Large Phantom Research Articles

Technical Note: T1 and T2 and complex permittivities of mineral oil, silicone oil, and glycerol at 0.35, 1.5, and 3 T.

To identify an inexpensive, low-dielectric liquid for large magnetic resonance imaging (MRI) phantoms that can be used at multiple magnetic field strengths. The T1 and T2 of four candidate phantom liquids (pure mineral oil, food-grade white mineral oil, silicone oil, and glycerol) with low dielectric constants were measured at three field strengths (0.35, 1.5, and 3T) and extrapolated for 7T. The complex permittivities of the liquids were measured for frequencies from 13 to 600MHz. Proton densities were calculated based on molecular weight, proton number, and density. The results were compared to the American College of Radiology (ACR) large MRI phantom electrolyte liquid (10mM NiCl2 and 75mM NaCl in water) and deionized water. The liquids were evaluated based on the NEMA standards (T1 <1200ms, T2 >50ms, proton density within 20% of water, and produces minimal dielectric artifacts). The radiofrequency (RF) wavelengths were computed for each liquid at the four field strengths to determine the risk of dielectric artifacts. The mineral oils were the only liquids to satisfy all of the NEMA guidelines. Excluding deionized water, silicone oil had the longest T1 and T2 , and was the most expensive liquid ($200/L). Glycerol had the shortest T1 and T2 , and the highest dielectric (excluding the ACR phantom electrolyte and deionized water). All of the liquids except silicone oil met the NEMA proton density guidelines. Food-grade white mineral oil is a good candidate for use in a phantom due to its relaxation times, low dielectric, high proton density, and low cost. Glycerol and deionized water are poorchoices for phantom liquids due to their relaxation times and high dielectric constants.

Funicular Flexible Crawler for Colonoscopy

The number of people with cancer of the large intestine has been increasing steadily. According to the global cancer observatory, its prevalence is second among all cancer types in world statistics over the past five years. Early detection and treatment of diseases by colonoscopy can enable complete recovery. Nevertheless, conventional colonoscopes require extremely high skill of the doctors who use them. Moreover, their invasiveness in the colon is high. Therefore, many robots to facilitate colonoscope-related operations have been developed recently. Unfortunately, no practical system exists which can truly overcome the proven effectiveness and lower costs presented by conventional colonoscopes. We therefore propose a novel autonomous propulsion mechanism for colonoscopy named the “Funicular Flexible Crawler.” It is suitable for propulsion through narrow spaces because it is driven by multiple small diameter crawler units connected to a flexible shaft. Experimentation with a large intestine phantom confirmed that the flexible crawler can run from the rectum to the cecum. As described herein, this propulsion mechanism is applicable for self-traveling colonoscopy.

Radiation dose and image quality comparison during spine surgery with two different, intraoperative 3D imaging navigation systems.

Careful protocol selection is required during intraoperative three‐dimensional (3D) imaging for spine surgery to manage patient radiation dose and achieve clinical image quality. Radiation dose and image quality of a Medtronic O‐arm commonly used during spine surgery, and a Philips hybrid operating room equipped with XperCT C‐arm 3D cone‐beam CT (hCBCT) are compared. The mobile O‐arm (mCBCT) offers three different radiation dose settings (low, standard, and high), for four different patient sizes (small, medium, large, and extra large). The patient's radiation dose rate is constant during the entire 3D scan. In contrast, C‐CBCT spine imaging uses three different field of views (27, 37, and 48 cm) using automatic exposure control (AEC) that modulates the patient's radiation dose rate during the 3D scan based on changing patient thickness. hCBCT uses additional x‐ray beam filtration. Small, medium, and large trunk phantoms designed to mimic spine and soft tissue were imaged to assess radiation dose and image quality of the two systems. The estimated measured “patient” dose for the small, medium, and large phantoms imaged by the mCBCT considering all the dose settings ranged from 9.4–27.6 mGy, 8.9–33.3 mGy, and 13.8–40.6 mGy, respectively. The “patient” dose values for the same phantoms imaged with hCBCT were 2.8–4.6 mGy, 5.7–10.0 mGy, and 11.0–15.2 mGy. The CNR for the small, medium, and large phantoms was 2.9 to 3.7, 2.0 to 3.0, and 2.5 to 2.6 times higher with the hCBCT system, respectively. Hounsfield unit accuracy, noise, and uniformity of hCBCT exceeded the performance of the mCBCT; spatial resolution was comparable. Added x‐ray beam filtration and AEC capability achieved clinical image quality for intraoperative spine surgery at reduced radiation dose to the patient in comparison to a reference O‐arm system without these capabilities.

