kVp Images Research Articles

Low kVp imaging for dose reduction in dual-source cardiac CT

With the widespread application of cardiac CT has come increasing concern over the effects of radiation dose associated with this exam. Dual source CT provides a number of methods for dose reduction in helical ECG-gated cardiac CT studies. This article discusses several of these methods, with a particular emphasis on low kVp scanning, which can be applied in a large percentage of patients.

The Image Quality and Radiation Dose of 100-kVp versus 120-kVp ECG-Gated 16-Slice CT Coronary Angiography

ObjectiveThis study was conducted to assess the feasibility of performing 100-kVp electrocardiogram (ECG)-gated coronary CT angiography, as compared to 120-kVp ECG-gated coronary CT angiography.Materials and MethodsWe retrospectively evaluated one hundred eighty five gender- and body mass index-matched 16-slice coronary CT sets of data, which were obtained using either 100 kVp and 620 effective mAs or 120 kVp and 500 effective mAs. The density measurements (image noise, vessel density, signal-to-noise ratio [SNR] and contrast-to-noise ratio [CNR]) and the estimated radiation dose were calculated. As a preference test, two image readers were independently asked to choose one image from each pair of images. The results of both protocols were compared using the paired t-test or the Wilcoxon signed rank test.ResultsThe 100-kVp images showed significantly more noise and a significantly higher vessel density than did the 120-kVp images. There were no significant differences in the SNR and CNR. The estimated reduction of the radiation dose for the 100-kVp protocol was 24%; 7.8 ± 0.4 mSV for 100-kVp and 10.1 ± 1.0 mSV for 120-kVp (p < 0.001). The readers preferred the 100-kVp images for reading (reader 1, p = 0.01; reader 2, p = 0.06), with their preferences being stronger when the subject's body mass index was less than 25.ConclusionReducing the tube kilovoltage from 120 to 100 kVp allows a significant reduction of the radiation dose without a significant change in the SNR and the CNR.

Image Fusion in Dual Energy Computed Tomography

The aim of this study was to evaluate the influence of different weighting factors on contrast enhancement, signal-to-noise ratio (SNR), and image quality in image fusion in dual energy computed tomography (DECT) angiography. Fifteen patients underwent a CT angiography of the aorta with a SOMATOM Definition Dual Source CT (DSCT; Siemens, Forchheim, Germany) in dual energy mode (DECT) (tube voltage: 80 and 140 kVp; tube current: 297 eff. mA and 70 eff. mA; collimation, 14 x 1.2 mm). Raw data were reconstructed using a soft convolution kernel (D30f). Fused images were calculated using a spectrum of weighting factors (0.0, 0.1, 0.3, 0.5, 0.7, 0.9, and 1.0) generating different ratios between the 80- and 140-kVp images (eg, factor 0.5 corresponds to 50% image information from the 140- and the 80-kVp image). Both CT values and SNR were measured in the descending aorta (levels of celiac trunk, renal arteries, and aortic bifurcation), in the right and left common iliac artery and in paraaortal fat. Image quality was evaluated using a 5-point grading scale. Results were compared using paired t-tests and nonparametric paired Wilcoxon tests. Statistically significant increases in mean CT values were seen in vessels when increasing weighting factors were used (all P <or= 0.001). For example, mean CT values derived from the aorta at the level of the celiac trunk were 273.8 +/- 25.8 Hounsfield units (HU), 304.0 +/- 24.3 HU, 361.4 +/- 22.5 HU, 418.3 +/- 25.8 HU, 477.8 +/- 32.2 HU, 536.2 +/- 41.2 HU, 564.6 +/- 45.3 HU, when the weighting factors 0.0, 0.1, 0.3, 0.5, 0.7, 0.9, and 1.0 were used. The highest SNR values were found in vessels when the weighting factor 0.5 was used. The highest SNR values of the paraaortal fat were obtained for the weighting factors 0.3 and 0.5. Visual image assessment for image quality showed the highest score for the data reconstructed using the weighting factor 0.5. Different weighting factors used to create fused images in DECT cause statistically significant differences in CT value, SNR, and image quality. Best results were obtained using the weighting factor 0.5, which we recommend for image fusion in DECT angiography.

