Reducing stray radiation with a novel detachable lead arm support in percutaneous coronary intervention.

BACKGROUND: Placing radioprotective devices near patients reduces stray radiation during percutaneous coronary intervention (PCI), a promising technique for treating coronary artery disease. Therefore, lead arm support may effectively reduce occupational radiation dose to cardiologists. PURPOSE: We aimed to estimate the reduction of stray radiation using a novel detachable lead arm support (DLAS) in PCI. MATERIALS AND METHODS: A dedicated cardiovascular angiography system was equipped with the conventional 0.5-mm lead curtain suspended from the table side rail. The DLAS was developed using an L-shaped acrylic board and detachable water-resistant covers encasing the 0.5-, 0.75-, or 1.0-mm lead. The DLAS was placed adjacent to a female anthropomorphic phantom lying on the examination tabletop at the patient entrance reference point. An ionization chamber survey meter was placed 100 cm away from the isocenter to emulate the cardiologist's position. Dose reduction using the L-shaped acrylic board, DLAS, lead curtain, and their combination each was measured at five heights (80-160 cm in 20-cm increments) when acquiring cardiac images of the patient phantom with 10 gantry angulations, typical for PCI. RESULTS: Median dose reductions of stray radiation using the L-shaped acrylic board were 9.0%, 8.8%, 12.4%, 12.3%, and 6.4% at 80-, 100-, 120-, 140-, and 160-cm heights, respectively. Dose reduction using DLAS with a 0.5-mm lead was almost identical to that using DLAS with 0.75- and 1.0-mm leads; mean dose reductions using these three DLASs increased to 16.2%, 45.1%, 66.0%, 64.2%, and 43.0%, respectively. Similarly, dose reductions using the conventional lead curtain were 95.9%, 95.5%, 83.7%, 26.0%, and 19.6%, respectively. The combination of DLAS with 0.5-mm lead and lead curtain could increase dose reductions to 96.0%, 95.8%, 93.8%, 71.1%, and 47.1%, respectively. CONCLUSIONS: DLAS reduces stray radiation at 120-, 140-, and 160-cm heights, where the conventional lead curtain provides insufficient protection.

Review and investigation of automatic brightness/dose rate control logic of fluoroscopic imaging systems in cardiovascular interventional angiography.

In this article, we review automatic brightness control (ABC) for fluoroscopy imaging systems. Starting from the simple manual control, the discussion is extended to the kV-primary ABC system, and then to the most recent contrast-to-noise ratio optimized (CNR Optimized) automatic dose rate control system (ADRC). The nature of this review article is trifold. First, it describes the ABC/ADRC and associated circuits governing the operation of the fluoroscopy imaging chain. Second, we show the characteristics of a control logic from a radiation physics point of view. Third, we introduce the most recent activities in the evaluation of CNR-optimized fluoroscopy systems and the phantom design that would be compatible with the design concept of the ADRC. Because of these three subject items in the discussion process, this article is also educational in nature written for medical physicists and radiological technologists who might be less familiar with the design concept of fluoroscopy operation, specifically on the ABC and ADRC. We insert a few related matters associated with fluoroscopy automatic control circuits where they seem applicable and appropriate to enhance the understanding of fluoroscopy operation logic.

AAPM Task Group Report 272: Comprehensive acceptance testing and evaluation of fluoroscopy imaging systems.

Modern fluoroscopes used for image guidance have become quite complex. Adding to this complexity are the many regulatory and accreditation requirements that must be fulfilled during acceptance testing of a new unit. Further, some of these acceptance tests have pass/fail criteria, whereas others do not, making acceptance testing a subjective and time-consuming task. The AAPM Task Group 272 Report spells out the details of tests that are required and gives visibility to some of the tests that while not yet required are recommended as good practice. The organization of the report begins with the most complicated fluoroscopes used in interventional radiology or cardiology and continues with general fluoroscopy and mobile C-arms. Finally, the appendices of the report provide useful information, an example report form and topics that needed their own section due to the level of detail.

