Nathan J. Packard

Initial Characterization of a Dedicated Breast PET/CT Scanner During Human Imaging

We have constructed a dedicated breast PET/CT scanner capable of high-resolution functional and anatomic imaging. Here, we present an initial characterization of scanner performance during patient imaging. Methods: The system consisted of a lutetium oxyorthosilicate–based dual–planar head PET camera (crystal size, 3 × 3 × 20 mm) and 768-slice cone-beam CT. The position of the PET heads (separation and height) could be adjusted for varying breast dimensions. For scanning, the patient lay prone on a specialized bed and inserted a single pendent breast through an aperture in the table top. Compression of the breast as used in mammography is not required. PET and CT systems rotate in the coronal plane underneath the patient sequentially to collect fully tomographic datasets. PET images were reconstructed with the fully 3-dimensional maximum a posteriori method, and CT images were reconstructed with the Feldkamp algorithm, then spatially registered and fused for display. Phantom scans were obtained to assess the registration accuracy between PET and CT images and the influence of PET electronics and activity on CT image quality. We imaged 4 women with mammographic findings highly suggestive of breast cancer (breast imaging reporting and data system, category 5) in an ongoing clinical trial. Patients were injected with 18F-FDG and imaged for 12.5 min per breast. From patient data, noise-equivalent counting rates and the singles-to-trues ratio (a surrogate for the randoms fraction) were calculated. Results: The average registration error between PET and CT images was 0.18 mm. PET electronics and activity did not significantly affect CT image quality. For the patient trial, biopsy-confirmed cancers were visualized on dedicated breast PET/CT on all patient scans, including the detection of ductal carcinoma in situ in 1 case. The singles-to-trues ratio was found to be inversely correlated with breast volume in the field of view, suggesting that larger breasts trend toward increased noise-equivalent counting rates for all other things equal. Conclusion: Scanning of the uncompressed breast with dedicated breast PET/CT can accurately visualize suspected lesions in 3 dimensions.

A dedicated cone-beam CT system for musculoskeletal extremities imaging: design, optimization, and initial performance characterization.

PURPOSE This paper reports on the design and initial imaging performance of a dedicated cone-beam CT (CBCT) system for musculoskeletal (MSK) extremities. The system complements conventional CT and MR and offers a variety of potential clinical and logistical advantages that are likely to be of benefit to diagnosis, treatment planning, and assessment of therapy response in MSK radiology, orthopaedic surgery, and rheumatology. METHODS The scanner design incorporated a host of clinical requirements (e.g., ability to scan the weight-bearing knee in a natural stance) and was guided by theoretical and experimental analysis of image quality and dose. Such criteria identified the following basic scanner components and system configuration: a flat-panel detector (FPD, Varian 3030+, 0.194 mm pixels); and a low-power, fixed anode x-ray source with 0.5 mm focal spot (SourceRay XRS-125-7K-P, 0.875 kW) mounted on a retractable C-arm allowing for two scanning orientations with the capability for side entry, viz. a standing configuration for imaging of weight-bearing lower extremities and a sitting configuration for imaging of tensioned upper extremity and unloaded lower extremity. Theoretical modeling employed cascaded systems analysis of modulation transfer function (MTF) and detective quantum efficiency (DQE) computed as a function of system geometry, kVp and filtration, dose, source power, etc. Physical experimentation utilized an imaging bench simulating the scanner geometry for verification of theoretical results and investigation of other factors, such as antiscatter grid selection and 3D image quality in phantom and cadaver, including qualitative comparison to conventional CT. RESULTS Theoretical modeling and benchtop experimentation confirmed the basic suitability of the FPD and x-ray source mentioned above. Clinical requirements combined with analysis of MTF and DQE yielded the following system geometry: a -55 cm source-to-detector distance; 1.3 magnification; a 20 cm diameter bore (20 x 20 x 20 cm3 field of view); total acquisition arc of -240 degrees. The system MTF declines to 50% at -1.3 mm(-1) and to 10% at -2.7 mm(-1), consistent with sub-millimeter spatial resolution. Analysis of DQE suggested a nominal technique of 90 kVp (+0.3 mm Cu added filtration) to provide high imaging performance from -500 projections at less than -0.5 kW power, implying -6.4 mGy (0.064 mSv) for low-dose protocols and -15 mGy (0.15 mSv) for high-quality protocols. The experimental studies show improved image uniformity and contrast-to-noise ratio (without increase in dose) through incorporation of a custom 10:1 GR antiscatter grid. Cadaver images demonstrate exquisite bone detail, visualization of articular morphology, and soft-tissue visibility comparable to diagnostic CT (10-20 HU contrast resolution). CONCLUSIONS The results indicate that the proposed system will deliver volumetric images of the extremities with soft-tissue contrast resolution comparable to diagnostic CT and improved spatial resolution at potentially reduced dose. Cascaded systems analysis provided a useful basis for system design and optimization without costly repeated experimentation. A combined process of design specification, image quality analysis, clinical feedback, and revision yielded a prototype that is now awaiting clinical pilot studies. Potential advantages of the proposed system include reduced space and cost, imaging of load-bearing extremities, and combined volumetric imaging with real-time fluoroscopy and digital radiography.