Palpreast—A New Wearable Device for Breast Self-Examination

Breast cancer is the most commonly diagnosed cancer in women worldwide. Although targeted screening programs using mammography have facilitated earlier detection and improved treatment has resulted in a significant reduction in mortality, some negative aspects related to cost, the availability of trained staff, the duration of the procedure, and its non-generalizability to all women must be taken into consideration. Breast palpation is a simple non-invasive procedure that can be performed by lay individuals for detecting possible malignant nodules in the breast. It is a simple test, based on the haptic perception of different stiffness between healthy and abnormal tissues. According to a survey we carried out, despite being safe and simple, breast self-examination is not carried by women because they are not confident of their ability to detect a lump. In this study, a non-invasive wearable device designed to mimic the process of breast self-examination using pressure sensing textiles and thus increase the confidence and self-awareness of women is proposed. Combined with other screening methods, the device can increase the odds of early detection for better prognosis. Here, we present the physical implementation of the device and a finite element analysis of the mechanics underlying its working principle. Characterization of the device using models of large and medium breast phantoms with rigid inclusions demonstrates that it can detect nodules in much the same way as does the human hand during breast self-examination.

Evaluating the impact of extended field‐of‐view CT reconstructions on CT values and dosimetric accuracy for radiation therapy

Wide bore CT scanners use extended field-of-view (eFOV) reconstruction algorithms to attempt to recreate tissue truncated due to large patient habitus. Radiation therapy planning systems rely on accurate CT numbers in order to correctly plan and calculate radiation dose. This study looks at the impact of eFOV reconstructions on CT numbers and radiation dose calculations in real patient geometries. A large modular phantom based on real patient geometries was created to surround a CIRS Model 062M phantom. The modular sections included a smooth patient surface, a skin fold in the patient surface, and the addition of arms for simulation of the patient in arms up or arms down position. This phantom was used to evaluate the accuracy of CT numbers for three extended FOV algorithms implemented on Siemens CT scanners: eFOV, HDFOV, and HDProFOV. Six different configurations of the phantoms were scanned and images were reconstructed for the three different extended FOV algorithms. The CIRS phantom inserts and overall phantom geometry were contoured in each image, and the Hounsfield units (HU) numbers were compared to an image of the phantom within the standard scan FOV (sFOV) without the modular sections. To evaluate the effect on dose calculations, six radiotherapy patients previously treated at our institution (three head and neck and three chest/pelvis) whose body circumferences extended past the 50cm sFOV in the treatment planning CT were used. Images acquired on a Siemens Sensation Open scanner were reconstructed using sFOV, eFOV and HDFOV algorithms. A physician and dosimetrist identified the radiation target, critical organs, and external patient contour. A benchmark CT was created for each patient, consisting of an average of the 3 CT reconstructions with a density override applied to regions containing truncation artifacts. The benchmark CT was used to create an optimal radiation treatment plan. The plan was copied onto each CT reconstruction without density override and dose was recalculated. Tissue extending past the sFOV impacts the HU numbers for tissues inside and outside the sFOV when using extended FOV reconstructions. On average, the HU for all CIRS density inserts in the arms up (arms down) position varied by 43HU (67HU), 39HU (73HU), and 18HU (51HU) for the eFOV, HDFOV, and HDProFOV scans, respectively. In the patient dose calculations, patients with a smooth patient contour had the least deviation from the benchmark in the HDFOV (0.1-0.5%) compared to eFOV (0.4-1.8%) reconstructions. In cases with large amounts of tissue and irregular skin folds, the eFOV deviated the least from the benchmark (range 0.2-0.6% dose difference) compared to HDFOV (range 1.3-1.8% dose difference). All reconstruction algorithms demonstrated good CT number accuracy in the center of the image. Larger artifacts are seen near and extending outside the scan FOV, however, dose calculations performed using typical beam arrangements using the extended FOV reconstructions were still mostly within 2.5% of best estimated reference values.