Determination of geometric information and radiation field overlaps on the skin in percutaneous coronary interventions with computer-aided design-based X-ray beam modeling.

PURPOSE: This study aimed to develop a method for the determination of the source-to-surface distance (SSD), the X-ray beam area in a plane perpendicular to the beam axis at the entrance skin surface (Ap ), and the X-ray beam area on the actual skin surface (As ) during percutaneous coronary intervention (PCI). MATERIALS AND METHODS: Male and female anthropomorphic phantoms were scanned on a computed tomography scanner, and the data were transferred to a commercially available computer-aided design (CAD) software. A cardiovascular angiography system with a 200 × 200 mm flat-panel detector with a field-of-view of 175 × 175 mm was modeled with the CAD software. Both phantoms were independently placed on 40 mm thick pads, and the examination tabletop at the patient entrance reference point. Upon panning, the heart center was aligned to the central beam axis. The SSD, Ap , and As were determined with the measurement tool and Boolean intersection operations at 10 gantry angulations. RESULTS: The means and standard deviations of the SSD, Ap , and As for the male and female phantoms were 573 ± 15 and 580 ± 15 mm, 8799 ± 1009 and 9661 ± 1152 mm2 , 10495 ± 602 and 11913 ± 600 mm2 , respectively. The number of As overlaps for the male and female phantoms were 15/45 and 21/45 view combinations, respectively. CONCLUSIONS: CAD-based X-ray beam modeling is useful for the determination of the SSD, Ap , and As . Furthermore, the knowledge of the As distribution helps to reduce the As overlap in PCI.

Accuracy of HVL measurements utilizing solid state detectors for radiography and fluoroscopy X-ray systems.

The half-value layer (HVL) is one of the regulatory required radiation safety parameters that needs to be measured annually. With the advent of solid state detectors and their associated electrometer assembly, the HVL measurement can be conducted with relative ease. In fact, various radiological technique parameters such as tube potential (kV), exposure time in millisecond (msec), air kerma (mGy), and air kerma rate (mGy/sec) can be obtained along with the HVL with just one exposure. The measured (or, calculated) HVL is based on radiation detection systems calibrated for conventional x-ray systems equipped with tungsten anode and added aluminum filters (molybdenum anode and filter in the case of mammography systems). However, a new generation of radiography and fluoroscopy (R/F) systems, inclusive of interventional angiography equipment, is equipped with varying thicknesses and materials of spectral shaping filters (SSF) to minimize the radiation exposure to the patients while image quality is maintained and optimized. The accuracy of HVL obtained with new generation of R/F systems has not been investigated in depth due to the addition of spectral filters yielding a harder beam quality with a higher HVL than the regulatory required value of 2.9 mm Al HVL at 80 kV. It would be of great interest to determine the accuracy of HVL as measured (or, calculated) by the solid state detector systems (SSDS), especially when accurate radiation dose delivered to the patient is required. In this investigation, the subject is limited to the accuracy of HVL measurement for conventional R/F systems.

Characterization and verification of lead thickness of commercially available lead foil tape for the measurements of lead equivalency of radio-protective shields.

PURPOSE: Radiation protective apparatus is normally specified in "millimeter" of lead equivalence. Typically, it is less than 0.5 mmPb with the exception of lead eyeglasses, which may be 0.75 mmPb equivalent. Upon discovery of commercially available lead foil tape, manufactured by 3M™ "Lead Foil Tape 421" (LFT) which is designed for industrial utility applications. We set out to determine if this LFT can, indeed, be employed as the reference lead in the evaluation of lead equivalency of various protective apparatus. METHOD: The LFT is cut to appropriate size (50 mm × 50 mm) and stacked for varying the total lead thickness for the transmission measurements. The transmission curves are obtained following the geometry spelled out in ASTM Designation F3094-14 standards. The radiation beam qualities corresponding to modern cardiovascular angiography equipment in the range of 60~120 kVp, in increments of 10 kVp, and in combination with the spectral shaping filters of 0, 0.1, 0.2, 0.3, 0.6 and 0.9 mmCu were employed for characterization of the lead foil tape. The transmission data of lead pieces with known thicknesses (1/64", 1/32" and 3/64") are superimposed on the lead foil tape transmission curves to validate that the 3M™ LFT is indeed usable as 0.1 mm lead. RESULTS: The transmission ratio (data points) of lead pieces with known thicknesses at various radiation beam qualities mentioned above, fall right onto the transmission curves of 3M™ LFT with better than 2% accuracy. Therefore, it is indeed behaving like 0.1 mm thick lead sheet, based on the superimposed transmission curves. The 3M™ "Lead Foil Tape 421" is employed as the reference lead for evaluation of radiation protective apparatus at this institution. Verification of lead protective apparatus with unknown lead equivalence can now be determined with a high accuracy and certainty.

