Reconsidering pregnancy screening policies for minors: patient-specific estimate of fetus and effective dose for potentially pregnant minors undergoing optimized dose CT of the pelvis.

BACKGROUND: Only verbal pregnancy screening is recommended for post-menarcheal females undergoing pelvic radiographs. In contrast, usually, a urine/serum pregnancy test for pelvic computed tomographic (CT) exams is required out of concern for higher radiation exposure. OBJECTIVE: To estimate patient-specific fetus absorbed dose to a potentially pregnant minor from an optimized dose CT of the pelvis for femoral version and surgical planning and provide evidence that such examinations of the pelvis can be performed with only verbal pregnancy screening. METHODS AND METHODS: A retrospective study was performed on 102 female patients between 12-18 years of age (15.4 ± 2.1 years) who underwent optimized dose CT of the pelvis for orthopedic evaluation of femoral version and surgical planning. Optimized CT exams were performed with weight-adjusted kVp and tube current modulation. Patient-specific dose from the optimized dose CT was calculated using the National Cancer Institute Dosimetry System for CT (NCICT) database by matching each patient to a phantom from the NCI non-reference phantom library based on patient sex, weight, and height. The calculated absorbed uterus dose was used as a surrogate for the fetus dose. Furthermore, patient-specific organ doses were used to estimate the effective dose. The strengths of the linear relationships between the dose metrics and patient characteristics were assessed using Pearson correlation coefficients through linear regression. RESULTS: The mean patient-specific effective dose for an optimized dose CT of the pelvis was 0.54 ± 0.20 mSv (range: 0.15-1.22 mSv). The mean estimated absorbed uterine dose was 1.57 ± 0.67 mGy (range: 0.42-4.81 mGy). Both effective dose and estimated uterine dose correlated poorly with patient physical characteristics (R = -0.26; 95% CI: [-0.43, -0.007] for age, R = 0.03; 95% CI: [-0.17, 0.22] for weight) but correlated strongly (R = 0.79, 95% CI: [0.7, 0.85]) with CTDIvol. CONCLUSION: The estimated fetus dose in case of pregnancy was significantly lower than 20 mGy for urine/serum pregnancy screening, suggesting that the pregnancy screening protocols in minors undergoing optimized dose CT require reassessment and may be safely performed by verbal attestation only.

Hip Imaging in Children With Cerebral Palsy: Estimation and Intrapatient Comparison of Patient-Specific Radiation Doses of Low-Dose CT and Radiography.

OBJECTIVES: Hip displacement is the second most common orthopedic problem affecting children with cerebral palsy (CP). Routine radiographic hip surveillance typically involves an anteroposterior (AP) pelvis radiograph. Unfortunately, this imaging protocol is limited by its projectional technique and the positioning challenges in children with CP. Alternatively, hip low-dose computed tomography (LDCT) has been advocated as a more accurate strategy for imaging surveillance as it provides biofidelic details of the hip that is independent of patient positioning. However, the tradeoff is the (presumed) higher radiation dose to the patient. The goal of this study is to estimate patient-specific radiation doses of hip LDCTs and AP pelvis radiographs in CP patients, and perform an intrapatient dose comparison. MATERIALS AND METHODS: A search of our imaging database was performed to identify children with CP who underwent hip LDCT and AP pelvis radiograph within 6 months of each other. The LDCTs were performed using weight-adjusted kVp and tube current modulation, whereas the radiographs were obtained with age-/size-adjusted kVp/mAs. The patient-specific organ and effective doses for LDCT were estimated by matching the patients to a nonreference pediatric phantom library from the National Cancer Institute Dosimetry System for Computed Tomography database with Monte Carlo-based dosimetry. The patient-specific organ and effective doses for radiograph were estimated using the National Cancer Institute Dosimetry System for Radiography and Fluoroscopy with Monte Carlo-based dose calculation. Dose conversion k-factors of dose area product for radiography and dose length product for LDCT were adapted, and the estimation results were compared with patient-specific dosimetry. RESULTS: Our study cohort consisted of 70 paired imaging studies from 67 children (age, 9.1 ± 3.3 years). The patient-specific and dose length product-based effective doses for LDCT were 0.42 ± 0.21 mSv and 0.59 ± 0.28 mSv, respectively. The patient-specific and dose area product-based effective doses for radiography were 0.14 ± 0.09 mSv and 0.08 ± 0.06 mSv, respectively. CONCLUSIONS: The radiation dose for a hip LDCT is ~4 times higher than pelvis radiograph, but it is still very low and poses minimal risk to the patient.

Flat Panel Detector c-Arms Are Associated with Dramatically Reduced Radiation Exposure During Ureteroscopy and Produce Superior Images.