132. Study of the dosimetric robustness of PVA-GTA based Fricke gels against manufacture parameters

PurposeFricke gel dosimeters (FGDs) based on poly-vinyl alcohol (PVA) as gelling agent and glutaraldehyde (GTA) as cross-linker, were developed with the aim to overcome the drawbacks affecting the FGDs prepared with traditional natural gel matrices like gelatin or agarose [1]. Here, we investigate the dosimetric robustness of PVA-GTA FGDs against parameters influencing their preparation, like pH and temperature of gelling. Materials and MethodsPVA-GTA matrices undergo gelling by heating instead of cooling. Gelling temperatures from 6 °C to 40 °C were considered. FGDs with PVA and GTA concentrations of 10% w/w and 1% w/w respectively, were arranged preparing Fricke solutions with different amounts of sulfuric acid in the interval 18–100 mM. FGDs in spectrophotometry cuvettes (10 mm optical path) were uniformly irradiated with a 137Cs source. The optical absorbance (OA) properties and dose-response in the interval 0–35 Gy were investigated. Furthermore, FGDs in form of thin layers (3 mm optical path, area 10 × 5 cm2) were irradiated with 80 kV X-rays producing a steep dose gradient. Light transmittance images were acquired before irradiation and at consecutive times up to 6 h post-irradiation to derive the Fe3+ diffusion coefficient. ResultsOA spectra and dose-response curves of FGDs were independent of the gelling temperature. A sensitivity of approximately 0.073 Gy-1 was obtained. By contrast, as in traditional FGDs, sulfuric acid concentration significantly affected the OA spectra as well as the sensitivity and linearity of the PVA-GTA FGDs. Both gelling temperature gradient and pH did not influence the Fe3+ diffusion coefficient, assessed equal to approximately 0.22 mm2/h, i.e. more than two times lower than in traditional FGDs. ConclusionsThe robustness of PVA-GTA-FGDs against preparation parameters represent a significant advantage over traditional FGDs. In particular, the response independence of the gelling temperature may enable the manufacture of large FGD phantoms without any gradient in sensitivity.

Assessment the effect of CT localizer parameter on radiation dose and image quality of GE CT spiral images, A phantom study

Introduction: Using computed tomography (CT) is increasing significantly in Iran and worldwide. Reduction of CT's radiation dose is a critical concern. The aim of this study was evaluating the effect of localizer kVp on spiral CT images when the tube current modulation (TCM) is activated. Materials and Methods: A special wedding cake phantom with circular intersection and diameters of 15 to 40 cm was scanned on a GE VCT 64 slice scanner. The whole length of phantom (84cm) was scanned with kVps of 80,100,120 and 140. The image noise was measured in SD of Ct numbers and the radiation dose was measured in term of CTDIvol. The tube current variation through the Z-Axis was extracted from DICOM images. Results: Function of TCM was affected by changing the scout view parameters. By increasing the localizer kVp the average used tube current was decreased and image noise was increased (8-32%, p<0.05); especially for larger phantom diameters and wide kVp difference between localizer and spiral kVps (up to 32%). On the other hand, the localizer with lower kVp than spiral leads to higher CTDIVOLs and lower image noises. The image noise was closer to chosen Noise Index when the localizer and spiral scan were in the same kVps. Conclusion: The attenuation of localizer X-ray is the criteria of tube current changing in spiral CT. The attenuation is variable in different kVps of X-ray. The ability of TCM system was the best in offering the constant image noise when the localizer and spiral scans were in the same kVp. Inappropriate localizer kVp may increase the patients’ radiation dose or degrade the image quality by miscalculations in TCM system

Escalation of optical transmittance and determination of diffusion co-efficient in low-bloom strength gelatin-based Fricke gel dosimeters

The goal of this work was to explore the action of ethylenediaminetetraacetic (EDTA) in pursuit of escalating the optical transmittance of the Fricke gel dosimeter prepared with low bloom strength gelatin. The Fricke-xylenol-gelatin (FXGConv) dosimeter recipe used in this study contained a mixture of 5% gelatin, 50 mM sulphuric acid (H2SO4), 0.3 mM ammonium ferrous sulphate (FAS), 0.05 mM xylenol orange (XO) and triple distilled water. Several batches of gels were prepared with varying concentrations of EDTA incorporated in the FXGConv gel. The gel cuvettes were stored at 4 °C for 12 h after preparation and irradiated using gamma rays from a telecobalt unit for doses ranging from 0 to 15 Gy. Dosimetric parameters including the optical background absorbance of the unirradiated gel, stability of dose after irradiation, self-oxygenation (auto-oxidation) and dose response were studied using a spectrophotometer at 585 nm. Based on the response of the EDTA infused FXGConv gels, further investigations were carried out on FXG-Glycine gels incorporated with EDTA. Diffusion coefficient measurements were performed on large volume phantoms irradiated in a linear accelerator. Optical transmittance of unirradiated FXG-Glycine gels improved approximately 40% with EDTA while maintaining acceptable control over Fe2+ auto-oxidation as well as Fe3+ diffusion. An optimal concentration of EDTA in the glycine-based FXG gel dosimeter can provide an overall benefit with low background absorbance and acceptable diffusion control while using lower bloom gelatin for preparation of gel dosimeters.