Evaluation and verification of a simplified lead equivalency measurement method.

PURPOSE: This technical note presents an inexpensive tool and method for determining lead equivalency using digital radiography x-ray equipment. METHODS: A test tool was developed using commercially available lead tape (3M™ Lead Foil Tape 421). The test tool consisted of nine varying lead thick squares arranged in a larger square (0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, and 1.0 mm). It was imaged on a DR plate with a digital portable x-ray unit across a range of energies (60-120 kVp) and two beam filtrations. Lead equivalency was determined by using the linear relationship between dose to the detector and pixel values in the raw images. The lead equivalency of the tape was validated using known lead thicknesses (physically measured with caliper). Additional lead equivalency measurements were made for protective eyewear, a thyroid shield, and a lead apron. RESULTS: The test tool and method measured the two known lead thicknesses to be -9.7% to 7.1% different from the actual values across the range of energies under normal x-ray beam conditions and under a 1-mm copper filtered x-ray beam. The additional lead equivalency measurements of radiation protection apparel across energies ranged from -6% to 20% for both beam conditions when compared with the values provided by the manufacturer. CONCLUSION: This work validates the test tool and methodology as an inexpensive alternative to checking the lead equivalency of radiation protection apparel in a clinical setting. The methodology is equipment independent with a few prerequisites.

COVERING THE PATIENT'S ARM SUPPORT IN LEAD REDUCED THE RADIATION DOSE RATE TO THE CARDIOLOGISTS DURING PERCUTANEOUS CORONARY INTERVENTIONS: A PHANTOM STUDY.

The aim of the study was to estimate organ dose rate reduction to a female anthropomorphic phantom, which simulated the cardiologist, during percutaneous coronary interventions (PCI) when the patient's arm support was covered with 0.4-mm lead foil. Organ dose rates were determined using five radiation detectors inserted into the left eye, left thyroid, left breast, left liver lobe and uterus of the phantom. A male anthropomorphic phantom was placed on the examination table of an angiography system. Heart images of the patient phantom were acquired under 10 gantry angulations typical for PCI. The lead-covered arm support did not interfere with any of the cardiac images. The median organ dose rate reductions to the left eye, left thyroid, left breast, left liver lobe and uterus were 7.8, 36.0, 28.8, 35.7 and 33.5%, respectively. The lead-covered arm support substantially reduced scattered radiation to the female cardiologist without interfering with clinical environments.

Evaluation of lead equivalence of radiation protection apparatuses as a function of tube potential and spectral shaping filter.