Background: We wished to determine whether newly available flat panel detector (FPD) c-arms were (1) associated with lower radiation dose during ureteroscopy (URS) than conventional image intensifier (CII) c-arms and (2) to compare fluoroscopic image quality between the units. Materials and Methods: We retrospectively reviewed 44 consecutive patients undergoing URS at a pediatric hospital, with c-arms assigned by availability in the operating room. We performed dosimetry experiments using the same c-arms on standard phantoms. Results: Patient and case characteristics did not differ significantly between the two groups of patients. The median dose in the FPD group was less than a quarter of the dose in the CII group, 0.48 [0.42, 0.97] mGy vs 2.2 [1.1, 3.8] mGy, p < 0.0001. The FPD dose remained at less than one-third of the CII dose accounting for any difference in fluoroscopy time, and remained significant in a multivariate model including fluoroscopy time and patient weight (ß = 2.4, p = 0.007). Phantom studies showed higher image quality for FPDs at all simulated patient sizes, even at lower radiation doses. Conclusions: This is the first report comparing radiation dose from c-arms of image intensifiers and FPDs in adults or children. Use of an FPD during URS was associated with a substantially decreased absorbed dose for patients while simultaneously improving image quality.

Development of a novel framework to evaluate the localization accuracy of tomosynthesis-guided breast biopsy units.

PURPOSE: To develop a scheme to quantitatively assess localization accuracy of tomosynthesis-guided vacuum-assisted breast biopsy apparatus. METHODS: A phantom containing a metallic pellet on a flexible plastic shaft was constructed and was tested in cranio-caudal (CC) and lateral (LAT) arm biopsy geometries following the standard clinical breast biopsy workflow. Three points were manually digitized on tomosynthesis images including: the center of the target, and the tip of the needle in pre- and postfire positions. The needle trajectory was determined and four error metrics were defined: (1) stroke length error (difference between the nominal and measured stroke lengths); (2) Euclidian distance between the target and center of trough (i.e., aperture); (3) longitudinal distance between target and center of trough; and (4) lateral distance between target and needle. The proposed methodology was also evaluated on a breast gel phantom and the complete biopsy procedure, including vacuum-assisted biopsy was performed. RESULTS: Three biopsy geometries were investigated: (i) LAT arm on a prone table unit (Hologic, Affirm Prone), (ii) CC- and (iii) LAT arm in an upright unit (Hologic Affirm Upright). Both biopsy units passed the vendor-provided daily localization accuracy test, with <1 mm nominal error in each dimension. The aforementioned error metrics (1) to (4) were (0.6, 1.8, 0.4, 1.7) mm, (0.4, 4.2, 4.1, 1.1) mm, and (0.3, 2.4, 0.7, 2.3) mm, respectively, for geometry-I, -II, and -III. The gel phantom was tested on the upright unit with lateral arm and the error metrics (1) to (4) were 0.4, 2.5, 0.8, and 2.4 mm respectively. CONCLUSIONS: A framework was developed to evaluate the tomosynthesis-guided breast biopsy localization error, allowing quantitative comparisons between different systems and biopsy configurations. The proposed framework can also be extended to the stereotactic breast biopsy units. We suggest that a quantitative tolerance level for localization accuracy of breast biopsy units be established.

Postimplant Dosimetry of Permanent Prostate Brachytherapy: Comparison of MRI-Only and CT-MRI Fusion-Based Workflows.

PURPOSE: The current magnetic resonance imaging-computed tomography (MRI-CT) fusion-based workflow for postimplant dosimetry of low-dose-rate (LDR) prostate brachytherapy takes advantage of the superior soft tissue contrast of MRI, but still relies on CT for seed visualization and detection. Recently an MR-only workflow has been proposed that employs standard MR sequences and visualizes conventional implanted seed with positive contrast solely through MR postprocessing. In this work, the novel MR-only based workflow is compared with the clinical CT-MRI fusion approach. METHODS AND MATERIALS: Twenty-four prostate patients with a total of 1775 implanted LDR seeds were scanned using a 3-dimensional multiecho gradient echo sequence on a 3 Tesla MR scanner within 30 days after implantation. Quantitative susceptibility mapping was used for seed visualization. Seeds were automatically segmented and localized on the quantitative susceptibility mapping using convolutional neural network and k-means clustering, respectively. To assess the MR-only seed localization error, CT and MR-derived seed positions were coregistered, and ultimately, the resulting dose-volume histograms were compared. RESULTS: The MR-based seed visualization, segmentation, and localization generated comparable results to the CT-MR registration approach. The accuracy of the MRI-only based seed identification was 99.1%. After a rigid registration between the MR and CT-derived seed centroids, the average localization error was 0.8 ± 0.8 mm. The average prostate D90, V100, V150, and V200 for MRI-only and CT-MR fusion based dosimetry were 114.3 ± 12.5% versus 113.9 ± 11.9%, 95.1 ± 3.7% versus 95.3 ± 3.8%, 54.5 ± 14.5% versus 55.0 ± 13.2% and 22.9 ± 6.8% versus 23.2 ± 6.7%, respectively. No significant differences were observed in 3-dimensional seed positions and dosimetric parameters between MR-only and CT-MR fusion-based workflows (P > 0.2). CONCLUSIONS: The MRI-only LDR postimplant dosimetry is feasible and has very good potential to eliminate the need for CT-based seed identification.

