL) were more uniform than those of AAA. There were no significant differences in V20 and mean dose of total lung. There was no large difference for the whole lung. However, the surrounding high-dose region of PTV became smaller in AAA. The difference in dose evaluation indexes extended between PBC (BPL) and AAA that as many as low CT value of lung. When the dose calculation algorithm is changed, it is necessary to consider difference dose distributions compared with those of established practice.

[Necessity of delivering the calibration coefficient of a dosimeter for low energy X-rays used in mammography X-ray units].

The calibration coefficient currently supplied by the National Institute of Advanced Industrial Science and Technology (AIST) is measured with an X-ray unit equipped with a W target/Al filter. We compared the calibration coefficient, which is measured with the X-ray unit equipped with a W target/Al filter and the X-ray unit for mammography. For the calibration coefficient, which is measured at the calibration fields of the X-ray units for mammography, the difference in the calibration coefficient measured with the X-ray unit equipped with a W target/Al filter that assumed a difference in the quality index-value of less than 1.5% was 24.2% at the maximum. In addition, many differences of more than unsureness 5% of the calibration coefficient were accepted. Thus, the correct dosimetry is not being performed for mammography. To improve the precision of dosimetry for mammography, at least, it is desirable to employ the calibration coefficient using the Mo target supplied by AIST. Furthermore, the indication of quality of radiation should be made severe, too. To perform equal dosimetry with a diagnosis domain, it is necessary to supply the calibration coefficient of the other target/filters.

[Total breathing phase scan (slow scan) method in the free breathing state by multi-detector-row computed tomography scanner in radiotherapy planning].

With the new high-speed computed tomography (CT) devices, the longest scan time for a conventional scan is about 2 seconds. Therefore, the all-breathing-phase scan (slow scan) in the free-breathing state, which includes an entire breathing phase of three or four seconds in one imaging section, is impossible. We performed spiral scanning with a low-speed scanning pitch using multi detector-row CT (4MDCT) and examined a method of scanning that was similar to the slow scan method. As a result, with the scanning diagram, we demanded the image data time than a combination of scanning pitch and X-ray tube rotation time, image data time of 6.66 seconds longest in scanning slice thickness 1 mm x 4 detectors and reconstruction slice thickness 2 mm was possible, and the scanning that was similar to the slow scan method was possible. Furthermore, it was good in the image data time when we demanded it from scanning diagram and, as a result of confirmation of image data time by the equal speed turn phantom, agreed. In addition, there is not increase of a radiation exposure dose by this method, and this method is a useful method.

[Clinical calibration dosimetry in JSMP-01: measurements using Farmer-type cylindrical ion chambers.].

The Japan Society of Medical Physics (JSMP) Task Group published Standard dosimetry of absorbed dose in external beam radiotherapy (Standard dosimetry 01) as a new high-energy photon and electron dosimetry protocol in 2002. In this study, we present Standard dosimetry 01 as the JSMP-01 protocol for the convenience of users. This protocol is based on using an ion chamber having a (60)Co absorbed dose to water calibration coefficient, N(D,w), which is calculated from a (60)Co exposure calibration coefficient, N(c). We present dose comparisons between a reference chamber and various Farmer-type cylindrical chambers with different wall materials. The absorbed dose to water was compared at the calibration depths of 5 cm for a (60)Co beam, 10 cm for photons, and d(c) = 0.6 R(50) - 0.1 (cm) for electrons according to JSMP-01. The JARP chamber in the Kyushu Regional Center which meets third-order standards in Japan was used as the reference chamber. The absorbed dose to water for the Farmer-type chambers determined according to JSMP-01 agreed with that for the JARP chamber within 1% for photon and electron beams. The doses obtained by JSMP-01 and the Japan Association of Radiological Physics protocol (JARP-86) were also compared for photon and electron beams. For the Farmer-type chambers with photon beams, JSMP-01 results were up to 1.5% higher than JARP-86 results. For electron beams JSMP-01 results were higher than JARP-86 results by 1.3-2.8%.

[Clinical calibration dosimetry in JSMP-01: measurements using plane-parallel ion chambers.].

The Japan Society of Medical Physics (JSMP) Task Group published Standard dosimetry of absorbed dose in external beam radiotherapy (Standard dosimetry 01) as a new high-energy photon and electron dosimetry protocol in 2002. In this study, we present Standard dosimetry 01 as the JSMP-01 protocol for the convenience of users. This protocol is based on using an ion chamber having a (60)Co absorbed dose to water calibration coefficient, N(D,w), which is calculated from a (60)Co exposure calibration coefficient, N(c). We present dose comparisons between a reference chamber and various plane-parallel chambers. The absorbed dose to water was compared at the calibration depth of 5 cm for a (60)Co beam and d(c) = 0.6R(50) - 0.1 (cm) for electron beams according to JSMP-01. The absorbed dose to water calibration coefficients, [N(D,w)](Co) and [N(D,w)](18E), for the plane-parallel chambers were also determined by (60)Co and electron beam cross-calibrations using a reference chamber. The dose for the plane-parallel chambers derived from [N(D,w)](Co) and [N(D,w)](18E) was compared to that for the reference chamber using electron beams. The JARP chamber in the Kyushu Regional Center which meets third-order standards in Japan was used as the reference chamber. The doses for the plane-parallel chambers determined according to JSMP-01 agreed with that for the JARP chamber within 1% and 2% for (60)Co and electron beams, respectively. For electron beams, the doses for the plane-parallel chambers calculated from [N(D,w)](Co) and [N(D,w)](18E) were within 1.5% and 1.0% compared to those for the JARP chamber, respectively, except for the Exradin A10 chamber.