Radiation reduction with prospective ECG-triggering acquisition using 64-multidetector computed tomographic angiography

Current 64-multidetector Computed Tomographic scanners (MDCT) utilize retrospective overlapping helical acquisition (RS-OHA) which imparts a higher than desired radiation dose. Although the radiation burden of computed tomographic angiography (CTA) can be efficiently reduced by dose modulation and limiting field of view, a further decrease in radiation without compromising diagnostic image quality would be indeed very desirable. An alternative imaging mode is the axial prospective ECG-triggering acquisition (prospective gating). This study was done to compare the effective radiation dose and the image quality with two techniques to reduce radiation doses with CTA studies utilizing 64-MDCT scanners. The study included 149 consecutive patients (48 females and 101 males) 64-MDCT (mean age = 67 +/- 11 years, 72.2% male). Patients underwent CT coronary angiography using one of three algorithms: retrospective triggering with dose modulation; prospective triggering with padding (step and shoot acquisition with additional adjacent phases); and prospective triggering without padding (single phase acquisition only). Based on body habitus, two different voltages were utilized: 100 kVp (<85 kg) or 120 kVp (>85 kg). Radiation doses and image quality (signal to noise ratio) was measured for each patient, and compared between different acquisition protocols. The signal to-noise ratio of the ascending aorta (SNR-AA) was calculated from the mean pixel values of the contrast-filled left ventricular chamber divided by the standard deviation of these pixel values. Use of 100 kVp reduced radiation dose 41.5% using prospective triggering and 39.6% using retrospective imaging as compared to 120 kVp (P < 0.001). Use of prospective imaging reduced radiation exposure by 82.6% as compared to retrospective imaging (P < 0.001). Using both prospective imaging and 100 kVp without padding (single phase data, no other phases obtained), radiation dose was reduced by 90% (P < 0.001). In terms of image quality, the coefficient of variation of ascending aortic contrast enhancement between kVp of 120 and kVp of 100 was 6% (1.05, 95 CI 0.93-1.17), and 7.8% (0.9, 95% CI 0.7-1.2) at the pulmonary artery. The prospective ECG-Triggered acquisition and 100 kVp images were of diagnostic quality, allowing adequate assessment in all patients. CTA using PA and 100 kVp reduced the radiation dose by up to 90% without compromising the image quality.

SU-GG-J-32: Analysis of Imaging Doses for Optimal Management of 4D-IGRT Treatments

Purpose: Introducing 4D‐IGRT potentially increases an accumulated dose to patients from imaging and verification processes compared to conventional practice. It is therefore essential to investigate the level of imaging dose to patients when 4D‐IGRT devices are installed. We monitored the imaging dose level and compared with that of pre‐IGRT practice. Method and Materials: A 4D‐CT (GE, Ultra Light Speed 16), a simulator (Varian Acuity), and a linear accelerator (Varian IX) equipped with kVp (OBI) and MVp (aSi 1000) imaging devices were installed for 4D‐IGRT. The surface doses to a RANDO phantom were measured with newly installed devices and with pre‐existing devices; single slice CT(GE,Light Speed), a simulator (Varian Ximatron) and L‐gram with a Varian 2100CLinac. The surface doses were measured using TLDs (HASHOW, Model T‐100) at 8 different sites of the phantom; the brain, eye, thyroid, chest, abdomen, ovary, prostate, pelvis. Results: Compared to the imaging with single slice non‐gated CT, the gated MDCT imaging increased the dose to the chest and abdomen more than tenfold (1.74 ± 0.34 vs 23.23 ± 3.67 cGy ). But the imaging doses with Acuity at all measured sites were smaller than those with Ximatron due to a function that reduced irradiation time for fluoroscopy, which were 0.91 ± 0.89 vs 6.77 ± 3.56 cGy, respectively. The portal imaging doses with MVp EPID were about the half of the dose with conventional L‐gram (1.83 ± 0.36 vs 3.80 ± 1.67 cGy). The dose from OBI was 0.97 ± 0.34 cGy for fluoroscopy mode. Conclusion: Gated CT is the major source of increasing the imaging dose to patients. OBI imaging dose was small, but the accumulated dose associated with everyday verification for accurate treatments needs to be taken into account.