Feasibility of an MRI-only workflow for postimplant dosimetry of low-dose-rate prostate brachytherapy: Transition from phantoms to patients.

PURPOSE: The lack of positive contrast from brachytherapy seeds in conventional MR images remains a major challenge toward an MRI-only workflow for postimplant dosimetry of low-dose-rate brachytherapy. In this work, the feasibility of our recently proposed MRI-only workflow in clinically relevant scenarios is investigated and the necessary modifications in image acquisition and processing pipeline are proposed for transition to the clinic. METHODS AND MATERIALS: Four prostate phantoms with a total of 321 I-125 implanted dummy seeds and three patients with a total of 168 implanted seeds were scanned using a gradient echo sequence on 1.5 T and 3T MR scanners. Quantitative susceptibility mapping (QSM) was performed for seed visualization. Before QSM, the seed-induced distortion correction was performed followed by edge enhancement. Seed localization was performed using spatial clustering algorithms and was compared with CT. In addition, feasibility of the proposed method on detection of prostatic calcifications was studied. RESULTS: The proposed susceptibility-based algorithm generated consistent positive contrast for the seeds in phantoms and patients. All the 321 seeds in the four phantoms were correctly identified; the MR-derived seeds centroids agreed well with CT-derived positions (average error = 0.5 ± 0.3 mm). The proposed algorithm for seed visualization was found to be orientation invariant. In patient cases, all seeds were visualized and correctly localized (average error = 1.2 ± 0.9 mm); no significant differences between dose volume histogram parameters were found. Prostatic calcifications were depicted with negative contrast on QSM and spatially agreed with CT. CONCLUSIONS: The proposed MRI-based approach has great potential to replace the current CT-based practices. Additional patient studies are necessary to further optimize and validate the workflow.

Potential applications of the quantitative susceptibility mapping (QSM) in MR-guided radiation therapy.

Magnetic resonance-guided radiation therapy (MR-GRT) offers great potential to improve radiation treatment outcomes by providing more accurate and patient-tailored therapy. Despite superior soft tissue contrast in MRI, one of the challenges towards MRI-only workflows is that the process often requires some sort of 'MR-invisible' metal-based devices. In this study, the feasibility of quantitative susceptibility mapping (QSM) for visualization of some MR-invisible radiation therapy devices was studied. Our recently proposed QSM-based algorithm for brachytherapy seed visualization was modified and the feasibility of the optimized algorithm for visualization of different devices including: brachytherapy seeds, plastic interstitial needles, CT-markers and obturators, and different types of fiducial markers in agar, prostate and meat phantoms were studied. All phantoms were scanned using 3T MR scanner with a 3D multi-echo gradient recalled echo (ME-GRE) pulse sequence. The QSM results in all phantoms were compared to CT images for spatial accuracy of the QSM. The applied post-processing algorithm was found to be insensitive to the seeds' type; also, presence of nearby calcifications had no effect on seed visualization. QSM successfully generated positive contrast for both types of investigated fiducial markers with high spatial accuracy compared to CT. Interstitial needles containing both aluminum-based CT-maker and titanium-based obturators were accurately depicted on the QSM. The proposed QSM-based technique relies on the standard MR pulse sequences and visualize the conventional MR-invisible metallic devices with CT-like positive contrast solely through post-processing. Upon in vivo validation of the technique, QSM may have the potential to replace CT for an MR-only guided radiation therapy.

MRI-based automated detection of implanted low dose rate (LDR) brachytherapy seeds using quantitative susceptibility mapping (QSM) and unsupervised machine learning (ML).

BACKGROUND AND PURPOSE: Permanent seed brachytherapy is an established treatment option for localized prostate cancer. Currently, post-implant dosimetry is performed on CT images despite challenging target delineation due to limited soft tissue contrast. This work aims to develop an MRI-only workflow for post-implant dosimetry of prostate brachytherapy seeds. MATERIAL AND METHODS: A prostate mimicking phantom containing twenty stranded I-125 dummy seeds and calcifications was constructed. A three-dimensional gradient-echo MR sequence was employed on 3T and 1.5T MR scanners. An optimized quantitative susceptibility mapping (QSM) technique was applied to generate positive contrast for the seeds and calcifications. Seed numbers, centroids, and orientations were determined using unsupervised machine learning algorithms (K-means and K-medoids clustering). The geometrical seed positions and the resulting dose distribution were compared to the clinical CT-based approach. RESULTS: The optimized QSM-based method generated high quality positive contrast for the seeds that were significantly different from that for calcifications and could be easily differentiated by thresholding. The estimated seed centroids from both 3T and 1.5T MR data were in perfect agreement with the standard CT-based seed detection algorithm (maximum difference of 0.7 mm). The estimated seed orientations were highly correlated with the actual orientations (R > 0.98). CONCLUSIONS: The proposed MRI-based workflow enabling an accurate and robust means to localize the seeds (position and orientation) upon validation on complex seed configurations, has the potential to replace the current widely practiced CT-based workflow.