PURPOSE: This study aims to evaluate the lead equivalence (LE) of radiation protective apparatuses under various combinations of tube potentials and spectral shaping filter. METHOD: In this study, the commercially available 3M™ Lead Foil Tape 421, with nominal lead thickness of 0.1 mm, was employed to determine the LE of four different radiation protective apparatuses. The LE of protective apparatus was determined by utilizing the X-ray transmission curves obtained with the lead foil tape at 60-120 kVp in combination with the spectral shaping filters of 0.1, 0.2, 0.3, 0.6, and 0.9 mmCu. The experimental setup and test method, for the transmission measurements with narrow beam geometry, was performed in accordance to ASTM Designation F2547-18 Standards. All measurements were obtained using cardiovascular interventional angiography system. RESULTS: A much larger discrepancies between the measured LE and stated (nominal) LE were observed at low tube potential (<70 kVp) for non-lead protective apparatus. At higher tube potentials (>80 kVp) and thicker spectral shaping filters, the measured LE appears to be more consistent with the manufacturer specified nominal thickness for the protective apparatus investigated. On the other hand, for the lead protective eyeglasses, the measured lead equivalence of both the lead side shield and the lens of eyeglasses (0.38 and 0.85 mmPb respectively) are consistent across all tube voltage. CONCLUSION: The conventional specification of LE without considering spectral shaping filter is a valid measure for tube voltages at and above 80 kVp. The measured LE generally exceed the specifications. The difference is most significant at lower tube potentials, and especially with thicker spectral shaping filters. At higher voltages (>100 kVp), the measured LE and the nominal LE are in good agreement with each other irrespective of the spectral shaping filter thickness.

Signal and contrast to noise ratio evaluation of fluoroscopic loops for interventional fluoroscope quality control.

Modern fluoroscopes pose a challenge for the clinical physicist for annual testing and continued upkeep. These fluoroscopes are critical to providing care to patients for complex interventions, and continue to evolve in automated image quality adjustments. Few tools in software or hardware currently exist to assist the physicist or technologist in gauging fluoroscope constancy or readiness for procedures. Many modalities such as mammography, computed tomography or even magnetic resonance imaging are much more evolved with respect to testing or quality control. In this work we sought to provide simple reproducible tools and methods for spot evaluating or continued quality testing of interventional fluoroscopes.

Estimation of primary radiation output for wide-beam computed tomography scanner.

PURPOSE: To estimate in-air primary radiation output in a wide-beam multidetector computed tomography (CT) scanner. MATERIALS AND METHODS: A 6-cc ionization chamber was placed free-in-air at the isocenter, and two sheets of lead (1-mm thickness) were placed on the bottom of the gantry cover, forming apertures of 40-80 mm in increments of 8 mm. The air-kerma rate profiles were measured with and without the apertures ( K ˙ w - A , K ˙ w / o - A ) for 4.8 s at tube potentials of 80, 100, 120, and 135 kVp, tube current of 50 mA, and rotation time of 0.4 s. The nominal beam width was varied from 40 to 160 mm in increments of 40 mm. Upon completion of data acquisition, the K ˙ w / o - A were plotted as a function of the measured beam width, and the extrapolated dose rates ( K ˙ 0 - w / o - A ) at zero beam width were calculated by second-order least-squares estimation. Similarly, the K ˙ w - A were plotted as a function of the radiation field (measured beam width × aperture size at the isocenter), and the extrapolated dose rates ( K ˙ 0 - w - A ) were compared with the K ˙ 0 - w / o - A . RESULTS: The means and standard errors of the K ˙ w / o - A with 40-, 80-, 120-, and 160-mm nominal beam widths at 120 kVp were 10.94 ± 0.01, 11.13 ± 0.01, 11.22 ± 0.01, and 11.31 ± 0.01 mGy/s, respectively, and the K ˙ 0 - w / o - A was reduced to 10.67 ± 0.02 mGy/s. The K ˙ 0 - w - A of 40-, 80-, 120-, and 160-mm beam widths were reduced to 10.6 ± 0.1, 10.6 ± 0.2, 10.5 ± 0.1, and 10.6 ± 0.1 mGy/s and were not significantly different from the K ˙ 0 - w / o - A . CONCLUSIONS: A method for describing the in-air primary radiation output in a wide-beam CT scanner was proposed that provides a means to characterize the scatter-to-primary ratio of the CT scanner.

Does gantry rotation time influence accuracy of volume computed tomography dose index (CTDIvol) in modern CT?

