Evaluation of different techniques for CT radiation profile width measurement

The use of pencil ionization chamber with temporal readout capabilities to measure CT beam full width half maximum

BACKGROUND

Measurement of Computed Tomography (CT) beam width is required by accrediting and regulating bodies for routine physics evaluations due to its direct correlation to patient dose. Current methods for performing CT beam width measurement require special hardware, software, and/or consumable films. Today, most 100-mm pencil chambers with a digital interface used to evaluate Computed Tomography Dose Index (CTDIvol) have a sufficiently high sampling rate to reconstruct a high-resolution dose profile for any acquisition mode.

PURPOSE

The goal of this study is to measure the CT beam width from the sampled dose profile under a single helical acquisition with the 100-mm pencil chamber used for CTDIvol measurements.

METHODS

The dose profiles for different scanners were measured for helical scans with varying collimation settings using a 100-mm pencil chamber placed at the isocenter and co-moving with the patient table. The measured dose profiles from the 100-mm pencil chamber were corrected for table attenuation by extracting a periodic correction function (PCF) to eliminate table interference. The corrected dose profiles were then deconvolved with the response function of the chamber to compute the beam profile. The beam width was defined by the full width half maximum (FWHM) of the resulting beam profile. Reference dose profiles were also measured using Gafchromic film for comparison.

RESULTS

The beam widths, estimated using the innovative deconvolution method from the 100-mm pencil chamber, exhibit an average percentage difference of 1.6 ± 1.8 when compared with measurements obtained through Gafchromic film for beam width assessment.

CONCLUSION

The proposed approach to deconvolve the pencil chamber response demonstrates the potential of obtaining the CT beam width at high accuracy without the need of special hardware, software, or consumable films. This technique can improve workflow for routine performance evaluation of CT systems.

Influence of Pitch on Surface Dose Distribution and Image Noise of Computed Tomography Scans

This study evaluated the effect of pitch on 256-slice helical computed tomography (CT) scans. Cylindrical water phantoms (CWP) were measured using axial and helical scans with various pitch values. The surface dose distributions of CWP were measured, and reconstructed images were obtained using filtered back-projection (FBP) and iterative model reconstruction (IMR). The image noise in each reconstructed image was decomposed into a baseline component and another component that varied along the z-axis. The baseline component of the image noise was highest at the center of the reconstructed image and decreased toward the edges. The normalized 2D power spectra for each pitch were almost identically distributed. Furthermore, the ratios of the 2D power spectra for IMR and FBP at different pitch values were obtained. The magnitudes of the components varying along the z-axis were smallest at the center of the reconstructed image and increased toward the edge. The ratios of the 3D power spectra on the f_x axis for IMR and FBP at different pitch values were obtained. The results showed that the effect of the pitch was related to the component that varied along the z-axis. Furthermore, the pitch had a smaller effect on IMR than on FBP.

THE CONCEPT OF X-RAY CT DOSE EVALUATION METHOD USING RADIOCHROMIC FILM AND FILM-FOLDING PHANTOM

In this paper, we propose a novel radiochromic film (RCF)-based computed tomography (CT) dosimetry method, which is different from the method based on CT dose index. RCF dosimetry using Gafchromic QA2 films was performed using two lengths of film-folding phantoms. The phantom was exposed to X-ray CT through a single scan, while the RCF was sandwiched between the phantoms. We analysed the dose profile curve in two directions to investigate the dose distribution. We observed a difference in the dose distribution as the phantom size changed. Our results contradict with the results of previous studies such as Monte Carlo simulation or direct measurement. The ability to visually evaluate 2D dose distributions is an advantage of RCF dosimetry over other methods. This research investigated the ability of 2D X-ray CT dose evaluation using RCF and film-folding phantom.

[Measurement Accuracy of CT Beam Width with a Portable Flat-panel Detector]

PURPOSE

X-ray film or computed radiography (CR) system has been employed in clinical setting, and these devices are gradually replaced by portable flat-panel detector (FPD) systems. They may be employed to measure the beam width instead of the traditional CR system. In this study, we estimated the accuracy of beam width measured by the portable FPD system.

METHOD

A CR cassette and FPD were placed at the isocenter, and the pixel values were measured in a single axial CT scanning at a tube potential of 80 kVp, tube currents of 10-40 mA (5 mA steps), and tube rotation time of 0.5 s. Then, the FPD was sandwiched between 0.5 mm copper plate and 2 mm lead plate to avoid the pixel saturation and artifact from the FPD electronic substrates. The beam widths were measured at selected nominal beam widths (40, 80, 120 and 160 mm) using a double exposure technique (tube currents of 10 and 20 mA).