P020] The influence of phantom diameter on X-ray CT dose evaluation using radiochromic film

Purpose In recent years, one of most important topic for X-ray CT examination is reducing radiation exposure. We have tried to establish the simple and accurate CT dosimetry method using radiochromic (RC) film. In this study, we investigated how the phantom diameter affects the pixel value distribution of RC film. This research is one step of quantifying dose distribution of X-ray CT using RC film. Methods The radiochromic film was exposed by a clinical CT scanner (Aquilion Lightning, Toshiba Medical Systems). Gafchromic XR-QA2 film was irradiated. The film-folding phantom consists of two acrylic semicylinder. In this study, the large phantom (cylinder cross-section diameter is 16 cm) and the small phantom (cylinder cross-section diameter is 10 cm) were used in this research. The scan parameters were as follows: non-helical scan, tube voltage = 120 kV, tube current = 300 mA, slice thickness = 2 cm, scan speed = 1 s per rotation. Image processing and analysis were performed by MATLAB (The Mathworks, Inc.) and Image J (National Institute of Health). The irradiated films were scanned by a commercial scanner (EPSON ES-10000G, Seiko Epson Co.). Results To compare the pixel value distribution using two differently sized phantom, we used the contour line analysis. The differences of distribution were observed between the large phantom and the small phantom. It was found that the distribution using the large phantom changes clearly. Conclusions In conclusion, our method may be useful for analyzing the distribution of pixel value on various conditions.

Technical Note: Adapting a GE SIGNA PET/MR scanner for radiotherapy.

Simultaneous collection of PET and MR data for radiotherapy purposes are useful for, for example, target definition and dose escalations. However, a prerequisite for using PET/MR in the radiotherapy workflow is the ability to image the patient in treatment position. The aim of this work was to adapt a GE SIGNA PET/MR scanner to image patients for radiotherapy treatment planning and evaluate the impact on signal-to-noise (SNR) of the MR images, and the accuracy of the PET attenuation correction. A flat tabletop and a coil holder were developed to image patients in the treatment position, avoid patient contour deformation, and facilitate attenuation correction of flex coils. Attenuation corrections for the developed hardware and an anterior array flex coil were also measured and implemented to the PET/MR system to minimize PET quantitation errors. The reduction of SNR in the MR images due to the added distance between the coils and the patient was evaluated using a large homogenous saline-doped water phantom, and the activity quantitation errors in PET imaging were evaluated with and without the developed attenuation corrections. We showed that the activity quantitation errors in PET imaging were within ±5% when correcting for attenuation of the flat tabletop, coil holder, and flex coil. The SNR of the MRI images were reduced to 74% using the tabletop, and 66% using the tabletop and coil holders. We present a tabletop and coil holder for an anterior array coil to be used with a GE SIGNA PET/MR scanner, for scanning patients in the radiotherapy work flow. Implementing attenuation correction of the added hardware from the radiotherapy setup leads to acceptable PET image quantitation. The drop in SNR in MR images may require adjustment of the imaging protocols.

Task-based quantification of image quality using a model observer in abdominal CT: a multicentre study

ObjectiveWe investigated the variability in diagnostic information inherent in computed tomography (CT) images acquired at 68 different CT units, with the selected acquisition protocols aiming to answer the same clinical question.MethodsAn anthropomorphic abdominal phantom with two optional rings was scanned on 68 CT systems from 62 centres using the local clinical acquisition parameters of the portal venous phase for the detection of focal liver lesions. Low-contrast detectability (LCD) was assessed objectively with channelised Hotelling observer (CHO) using the receiver operating characteristic (ROC) paradigm. For each lesion size, the area under the ROC curve (AUC) was calculated and considered as a figure of merit. The volume computed tomography dose index (CTDIvol) was used to indicate radiation dose exposure.ResultsThe median CTDIvol used was 5.8 mGy, 10.5 mGy and 16.3 mGy for the small, medium and large phantoms, respectively. The median AUC obtained from clinical CT protocols was 0.96, 0.90 and 0.83 for the small, medium and large phantoms, respectively.ConclusionsOur study used a model observer to highlight the difference in image quality levels when dealing with the same clinical question. This difference was important and increased with growing phantom size, which generated large variations in patient exposure. In the end, a standardisation initiative may be launched to ensure comparable diagnostic information for well-defined clinical questions. The image quality requirements, related to the clinical question to be answered, should be the starting point of patient dose optimisation.Key Points• Model observers enable to assess image quality objectively based on clinical tasks.• Objective image quality assessment should always include several patient sizes.• Clinical diagnostic image quality should be the starting point for patient dose optimisation.• Dose optimisation by applying DRLs only is insufficient for ensuring clinical requirements.