[Study of improvement in X-ray computed tomography images with a radiation treatment planning system: image noise and slice thickness].

In radiotherapy, dose distribution and calculation of dose monitor units (DMU) are generally performed by a radiation treatment planning system using CT images. Therefore, differences in calculation can arise as a result of the quality of the CT image data. The quality of CT images involves contrast resolution, resolution, noise, slice thickness, and other factors. Among these factors, we examined noise and slice thicknesses. Results demonstrated that, even if noise increased, CT value did not change, and, therefore, did not influence DMU. Examination of slice thickness showed that, when the radiation field was rectangular, it was not influenced by slice thickness. However, when a multi-leaf collimator (MLC) was used, if slice thickness was thicker than the size of the MLC, a difference arose in the position of the MLC, and, therefore, some difference arose in dose. Therefore, slice thickness should be thinner than the size of the MLC.

[Patient dose measurement with fluorescent glass dosimeter: characteristics evaluation and patient skin dose measurement in abdominal interventional radiology].

In this study, we investigated the usefulness of the fluorescent glass dosimeter for measuring patient dose. The fluorescent glass dosimeter is constructed of a glass element and its holder. One type has a tin (Sn) filter and the other does not. The characteristics of these two types of fluorescent glass dosimeters were studied in the range of diagnostic X-ray energy. The result was excellent for each characteristic. Directional dependency, however, was recognized in the fluorescent glass dosimeter with tin (Sn) filter. Based on these evaluations, patient skin dose was measured for abdominal interventional radiology and diagnostic digital subtraction angiography using the holder without filter, which is less direction-dependent and eliminates obstructive shadows in radiography and fluoroscopy. The average skin dose of 30 patients for abdominal IVR was 1.17+/-0.44 Gy (0.51-1.94 Gy), while those for diagnostic DSA examination was 0.54+/-0.21 Gy (0.15-1.02 Gy). The fluorescent glass dosimeter provides high capability for skin dose measurement. The fluorescent glass dosimeter is also useful for controlling patient dose during IVR procedures.

[Form reappearance of the section sensitivity profile on Z-axis in multi-detector spiral computed tomography].

The filter is hung in the direction of slice thickness (Z-axis) in the reconstruction of multi-detector spiral CT. Because of this, performing several time scans is considered useful from the standpoint of reappearance of the section sensitivity profile curves. However, when examination is done at full width half maximum (FWHM) and the position of the center of FWHM, there is non-symmetry and the level of hem extent of the section sensitivity profile curves. Change in FWHM and the position of the center of FWHM increases as pitch increases. Although the change in FWHM was less when the reconstruction slice thickness was increased, the change became larger in the center of FWHM as well. As for the non-symmetry of the section sensitivity profile curves and the level of hem extent, the change decreased when the reconstruction slice thickness was increased, although it increased when pitch was enlarged. It is considered that the cause of these changes is change in table movement speed during scanning.

[Absorption and scatter with an acrylic plate attached to the front of an X-ray beam limiting device].

We measured X-ray absorption and scatter with an acrylic plate attached to the front of the X-ray beam limiting device. The acrylic plate played the role of an added filter, absorbing the low energy element of the X-ray beam and decreasing the surface dose to 4.3% of the extreme height as a result. However, the surface dose outside the irradiation field increased to 35.3% of the extreme height, and the scatter dose increased to 47.8% of the extreme height. An acrylic plate with the minimum thickness necessary should be used to decrease unnecessary exposure outside the irradiation field.

[An absorbed dose conversion factor due to the x-ray spectrum compare with the method by the half-value layer].

When the absorbed dose conversion factor of the x-rays is looked for, generally it is being done how to measure the effective energy (keV) from the half-value layer. But, spectrum measurement is necessary to evaluate quality of x-ray precisely. So, 2.94% of the maximum differences were in the water, soft tissue, and 20.93% of the maximum differences were in search of absorbed dose conversion factor due to the half-value layer and the spectrum as a compared result by cortical bone. And, when a x-ray tube voltage rose, this difference showed a tendency of spreading out. A cause was because the rates of the photon of the higher energy than the energy measured with effective energy increased by a x-ray tube voltage's rising. The ratio of the mass energy absorption coefficient which faces air like cortical bone should be careful because an error by the absorbed dose conversion factor grows big when a absorbed dose conversion factor is measured by the big absorption medium, though it is as the difference in absorbed dose conversion factor is compared with an error by the dosimeter and there is no problem in the water, soft tissue.