PURPOSE: The purpose of this study was to develop a gantry overrun corrected CTDIvol (cCTDIvol) dosimetry and evaluate the differences between the displayed CTDIvol (dCTDIvol), measured CTDIvol (mCTDIvol), and the cCTDIvol. METHODS AND MATERIALS: The each 8 rotation times between 275 and 1000ms of two CT scanners were investigated. Rotation time (Trot) and the beam-on time (Tbeam) in axial scanning were measured accurately to determine the gantry overrun time (Tover) as Tbeam-Trot. Subsequently, mCTDIvol was measured by using a 100mm ionization chamber and CTDI phantoms. Furthermore, we introduced a gantry overrun correction factor (Co=Trot/Tbeam) to obtain cCTDIvol. Upon completion of the data acquisition, the dCTDIvol and mCTDIvol were compared with the cCTDIvol. RESULTS: The discrepancies of Trot were 0.2±0.2ms as compared to the preset rotation times, and Tover was machine-specific and almost constant (22.4±0.5ms or 45.1±0.3ms) irrespective of the preset rotation time. Both dCTDIvol and mCTDIvol were increasingly overestimated compared to cCTDIvol as the faster the preset rotation time was selected (1.7-23.5%). CONCLUSION: The rotation time influences the accuracy of CTDIvol in modern CT, and should be taken into consideration when assessing the radiation output in modern CT.

Low dose chest CT protocol (50 mAs) as a routine protocol for comprehensive assessment of intrathoracic abnormality.

PURPOSE: To determine the diagnostic capability of low-dose CT (50 mAs) in comparison to standard-dose CT (150 mAs). MATERIALS AND METHODS: Fifty-nine consecutive patients underwent two non-contrast chest CT scans with different current-time products (50 and 150 mAs at 120 kVp) on a 64-detector row CT scanner. Three board certified chest radiologists independently reviewed 118 series of 2 mm-thick images (2 series for each of 59 patients) in a random order. The readers assessed abnormal findings including emphysema, ground-glass opacity, reticular opacity, micronodules, bronchiectasis, honeycomb, nodules (>5 mm), aortic aneurysm, coronary artery calcification, pericardial and pleural effusion, pleural thickening, mediastinal tumor and lymph node enlargement. Five-point scale from 1 (definitely absent) to 5 (definitely present) was used to record the results. The rates of score agreement between two images were calculated. Deviation of one observer's score from other two observers was compared between low dose CT and standard dose CT. RESULTS: Mean agreement rate of the lung parenchymal findings between low dose CT and standard dose CT images was 0.836 (range, 0.746-0.926). Mean agreement rates for mediastinal and pleural findings were 0.920 (range, 0.735-1.000). There was no statistically significant difference in the deviation of the observers' scores between low-dose CT and standard-dose CT. CONCLUSION: Low dose CT protocol at 50 mAs can produce the screening results consistent with standard dose CT protocol (150 mAs), supporting routine use of low dose chest CT protocol.

Accuracy and calibration of integrated radiation output indicators in diagnostic radiology: A report of the AAPM Imaging Physics Committee Task Group 190.

Due to the proliferation of disciplines employing fluoroscopy as their primary imaging tool and the prolonged extensive use of fluoroscopy in interventional and cardiovascular angiography procedures, "dose-area-product" (DAP) meters were installed to monitor and record the radiation dose delivered to patients. In some cases, the radiation dose or the output value is calculated, rather than measured, using the pertinent radiological parameters and geometrical information. The AAPM Task Group 190 (TG-190) was established to evaluate the accuracy of the DAP meter in 2008. Since then, the term "DAP-meter" has been revised to air kerma-area product (KAP) meter. The charge of TG 190 (Accuracy and Calibration of Integrated Radiation Output Indicators in Diagnostic Radiology) has also been realigned to investigate the "Accuracy and Calibration of Integrated Radiation Output Indicators" which is reflected in the title of the task group, to include situations where the KAP may be acquired with or without the presence of a physical "meter." To accomplish this goal, validation test protocols were developed to compare the displayed radiation output value to an external measurement. These test protocols were applied to a number of clinical systems to collect information on the accuracy of dose display values in the field.