RESULT

Log-linear relationships for two systems were obtained between the pixel value and radiation exposure for parameters less than or equal to 12.5 mAs. A test for the equivalence with confidence intervals showed that the measurement accuracy of the CR and FPD systems was equivalent.

CONCLUSION

The portable FPD system could be utilized for the measurement of the CT beam width as well as CR system.

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.

An evaluation of computed tomography dose index measurements using a pencil ionisation chamber and small detectors

The aim of this study was to compare the values of the computed tomography dose index 100 (CTDI₁₀₀) obtained using two small detectors (i.e. a small ionisation chamber and a small solid state detector) with those obtained from a 100 mm pencil ionisation chamber for various input CT parameters: beam width, kVp, mAs, pitch, and head-body phantom variation. The measurement of CTDI₁₀₀ using the 100 mm pencil chamber was carried out in a single rotation of axial mode, while the measurement using small detectors was carried out in helical mode. The differences of CTDI₁₀₀ values obtained with two small detectors were about 7% for all variations. The differences of CTDI₁₀₀ values obtained with small detectors and a 100 mm pencil ionisation chamber for beam widths of more than 4 mm were within 40%. However, for the narrowest beam widths (4 mm), the difference between them was very large (about 150%).

CT radiation profile width measurement using CR imaging plate raw data

This technical note demonstrates computed tomography (CT) radiation profile measurement using computed radiography (CR) imaging plate raw data showing it is possible to perform the CT collimation width measurement using a single scan without saturating the imaging plate. Previously described methods require careful adjustments to the CR reader settings in order to avoid signal clipping in the CR processed image. CT radiation profile measurements were taken as part of routine quality control on 14 CT scanners from four vendors. CR cassettes were placed on the CT scanner bed, raised to isocenter, and leveled. Axial scans were taken at all available collimations, advancing the cassette for each scan. The CR plates were processed and raw CR data were analyzed using MATLAB scripts to measure collimation widths. The raw data approach was compared with previously established methodology. The quality control analysis scripts are released as open source using creative commons licensing. A log‐linear relationship was found between raw pixel value and air kerma, and raw data collimation width measurements were in agreement with CR‐processed, bit‐reduced data, using previously described methodology. The raw data approach, with intrinsically wider dynamic range, allows improved measurement flexibility and precision. As a result, we demonstrate a methodology for CT collimation width measurements using a single CT scan and without the need for CR scanning parameter adjustments which is more convenient for routine quality control work.

PACS numbers: 87.57.Q‐, 87.59.bd, 87.57.uq

Evaluation of an X-Ray Dose Profile Derived from an Optically Stimulated Luminescent Dosimeter during Computed Tomographic Fluoroscopy

The purpose of this study was to evaluate scatter radiation dose to the subject surface during X-ray computed tomography (CT) fluoroscopy using the integrated dose ratio (IDR) of an X-ray dose profile derived from an optically stimulated luminescent (OSL) dosimeter. We aimed to obtain quantitative evidence supporting the radiation protection methods used during previous CT fluoroscopy. A multislice CT scanner was used to perform this study. OSL dosimeters were placed on the top and the lateral side of the chest phantom so that the longitudinal direction of dosimeters was parallel to the orthogonal axis-to-slice plane for measurement of dose profiles in CT fluoroscopy. Measurement of fluoroscopic conditions was performed at 120 kVp and 80 kVp. Scatter radiation dose was evaluated by calculating the integrated dose determined by OSL dosimetry. The overall percent difference of the integrated doses between OSL dosimeters and ionization chamber was 5.92%. The ratio of the integrated dose of a 100-mm length area to its tails (−50 to −6 mm, 50 to 6 mm) was the lowest on the lateral side at 80 kVp and the highest on the top at 120 kVp. The IDRs for different measurement positions were larger at 120 kVp than at 80 kVp. Similarly, the IDRs for the tube voltage between the primary X-ray beam and scatter radiation was larger on the lateral side than on the top of the phantom. IDR evaluation suggested that the scatter radiation dose has a high dependence on the position and a low dependence on tube voltage relative to the primary X-ray beam for constant dose rate fluoroscopic conditions. These results provided quantitative evidence supporting the radiation protection methods used during CT fluoroscopy in previous studies.