Image quality of conventional images of dual-layer SPECTRAL CT: A phantom study.

Spectral CT using a dual layer detector offers the possibility of retrospectively introducing spectral information to conventional CT images. In theory, the dual-layer technology should not come with a dose or image quality penalty for conventional images. In this study, we evaluate the influence of a dual-layer detector (IQon Spectral CT, Philips Healthcare) on the image quality of conventional CT images, by comparing these images with those of a conventional but otherwise technically comparable single-layer CT scanner (Brilliance iCT, Philips Healthcare), by means of phantom experiments. For both CT scanners, conventional CT images were acquired using four adult scanning protocols: (a) body helical, (b) body axial, (c) head helical, and (d) head axial. A CATPHAN 600 phantom was scanned to conduct an assessment of image quality metrics at equivalent (CTDI) dose levels. Noise was characterized by means of noise power spectra (NPS) and standard deviation (SD) of a uniform region, and spatial resolution was evaluated with modulation transfer functions (MTF) of a tungsten wire. In addition, contrast-to-noise ratio (CNR), image uniformity, CT number linearity, slice thickness, slice spacing, and spatial linearity were measured and evaluated. Additional measurements of CNR, resolution and noise were performed in two larger phantoms. The resolution levels at 50%, 10%, and 5% MTF of the iCT and IQon showed small, but significant differences up to 0.25 lp/cm for body scans, and up to 0.2 lp/cm for head scans in favor of the IQon. The iCT and IQon showed perfect CT linearity for body scans, but for head scans both scanners showed an underestimation of the CT numbers of materials with a high opacity. Slice thickness was slightly overestimated for both scanners. Slice spacing was comparable and reconstructed correctly. In addition, spatial linearity was excellent for both scanners, with a maximum error of 0.11 mm. CNR was higher on the IQon compared to the iCT for both normal and larger phantoms with differences up to 0.51. Spatial resolution did not change with phantom size, but noise levels increased significantly. For head scans, IQon had a noise level that was significantly lower than the iCT, on the other hand IQon showed noise levels significantly higher than the iCT for body scans. Still, these differences were well within the specified range of performance of iCT scanners. At equivalent dose levels, this study showed similar quality of conventional images acquired on iCT and IQon for medium-sized phantoms and slightly degraded image quality for (very) large phantoms at lower tube voltages on the IQon. Accordingly, it may be concluded that the introduction of a dual-layer detector neither compromises image quality of conventional images nor increases radiation dose for normal-sized patients, and slightly degrades dose efficiency for large patients at 120 kVp and lower tube voltages.

Locoregional hyperthermia of deep-seated tumours applied with capacitive and radiative systems: a simulation study

Background: Locoregional hyperthermia is applied to deep-seated tumours in the pelvic region. Two very different heating techniques are often applied: capacitive and radiative heating. In this paper, numerical simulations are applied to compare the performance of both techniques in heating of deep-seated tumours.Methods: Phantom simulations were performed for small (30 × 20 × 50 cm3) and large (45 × 30 × 50 cm3), homogeneous fatless and inhomogeneous fat-muscle, tissue-equivalent phantoms with a central or eccentric target region. Radiative heating was simulated with the 70 MHz AMC-4 system and capacitive heating was simulated at 13.56 MHz. Simulations were performed for small fatless, small (i.e. fat layer typically <2 cm) and large (i.e. fat layer typically >3 cm) patients with cervix, prostate, bladder and rectum cancer. Temperature distributions were simulated using constant hyperthermic-level perfusion values with tissue constraints of 44 °C and compared for both heating techniques.Results: For the small homogeneous phantom, similar target heating was predicted with radiative and capacitive heating. For the large homogeneous phantom, most effective target heating was predicted with capacitive heating. For inhomogeneous phantoms, hot spots in the fat layer limit adequate capacitive heating, and simulated target temperatures with radiative heating were 2–4 °C higher. Patient simulations predicted therapeutic target temperatures with capacitive heating for fatless patients, but radiative heating was more robust for all tumour sites and patient sizes, yielding target temperatures 1–3 °C higher than those predicted for capacitive heating.Conclusion: Generally, radiative locoregional heating yields more favourable simulated temperature distributions for deep-seated pelvic tumours, compared with capacitive heating. Therapeutic temperatures are predicted for capacitive heating in patients with (almost) no fat.