Improvements in Patient Alignment With Stereoscopic kVp Image Guidance During Linac-Based Intracranial Stereotactic Radiotherapy–A Prospective Study

Improvements in Patient Alignment With Stereoscopic kVp Image Guidance During Linac-Based Intracranial Stereotactic Radiotherapy–A Prospective Study

WE-E-L100J-06: Simulation Studies On Contrast Enhanced Dual Energy Mammography

Purpose: Technique optimization for contrast-enhanced dual energy mammography in a breast CT scanner. Method and Materials: Using kVp accuracy and HVL measurements physically performed on the x-ray source of a dedicated cone beam breast CT system, a mathematical spectral model was calibrated to have the same spectral and output properties as the physical system. Mathematical breast phantoms were fabricated by generating random sized and randomly placed glandular spheres on an adipose background to approximate the anatomical noise of the breast. The simulated imaging geometry matched that of the breast CT system. Quantum noise was simulated using Poisson model and a validated random number generator. Contrast-enhanced dual energy mammography was optimized for parameters including kVp, x-ray spectral filtration, and mAs distribution between high and low energy images. Mean glandular dose to the breast was calculated using dose coefficients generated from previously published Monte Carlo simulations. A figure of merit of the contrast-to-noise ratio at constant mean glandular dose was chosen for the optimization task to maximize image quality at minimal radiation dose levels. Optimization was carried out over a range of breast diameters, breast compositions, and iodine concentrations. Results: Spectra study suggested the optimal kVp to be between 120kV and 140kV for the high energy image, and between 50kV and 60kV for the low energy image, for the studied ranges of breast diameters and compositions. Added copper filtration up to 1mm for the high kVp spectra continued to improve CNR at set dose levels. Iodine as diluted as 0.05% of the 350mg/mL standard solution could be detected with a 2.5mGy total dose. Optimized mAs distribution suggested a higher dose proportion for the low kVp image than for the high kVp image. Conclusion: An optimized contrast-enhanced dual energy mammography can efficiently suppress anatomical noise and significantly improve contrast detection.

SU‐FF‐J‐21: A Comparison of a Point Based Tool with An Image Overlay Tool for Fiducial Based Setup

Purpose: To compare interactive image overlay tools for fiducial based setup with a point based system. Method and Materials: A single implanted fiducial was used for localization. Gated kVp images were acquired for daily setup; if a shift was made, a 2nd set of images were acquired subsequent to the shift. Shifts were determined using both a commercial software package designed for point matching (ISOLOC, CIVCO, Kalona, IA) and also by dragging the projection of the fiducial contour over the image of the fiducial using tools integrated into the treatment delivery console(4DTC, Varian, Palo Alto, CA). Data was collected for 29 shifts on 15 different days. Results: The mean magnitude of the initial shifts were 1.5, 5.7, and 1.5 mm and the means of the differences between the shifts were 0.2, 1.1 and 0.8 mm in the lateral, cranial‐caudal, and AP directions respectively. On many days there was a discrepancy of 1–2 mm between the AP and lateral gated kVp X‐rays. The interactive tools allowed the user to compromise between the two projections, while the automatic system forced a compromise. The interactive tools also displayed a projection of the tumor allowing validation of the position of the fiducial with respect to the bulk tumor. The interactive tools were quicker to use and automatically applied the shifts; these tools are somewhat limited by the accuracy of the contouring of the fiducials. Conclusion: Both the interactive tools and the automatic system are practical for daily setup of patients with fiducials, but the integration into the console, the speed of localization, and most importantly the ability to correlate the fiducials with radioopaque anatomy make the interactive tools superior.

