Measurement of gantry rotation time in modern ct

Visualization of X-ray fields, overlaps, and over-beaming on surface of the head in spiral computed tomography using computer-aided design-based X-ray beam modeling

To visualize the X-ray fields, overlaps, and over-beaming on the skin surface during spiral head CT scanning. The measured pitch factors were determined by measuring 3 rotation times, 11 table-feed speeds, and an X-ray beam width. The X-ray fields, overlaps, and over-beaming on the skin surface were calculated via computer-aided design-based X-ray beam modeling, and the values obtained using the nominal pitch and measured pitch factors were compared. The X-ray fields with measured pitch factors exceeded those with nominal pitch factors. The overlaps increased with a decrease in the nominal pitch and measured pitch factors and were observed even at a nominal pitch factor of 1.0. The most stretched over-beaming field was observed with the measured pitch factor of 0.670. The technique can show the overlaps of the X-ray fields and may determine the adequate start angle to prevent overlaps to the eye lens.

Half-value layer measurements using solid-state detectors and single-rotation technique with lead apertures in spiral computed tomography with and without a tin filter

Solid-state detectors (SSDs) may be used along with a lead collimator for half-value layer (HVL) measurement using computed tomography (CT) with or without a tin filter. We aimed to compare HVL measurements obtained using three SSDs (AGMS-DM+ , X2 R/F sensor, and Black Piranha) with those obtained using the single-rotation technique with lead apertures (SRTLA). HVL measurements were performed using spiral CT at tube voltages of 70-140 kV without a tin filter and 100-140 kV (Sn 100-140 kV) with a tin filter in increments of 10 kV. For SRTLA, a 0.6-cc ionization chamber was suspended at the isocenter to measure the free-in-air kerma rate (K ˙ air ) values. Five apertures were made on the gantry cover using lead sheets, and four aluminum plates were placed on these apertures. HVLs in SRTLA were obtained fromK ˙ air decline curves. Subsequently, SSDs inserted into the lead collimator were placed on the gantry cover and used to measure HVLs. Maximum HVL differences of AGMS-DM+ , X2 R/F sensor, and Black Piranha with respect to SRTLA without/with a tin filter were - 0.09/0.6 (only two Sn 100-110 kV) mm, - 0.50/ - 0.6 mm, and - 0.17/(no data available) mm, respectively. These values were within the specification limit. SSDs inserted into the lead collimator could be used to measure HVL using spiral CT without a tin filter. HVLs could be measured with a tin filter using only the X2 R/F sensor, and further improvement of its calibration accuracy with respect to other SSDs is warranted.

Multimodal imaging approach for the diagnosis of intracranial atherosclerotic disease (ICAD): Basic principles, current and future perspectives

PURPOSE

To review the different imaging modalities utilized in the diagnosis of Intracranial Atherosclerotic Disease (ICAD) including their latest development and relevance in management of ICAD.

METHODS

A review of the literature was conducted through a search in google scholar, PubMed/Medline, EMBASE, Scopus, clinical trials.gov and the Cochrane Library. Search terms included, "imaging modalities in ICAD," "ICAD diagnostic," "Neuroimaging of ICAD," "Evaluation of ICAD". A summary and comparison of each modality's basic principles, advantages and disadvantages were included.

RESULTS

A total of 144 articles were identified and reviewed. The most common imaging used in ICAD diagnoses were DSA, CTA, MRA and TCD. They all had proven accuracy, their own benefits, and limitations. Newer modalities such as VWI, IVUS, OCT, PWI and CFD provide more detailed information regarding the vessel walls, plaque characteristics, and flow dynamics, which play a tremendous role in treatment guidance. In certain clinical scenarios, using more than one modality has been shown to be helpful in ICAD identification. The rapidly evolving software related to imaging studies, such as virtual histology, are very promising for the diagnostic and management of ICAD.

CONCLUSIONS

ICAD is a common cause of recurrent ischemic stroke. Its management can be both medical and/or procedural. Many different imaging modalities are used in its diagnosis. In certain clinical scenario, a combination of two more modalities can be critical in the management of ICAD. We expect that continuous development of imaging technique will lead to individualized and less invasive management with adequate outcome.