Accuracy of gantry rotation time of less than 300 ms for modern MDCT systems.

The accuracy of gantry rotation times of less than 300 ms has been assessed for two "state-of-the art" MDCT systems. The rotation time was measured at selected nominal rotation times (275 and 280 ms) with a solid-state detector; Unfors Xi probe. The detector was positioned on the inner bottom of the gantry bore. Because a pair of two successive radiation peaks is necessary for determination of the rotation time, the radiation detection was performed with the helical scan mode of operation. Upon completion of the data acquisition, we determined the peak times with the Unfors Xi View software program to obtain the rotation time. The means and standard deviations of the measured rotation times were 275.3 ± 0.5 and 285.1 ± 0.4 ms, respectively. The inaccuracy of the rotation time was approximately 5 ms at most, which was comparable to that previously reported for slower rotation times.

Evaluation of gantry rotation overrun in axial CT scanning.

The purpose of this study was to develop and evaluate a simple method to assess gantry rotation overrun in a single axial CT scanning. The exposure time in the axial scanning was measured at selected nominal rotation times (400, 700, and 1000 ms) using a solid-state detector, the RTI's CT dose profiler (CTDP). CTDP was placed at the isocenter and the radiation dose rate signal (profile) was recorded. Subsequently, the full width of this profile was determined as the exposure time (Taxial). Next, CTDP was positioned on the inner cover of the gantry with a sheet of lead (1 mm thick) placed on top of the detector. Gantry rotation time (Thelical) was determined by the time between two successive radiation peaks during continuous helical scanning. The gantry overrun time (Toverrun) is, thus, determined as Taxial - Thelical. The exposure times in the axial scanning, Taxial, obtained with CTDP for nominal rotation times of 400, 700, and 1000 ms were 409.5, 709.6, and 1008.7 ms, respectively. On the other hand, the measured gantry rotation times, Thelical, were 400.0, 700.3, and 999.8 ms, respectively. Therefore, the overruns were 9.5, 9.3, and 8.9 ms for nominal rotation times of 400, 700, and 1000 ms, respectively. The evaluation of overrun in axial scanning can be accomplished with the measurements of both the exposure time in axial scanning and the gantry rotation time. It is also noteworthy that in this context, overrun implies overexposure in axial scanning, which is still used, particularly, in head CT examination.

Cancerogenesis Risks between 64 and 320 Row Detector CT for Coronary CTA Screening.

OBJECTIVES: This study compares cancerogenesis risks posed by the 64 row detector and the 320 row detector computed tomography scanners used during coronary computed tomography angiography (CCTA) following decennial screening guidelines. MATERIAL AND METHODS: Data of the radiation absorbed after CCTA by lung, thyroid, and female breast in patients between 50 and 70 years of age obtained from prior published literature for the 64 row CT scanner were compared with data from our study using 320 row detector CT scanner. Data from the 64 row and the 320 row detector CT scanners was used to determine lifetime attributable risks (LAR) of cancer based on the biological effects of ionizing radiation (BEIR) VII report. RESULTS: The relative reduction of LAR (%) for 50-, 60-, and 70-year-old patients undergoing scanning with the 320 row detector CT scanner was 30% lower for lung, and more than 50% lower for female breast when compared with results from 64 row detector CT scanner. The use of 320 row detector CT would result in a combined cumulative cancer incidence of less than 1/500 for breast in women and less than 1/1000 for lung in men; By comparison, this is much lower than other more common risk factors: 16-fold for lung cancer in persistent smokers, 2-fold for breast cancer with a first degree family member history of breast cancer, and 10-fold for thyroid cancer with a family member with thyroid cancer. Decennial screening would benefit at least 355,000 patients from sudden cardiac death each year, 94% of whom have significant coronary artery disease, with at least one stenosis >75%. LAR for thyroid cancer was negligible for both scanners. CONCLUSION: Lung and female breast LAR reductions with 320 row detector compared with 64 row detector CT are substantial, and the benefits would outweigh increased cancer risks with decennial screening in the age group of 50-70 years.