Po-Thur Eve General-14: 3rd Generation Co-60 based Megavoltage Computed Tomography

Patient set up verification is a necessary step for high precision radiation therapy. There has been considerable work to develop such verification using megavoltage (MV) imaging on Electronic Portal Imaging Devices (EPIDs). Recently, MV image guidance has been advanced with the extension to MV computed tomography (MVCT). At the CCSEO we have been investigating MVCT with a Cobalt-60 unit, in the context of developing Cobalt-60 tomotherapy. The system is comprised of an automated rotary stage added to a Theratronics T780C Co-60 unit; recently our imaging has advanced with the development of a 3rd generation imaging system using a Varian PortalVision LC250 EPID. The EPID was calibrated for relative dose-response using a parallel-plate ion chamber; the calibration curve follows a quadratic form. The image reconstruction is based on a generalized Feldkamp technique; executed in MATLAB. Consistent with previous 1st generation studies, results on various phantoms show that Co-60 based EPID — MVCT is free from beam hardening and metallic artifacts. Phantoms included a variety of orthopaedic hip implants, Rando head phantom with stainless steel and tungsten inserts, and a plug phantom containing materials of known atomic number. EPID determined MVCT numbers also correspond linearly with relative electron density. The work will summarize the differences between kVp and MV CT images, while highlighting the improvements of EPID based imaging over 1st generation Cobalt MVCT.

Resolution enhancement in digital x-ray imaging

We have developed a restoration method for radiographs that enhances image sharpness and reveals bone microstructures that were initially hidden in the soft-tissue glare. The method is two fold: the image is first deconvolved using the Richardson–Lucy algorithm and is then divided with a signal modelling the soft-tissue distribution to increase the overall contrast. Each step has its own merits but the power of the restoration method lies in their combination. The originality of the method is its reliance on a priori information at each step in the processing. We have measured and modelled analytically the point-spread function of a low-dose gas microstrip x-ray detector at several beam energies. We measured the relationship between the local image intensity and the noise variance for these images. The soft-tissue signal was also modelled using a minimum-curvature filtering technique. These results were then combined into an image deconvolution procedure that uses wavelet filtering to reduce restoration noise while keeping the enhanced small-scale features. The method was applied successfully to images of a human-torso phantom and improved the contrast of small details on the bones and in the soft tissues. We measured a mean 54% increase in signal to noise ratio and a mean 105% increase in contrast to noise ratio in the 70 and 140 kVp images we analysed. The method was designed to facilitate the analysis of radiographs by relying on two levels of visual inspection. The contrast of the full image is first enhanced by division with the signal modelling the soft-tissue distribution. Based on the result, a radiologist might decide to zoom in on a given image section. The full restoration method is then applied to that region of interest. Indeed, full image deconvolution is often unnecessary since enhanced small-scale details are not visible at large scale; only the section of interest is processed which is more efficient.

Comparison of the Image Quality on Low-dose Urinary Tract Multidetector CT Examinations with 80 kVp and 120 kVp (Original Research)

Purpose: To compare the image quality on low-dose urinary tract multidetector CT examinations with 80 kVp and 120 kVp. Materials and Methods: Seventy-six patients with urinary tract symptoms were evaluated for urinary tract calculi by unenhanced multidetector CT. The examinations were performed with a four-row detector CT scanner. In all patients, two low-dose CT examinations with different kVp and mAs values (120 kVp, 30 mAs and 80 kVp, 95 mAs) were performed. The other scanning parameters were kept constant. Weighted-CT dose index was equal in both examinations. Both CT examinations were quantitatively compared with calculating the HU values of kidneys, subcutaneous fat, calculus, and phleboliths and the HU ratio of urinary calculi to the kidney. Both scans were also qualitatively compared for image quality of upper and lower abdomen. Results: One hundred and two calculi were detected in 50 of 76 patients. One or more phleboliths were present in 20 patients. HU values of the phleboliths and urinary calculi were higher on images obtained with 80 kVp than those obtained with 120 kVp. HU ratio of the calculi to the kidney was higher on scans with 80 kVp. Adipose tissue was less noisy and had lower HU value on 80 kVp images. Image quality of the upper abdomen and major pelvis were better on 80 kVp images, and the majority of these patients were normal (n=28) and overweight (n=20). Minor pelvis mostly (60%) revealed poorer image quality on 80 kVp images. However, the majority were overweight (n=22) or obese (n=6). In patients with osteoporosis, image quality was relatively better when compared with similar weighed patients without osteoporosis on 80 kVp images. Small renal, proximal and middle ureteral calculi were better demarcated on 80 kVp images. Conclusion: Low-dose urinary tract multidetector CT examination with 80 kVp, compared to 120 kVp provides better image quality in the upper abdomen and major pelvis. In overweight and obese patients, minor pelvis may not be evaluated thoroughly on 80 kVp images. Detection of small renal, proximal and middle ureteral calculi could be improved by using 80 kVp setting. Keywords: urinary calculi, low-dose multidetector ct, lower kilovoltage, 80 kvp