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 CTDI_vol (cCTDI_vol) dosimetry and evaluate the differences between the displayed CTDI_vol (dCTDI_vol), measured CTDI_vol (mCTDI_vol), and the cCTDI_vol.

METHODS AND MATERIALS

The each 8 rotation times between 275 and 1000ms of two CT scanners were investigated. Rotation time (T_rot) and the beam-on time (T_beam) in axial scanning were measured accurately to determine the gantry overrun time (T_over) as T_beam-T_rot. Subsequently, mCTDI_vol was measured by using a 100mm ionization chamber and CTDI phantoms. Furthermore, we introduced a gantry overrun correction factor (C_o=T_rot/T_beam) to obtain cCTDI_vol. Upon completion of the data acquisition, the dCTDI_vol and mCTDI_vol were compared with the cCTDI_vol.

RESULTS

The discrepancies of T_rot were 0.2±0.2ms as compared to the preset rotation times, and T_over was machine-specific and almost constant (22.4±0.5ms or 45.1±0.3ms) irrespective of the preset rotation time. Both dCTDI_vol and mCTDI_vol were increasingly overestimated compared to cCTDI_vol as the faster the preset rotation time was selected (1.7-23.5%).

CONCLUSION

The rotation time influences the accuracy of CTDI_vol in modern CT, and should be taken into consideration when assessing the radiation output in modern CT.

Measurement of image rotation angle in CT for radiotherapy treatment planning

The geometric accuracy of CT images is essential when they are used for planning radiation therapy. In this study, a method of quantitative testing of image rotation in CT is presented, based on the automated analysis of an image of a phantom half‐filled with water. A plug‐in was written for ImageJ software for automated detection of a water surface in an image and measurement of its angle relative to a horizontal line. A water phantom can be used for the evaluation of image rotation for axial mode. In helical mode the movement of the table would cause movement of the water. The difference between image rotation for axial and helical scans can be evaluated by measuring and comparing the angles of the tabletop surface for both modes. Preliminary results of measurements for three CT scanners show that image rotation does not exceed 0.5°, and is less than 0.1° for the dedicated CT simulator. It was observed that, for one CT scanner image, the rotation angle depended on tube rotation time.

PACS number(s): 87.57.Q‐, 87.55.Qr, 06.30.Bp

Temporal resolution measurement of 128-slice dual source and 320-row area detector computed tomography scanners in helical acquisition mode using the impulse method

PURPOSE

To analyse the temporal resolution (TR) of modern computed tomography (CT) scanners using the impulse method, and assess the actual maximum TR at respective helical acquisition modes.

METHODS

To assess the actual TR of helical acquisition modes of a 128-slice dual source CT (DSCT) scanner and a 320-row area detector CT (ADCT) scanner, we assessed the TRs of various acquisition combinations of a pitch factor (P) and gantry rotation time (R).

RESULTS

The TR of the helical acquisition modes for the 128-slice DSCT scanner continuously improved with a shorter gantry rotation time and greater pitch factor. However, for the 320-row ADCT scanner, the TR with a pitch factor of <1.0 was almost equal to the gantry rotation time, whereas with pitch factor of >1.0, it was approximately one half of the gantry rotation time. The maximum TR values of single- and dual-source helical acquisition modes for the 128-slice DSCT scanner were 0.138 (R/P=0.285/1.5) and 0.074s (R/P=0.285/3.2), and the maximum TR values of the 64×0.5- and 160×0.5-mm detector configurations of the helical acquisition modes for the 320-row ADCT scanner were 0.120 (R/P=0.275/1.375) and 0.195s (R/P=0.3/0.6), respectively.

CONCLUSION

Because the TR of a CT scanner is not accurately depicted in the specifications of the individual scanner, appropriate acquisition conditions should be determined based on the actual TR measurement.

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.

PACS number: 87.57.Q‐

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.