Impact of immobilization base plates composed of various materials on the imaging quality of MR simulation during radiotherapy

Objective To compare the impact of immobilization base plates composed of 7 types of materials on the MR-simulation imaging quality used for radiotherapy, aiming to provide reference data for clinical applications. Methods Using identical T1 and T2 sequences of Siemens MR-simulator, the MR images of ACR Large Phantom were acquired on the Orfit carbon fiber laminate, polycarbonate (PC), high precision base plate (HP), Jinan Huayuxin BR, WR, KP and SP materials, respectively. The imaging quality without any plate was used as the baseline data. The following metrics were compared: 1.High-contrast spatial resolution: the sharpness of 3 pairs of hole arrays was observed, which represented resolutions of 1.1 mm, 1.0 mm and 0.9 mm on the LR and AP directions; 2.Image intensity uniformity in terms of percent integral uniformity (PIU): PIU=100×[1-((high-low)/(high+ low)]; 3. Low-contrast resolution: distinguishable spokes representing resolutions of 5.1%, 3.6%, 2.5% and 1.4% were recorded. According to ACR recommendations, high-contrast resolution of 1.0 mm, PIU of T1WI and T2>82%and low-contrast spoke difference<3 were considered as clinically acceptable. Results High-contrast T1WI and T2WI resolutions of no plate, PC plate, HP plate and KP material were all 0.9 mm, and those of the remaining materials were worse than 0.9 mm. The T1WI and T2WI PIU of no plate was>87%, and the PIU of carbon fiber plate was reduced by> 25%. The PIU decrease of remaining materials was within ±4%. Except the carbon fiber plate, the low-contrast spokes of the remaining materials were within ±3 compared with the reference. Conclusions In clinical settings, carbon fiber plate exerts the most significant effect upon the MR-simulation imaging quality, which is unsuitable for MR simulation. Imaging quality of PC and HP plates are consistent with that of no plate. BR and KP materials exert slight effect upon MR signal. The remaining materials are of potential values for the manufacture of immobilization devices and accessories. Key words: Magnetic resonance simulation image; Immobilization device material; Imaging quality; Quality assurance

Evaluating the repeatability and set-up sensitivity of a large field of view distortion phantom and software for magnetic resonance-only radiotherapy.

Background and purposeMagnetic Resonance (MR)-only radiotherapy requires geometrically accurate MR images over the full scanner Field of View (FoV). This study aimed to investigate the repeatability of distortion measurements made using a commercial large FoV phantom and analysis software and the sensitivity of these measurements to small set-up errors.Materials and methodsGeometric distortion was measured using a commercial phantom and software with 2D and 3D acquisition sequences on three different MR scanners. Two sets of repeatability measurements were made: three scans acquired without moving the phantom between scans (single set-up) and five scans acquired with the phantom re-set up in between each scan (repeated set-up). The set-up sensitivity was assessed by scanning the phantom with an intentional 1 mm lateral offset and independently an intentional 1° rotation.ResultsThe mean standard deviation of distortion for all phantom markers for the repeated set-up scans was for all scanners and sequences. For the lateral offset scan of the markers agreed within two standard deviations of the mean of the repeated set-up scan (median of all scanners and sequences, range 78%–93%). For the 1° rotation scan, 80% of markers agreed within two standard deviations of the mean (range 69%–93%).ConclusionsGeometric distortion measurements using a commercial phantom and associated software appear repeatable, although with some sensitivity to set-up errors. This suggests the phantom and software are appropriate for commissioning a MR-only radiotherapy workflow.

Quantitative analysis of image quality for acceptance and commissioning of an MRI simulator with a semiautomatic method.