Measurement of table feed speed in modern CT.

The purpose of this study was to develop and evaluate a noninvasive method to assess table feed speed (mm/s) in modern commercial computed tomography (CT) systems. The table feed (mm/rotation) was measured at selected nominal table feed speeds, given as low (26.67 mm/s), intermediate (48.00 mm/s), and high (64.00 mm/s), by utilizing a computed radiography (CR) cassette installed with a photostimulable phosphor plate. The cassette was placed on the examination table to travel through the isocenter longitudinally, with a total scan length of over 430 mm. The distance travelled was employed to determine the total table feed length. To calculate the table feed speed, gantry rotation time was measured concurrently at a preselected nominal rotation time of 750 ms. Upon completion of data acquisition, the table feed and gantry rotation time were analyzed and used to calculate the actual table feed speed (mm/s). Under the low table feed speed setting, the table feed speed was found to be 26.67 mm/s. Similarly, under the intermediate and high table feed speed settings, the table feed speed was found to be 48.10 and 64.07 mm/s, respectively. Measurements of the table feed speed can be accomplished with a CR system and solid-state detector, and the table feed speed results were in excellent agreement with the nominal preset values.

Differences in behavior of tube current modulation techniques for thoracic CT examinations between male and female anthropomorphic phantoms.

Our objective was to investigate the differences in behavior of tube current modulation (TCM) techniques for thoracic CT examinations between male and female anthropomorphic phantoms. The phantoms were scanned with an automatic exposure control system in the longitudinal (z-) and angular-longitudinal (xyz-) TCM, in addition to the fixed-mA which was used as a reference. Axial dose distributions were measured at the levels of the breasts and the diaphragm, and longitudinal dose distributions were measured from the thoracic-inlet level to the diaphragm level at the center and periphery of the phantoms by use of eight solid-state detectors. Image noise was quantitatively measured continuously from the top to the bottom images of the phantoms. With the male phantom, the percentage of average absorbed dose with the xyz-TCM mode compared to the z-TCM mode was 90.2 % at the level of the nipples. This value was significantly smaller than that for the female phantom (95.6 %, P < 0.0001). With either phantom, the percentage of absorbed doses in the longitudinal direction with the xyz-TCM mode compared to the z-TCM mode at the center of the phantom was almost the same as the percent ratio at the periphery of the phantom. Therefore, the effect of xyz-TCM was less pronounced with the female phantom, especially on the reduction of the breast dose. The increase of image noise at the level of the supraclavicular fossa (in the male phantom) and at the level of the diaphragm (both phantoms) could not be avoided with the use of TCM techniques.

Measurement of gantry rotation time in modern ct.

The purpose of this study was to develop and evaluate a noninvasive method to assess rotation time in modern commercial computed tomography (CT) systems. The rotation time was measured at a selected nominal rotation time (400 ms) utilizing two types of solid-state detectors: the RTI's CT Dose Profiler (CTDP) and Unfors' Xi (Xi) probes. Either CTDP or Xi was positioned on the inner cover of the gantry and a sheet of lead (1 mm thick) placed on top of the detector. Since a pair of two successive peaks is used to determine the gantry rotation time, by necessity the helical scan must be employed. Upon completion of the data acquisition, these peak times were determined with the dedicated software to obtain rotation time. The average rotation time obtained with CTDP and Xi operated under the dedicated software was found to be 400.6 and 400.5 ms, respectively. The detector for this measurement need not be specifically designed for CT dosimetry. The measurements of CT scanner rotation time can be accomplished with a radiation probe designed for the CT application or a conventional radiation probe designed for radiography and fluoroscopy applications. It is also noteworthy to point out that the measurement results are in good agreement between the two radiation detector systems. Finally, clinical medical physicists should be aware of the accuracy and precision of gantry rotation time, and take into consideration for QA where and when applicable.