Detection of glass contamination in horticultural peat with dual-energy X-ray absorptiometry (DXA)

The application of dual-energy X-ray absorptiometry (DXA) to the detection of glass contamination in horticultural peat was studied using contaminated peat specimens, imaged with X-ray energies of 40 and 70 kVp and without any physical disruption of the peat samples. The source was an uncollimated beam from an industrial X-ray non-destructive testing system. X-ray transmission characteristics of peat and glass were determined from chemical composition data using XCOM software. The DXA images were analysed for detectivity figure (DF), signal-to-noise ratio (SNR) and contrast. These quantities were compared with those of the unprocessed 40 kVp images. There was >75% detection of glass presence if thicker than 3 mm, but 100% detection of glass fragments thicker than 1 mm. It was not possible to distinguish between glass and stone. DF values were dependent on peat and glass type, and contrast more than doubled with the DXA-derived images compared with their 40 kVp counterparts. Comparison with reported figures for a luggage inspection system shows that this system performs better than expected for commercial security systems. Better DXA results may be possible using simultaneous energy detection, and such a multi-energy sensing system could allow glass/stone detection as part of peat quality assurance, and the determination of peat chemical composition.

Lung Nodules: Dual-Kilovolt Peak Analysis with CT—Multicenter Study

To test the following hypothesis: The greater the increase in the mean computed tomographic (CT) number of a radiologically indeterminate lung nodule from the CT number on a 140-kVp CT image to that on an 80-kVp CT image, the more likely the nodule is benign (ie, contains calcium). Two hundred forty indeterminate lung nodules were prospectively studied at four institutions: Mayo Clinic Scottsdale, Ariz (n = 160); Mayo Clinic Rochester, Minn (n = 50); Shiga Health Insurance Hospital, Otsu, Japan (n = 25); and Duke University Medical Center, Durham, NC (n = 5). Of the 240 nodules, 157 met the entrance criteria for this study and had a diagnosis. All nodules included were solid, 5-40-mm diameter, relatively spherical, homogeneous, and without visible evidence of calcification or fat. Each nodule was evaluated by using 3-mm-collimation, nonenhanced CT scans with both 140- and 80-kVp x-ray beams. There were 86 (55%) benign and 71 (45%) malignant nodules. The median increase in the nodule mean CT number from the CT number on 140-kVp images to that on 80-kVp images was 2 HU for benign nodules and 3 HU for malignant nodules. This difference was not statistically significant. The area under the receiver operating characteristic curve was 0.505. Dual-kilovolt peak analysis with current CT technology does not appear to be helpful in the identification of benign lung nodules.

The Status of Film/Screen Mammography

Results of the Breast Exposure: Nationwide Trends (BENT) study are presented. In this study, data were collected from 42 states and several other jurisdictions and the number of facilities found to have unusually high or low exposures during mammography was determined. Surveyors trained by representatives of the Bureau of Radiological Health then worked with these facilities to determine the source of the problem and recommend corrective measures. Contrast was found to be reduced by high "inherent" filtration, high kVp, or inappropriate imaging conditions, while resolution was impaired by excessive exposure time, as well as inappropriate imaging conditions. These and other problems are discussed and correlated with image quality.