Magnetic Resonance Imaging (MRI) simulation differs from diagnostic MRI in purpose, technical requirements, and implementation. We propose a semiautomatic method for image acceptance and commissioning for the scanner, the radiofrequency (RF) coils, and pulse sequences for an MRI simulator. The ACR MRI accreditation large phantom was used for image quality analysis with seven parameters. Standard ACR sequences with a split head coil were adopted to examine the scanner's basic performance. The performance of simulation RF coils were measured and compared using the standard sequence with different clinical diagnostic coils. We used simulation sequences with simulation coils to test the quality of image and advanced performance of the scanner. Codes and procedures were developed for semiautomatic image quality analysis. When using standard ACR sequences with a split head coil, image quality passed all ACR recommended criteria. The image intensity uniformity with a simulation RF coil decreased about 34% compared with the eight‐channel diagnostic head coil, while the other six image quality parameters were acceptable. Those two image quality parameters could be improved to more than 85% by built‐in intensity calibration methods. In the simulation sequences test, the contrast resolution was sensitive to the FOV and matrix settings. The geometric distortion of simulation sequences such as T1‐weighted and T2‐weighted images was well‐controlled in the isocenter and 10 cm off‐center within a range of ±1% (2 mm). We developed a semiautomatic image quality analysis method for quantitative evaluation of images and commissioning of an MRI simulator. The baseline performances of simulation RF coils and pulse sequences have been established for routine QA.

Phantom-based field maps for gradient nonlinearity correction in diffusion imaging.

Gradient coils in magnetic resonance imaging do not produce perfectly linear gradient fields. For diffusion imaging, the field nonlinearities cause the amplitude and direction of the applied diffusion gradients to vary over the field of view. This leads to site- and scan-specific systematic errors in estimated diffusion parameters such as diffusivity and anisotropy, reducing reliability especially in studies that take place over multiple sites. These errors can be substantially reduced if the actual scanner-specific gradient coil magnetic fields are known. The nonlinearity of the coil fields is measured by scanner manufacturers and used internally for geometric corrections, but obtaining and using the information for a specific scanner may be impractical for many sites that operate without special-purpose local engineering and research support. We have implemented an empirical field-mapping procedure using a large phantom combined with a solid harmonic approximation to the coil fields that is simple to perform and apply. Here we describe the accuracy and precision of the approach in reproducing manufacturer gold standard field maps and in reducing spatially varying errors in quantitative diffusion imaging for a specific scanner. Before correction, median B value error ranged from 33 - 41 relative to manufacturer specification at 100 mm from isocenter; correction reduced this to 0 - 4. On-axis spatial variation in the estimated mean diffusivity of an isotropic phantom was 2.2% - 4.1% within 60 mm of isocenter before correction, 0.5% - 1.6% after. Expected fractional anisotropy in the phantom was 0; highest estimated fractional anisotropy within 60 mm of isocenter was reduced from 0.024 to 0.012 in the phase encoding direction (48% reduction) and from 0.020 to 0.006 in the frequency encoding direction (72% reduction).

How Well Does Dual-energy CT with Fast Kilovoltage Switching Quantify CT Number and Iodine and Calcium Concentrations?

Because it is imperative for understanding the performance of dual-energy computed tomography scanner to determine clinical diagnosis, we aimed to assess the accuracy of quantitative measurements using dual-energy computed tomography with fast kilovoltage switching. Quantitative measurements were performed for 16 reference materials (physical density, 0.965-1.550 g/cm3; diameter of rod, 2.0-28.5 mm; iodine concentration, 2-15 mg/mL; and calcium concentration, 50-300 mg/mL) with varying scanning settings, and the measured values were compared to their theoretical values. For high-density material, the maximum differences in Hounsfield unit values in the virtual monochromatic images at 50, 70, and 100 keV were -176.2, 61.0, and -35.2 HU, respectively, and the standard deviations over short- and long-term periods were 11.1, 6.1, and 3.5 HU at maximum. The accuracy of the Hounsfield unit measurement at 50 and 70 keV was significantly higher (P < 0.05) with higher radiation output and smaller phantom size. The difference in the iodine and calcium measurements in the large phantom were up to -2.6 and -60.4 mg/mL for iodine (5 mg/mL with 2-mm diameter) and calcium (300 mg/mL) materials, and the difference was improved with a small phantom. Metal artifact reduction software improved subjective image quality; however, the quantitative values were significantly underestimated (P < 0.05) (-49.5, -26.9, and -15.3 HU for 50, 70, and 100 keV, respectively; -1.0 and -17 mg/mL for iodine and calcium concentration, respectively) compared to that acquired without a metal material. The accuracy of quantitative measurements can be affected by material density and the size of the object, radiation output, phantom size, and the presence of metal materials.

21. Comparison of abdominal CT protocols: A multi-center study on image quality and radiation dose levels

Introduction (1) To highlight the spread of patient exposure and image quality performances for various abdominal protocols. (2) To ensure that radiation dose reductions in abdominal CT protocols do not impair the detection of low-contrast structures. Methods An anthropomorphic abdominal phantom (QRM, Moehrendorf, Germany) with two optional rings (2.5 cm and 5 cm), representing the attenuation of an adult abdomen of 50 kg (small phantom), 75 kg (medium phantom) and 100 kg (large phantom), was scanned on 70 CT machines in the Western part of Switzerland. The phantom contains in the axial plane four spheres of 5, 6 and 8 mm in diameter with a contrast of 20 HU relative to the background at 120 kV. For statistical reasons, each phantom size was systematically scanned 10 times using local clinical settings of the portal phase for the detection of focal liver lesions (FLL); 40 regions of interest (ROIs) including the target and 150 ROIs with background noise only were extracted. Low contrast detectability (LCD) was objectively assessed using a Channelized Hotelling mathematical model Observer (CHO) with ten dense differences of Gaussian channels for the calculation of a receiver operating characteristic (ROC) curve. For each lesion size, the area under the ROC curve (AUC) was used as a figure of merit (FOM). A new image quality metric, called the weighted AUC (AUC), was proposed to combine the AUC results of all 3 different lesion sizes in a single metric. The displayed CTDIvol was used as a radiation dose metric. Results The median dose used for acquisitions is equal to 5.8 mGy, 10.5 mGy and 16.3 mGy, respectively for the small, medium and large phantoms. For the small phantom, the median image quality is equal to 0.977 with an interquartile range (IQR) equal to 0.027. For the medium phantom the dispersion increased and the median image quality decreased (AUCw = 0.926 and IQR = 0.05). For the large phantom, despite a large range of CTDIvol, the median image quality also decreased (AUCw = 0.89 and IQR = 0.068) and six centers appeared as outliers with relatively limited AUCw scores (AUCw inferior to 0.75), due to very low CTDIvol values. Conclusions The use of a CHO model showed that the majority of institutions performed reasonably well when searching FLL. However, for some outliers, the CTDIvol values were so small that image quality outcomes were clearly not adequate for the intended task. Moreover, the spread of image quality levels was associated with a large CTDIvol distribution, showing that the concept of diagnostic reference levels should be revisited to take into account the image quality aspect. This would ensure comparable diagnostic performance image information content whatever the type of CT unit.

Optimization of a shorter variable-acquisition time for legs to achieve true whole-body PET/CT images.

The present study aimed to qualitatively and quantitatively evaluate PET images as a function of acquisition time for various leg sizes, and to optimize a shorter variable-acquisition time protocol for legs to achieve better qualitative and quantitative accuracy of true whole-body PET/CT images. The diameters of legs to be modeled as phantoms were defined based on data derived from 53 patients. This study analyzed PET images of a NEMA phantom and three plastic bottle phantoms (diameter, 5.68, 8.54 and 10.7cm) that simulated the human body and legs, respectively. The phantoms comprised two spheres (diameters, 10 and 17mm) containing fluorine-18 fluorodeoxyglucose solution with sphere-to-background ratios of 4at a background radioactivity level of 2.65kBq/mL. All PET data were reconstructed with acquisition times ranging from 10 to 180, and 1200s. We visually evaluated image quality and determined the coefficient of variance (CV) of the background, contrast and the quantitative %error of the hot spheres, and then determined two shorter variable-acquisition protocols for legs. Lesion detectability and quantitative accuracy determined based on maximum standardized uptake values (SUVmax) in PET images of a patient using the proposed protocols were also evaluated. A larger phantom and a shorter acquisition time resulted in increased background noise on images and decreased the contrast in hot spheres. A visual score of ≥ 1.5 was obtained when the acquisition time was ≥ 30s for three leg phantoms, and ≥ 120s for the NEMA phantom. The quantitative %errors of the 10- and 17-mm spheres in the leg phantoms were ± 15 and ± 10%, respectively, in PET images with a high CV (scan < 30s). The mean SUVmax of three lesions using the current fixed-acquisition and two proposed variable-acquisition time protocols in the clinical study were 3.1, 3.1 and 3.2, respectively, which did not significantly differ. Leg acquisition time per bed position of even 30-90s allows axial equalization, uniform image noise and a maximum ± 15% quantitative accuracy for the smallest lesion. The overall acquisition time was reduced by 23-42% using the proposed shorter variable than the current fixed-acquisition time for imaging legs, indicating that this is a useful and practical protocol for routine qualitative and quantitative PET/CT assessment in the clinical setting.