The effect of skin thickness determined using breast CT on mammographic dosimetry

The effect of breast skin thickness on dosimetry in mammography was investigated. Breast computed tomography (CT) acquisition techniques, combined with algorithms designed for determining specific breast metrics, were useful for estimating skin thickness. A radial-geometry edge detection scheme was implemented on coronal reconstructed breast CT (bCT) images to measure the breast skin thickness. Skin thickness of bilateral bCT volume data from 49 women and unilateral bCT volume data from 2 women (10 healthy women and 41 women with BIRADS 4 and 5 diagnoses) was robustly measured with the edge detection scheme. The mean breast skin thickness (+/-inter-breast standard deviation) was found to be 1.45 +/- 0.30 mm. Since most current published normalized glandular dose (DgN) coefficients are based on the assumption of a 4-mm breast skin thickness, the DgN values computed with Monte Carlo techniques will increase up to 18% due to the thinner skin layers (e.g., 6-cm 50% glandular breast, 28 kVp Mo-Mo spectrum). The thinner skin dimensions found in this study suggest that the current DgN values used for mammographic dosimetry lead to a slight underestimate in glandular dose.

Dedicated Cone-Beam CT System for Extremity Imaging

PURPOSE To provide initial assessment of image quality and dose for a cone-beam computed tomographic (CT) scanner dedicated to extremity imaging. MATERIALS AND METHODS A prototype cone-beam CT scanner has been developed for imaging the extremities, including the weight-bearing lower extremities. Initial technical assessment included evaluation of radiation dose measured as a function of kilovolt peak and tube output (in milliampere seconds), contrast resolution assessed in terms of the signal difference-to-noise ratio (SDNR), spatial resolution semiquantitatively assessed by using a line-pair module from a phantom, and qualitative evaluation of cadaver images for potential diagnostic value and image artifacts by an expert CT observer (musculoskeletal radiologist). RESULTS The dose for a nominal scan protocol (80 kVp, 108 mAs) was 9 mGy (absolute dose measured at the center of a CT dose index phantom). SDNR was maximized with the 80-kVp scan technique, and contrast resolution was sufficient for visualization of muscle, fat, ligaments and/or tendons, cartilage joint space, and bone. Spatial resolution in the axial plane exceeded 15 line pairs per centimeter. Streaks associated with x-ray scatter (in thicker regions of the patient--eg, the knee), beam hardening (about cortical bone--eg, the femoral shaft), and cone-beam artifacts (at joint space surfaces oriented along the scanning plane--eg, the interphalangeal joints) presented a slight impediment to visualization. Cadaver images (elbow, hand, knee, and foot) demonstrated excellent visibility of bone detail and good soft-tissue visibility suitable to a broad spectrum of musculoskeletal indications. CONCLUSION A dedicated extremity cone-beam CT scanner capable of imaging upper and lower extremities (including weight-bearing examinations) provides sufficient image quality and favorable dose characteristics to warrant further evaluation for clinical use.

The characterization of breast anatomical metrics using dedicated breast CT

PURPOSE Accurate anatomical characterization of the breast is useful in breast phantom development and computer modeling of breast imaging technologies. Capitalizing on the three-dimensional capabilities of dedicated breast CT (bCT), a number of parameters which describe breast shape and fibroglandular distribution are defined. METHODS Among 219 bCT data sets, the effective diameter and length of the pendant breast as well as the breast volume were measured and characterized for each bra cup size. The volume glandular fraction (VGF) was determined as a function of patient age, BIRADS density, bra cup size, and breast diameter. The glandular fraction was examined in coronal and sagittal planes of the breast, and the radial distribution of breast glandular fraction within a coronal bCT image was examined for three breast regions. The areal glandular fraction (AGF) was estimated from two-dimensional projections of the breast (simulated by projecting bCT data sets) and was compared to the corresponding VGF. RESULTS The effective breast diameter and length increase with increasing bra cup size. The mean breast diameters (+/- standard error) of bra cup sizes A/AA, B, C, and D/DD were 11.1 +/- 0.5, 11.4 +/- 0.3, 13.0 +/- 0.2, and 13.7 +/- 0.2 cm, respectively. VGF was lower among older women and those with larger breast diameter and larger bra cup size. VGF increased as a function of the reported BIRADS density. AGF increased with VGF. Fibroglandular tissue was distributed primarily in the central portion of the breast. CONCLUSIONS Breast metrics were examined and a number of parameters were defined which may be useful for breast modeling. The reported data may provide researchers with useful information for characterizing the breast for various imaging or dosimetry tasks.

Validation of a method for measuring the volumetric breast density from digital mammograms

The purpose of this study was to evaluate the performance of an algorithm used to measure the volumetric breast density (VBD) from digital mammograms. The algorithm is based on the calibration of the detector signal versus the thickness and composition of breast-equivalent phantoms. The baseline error in the density from the algorithm was found to be 1.25 +/- 2.3% VBD units (PVBD) when tested against a set of calibration phantoms, of thicknesses 3-8 cm, with compositions equivalent to fibroglandular content (breast density) between 0% and 100% and under x-ray beams between 26 kVp and 32 kVp with a Rh/Rh anode/filter. The algorithm was also tested against images from a dedicated breast computed tomography (CT) scanner acquired on 26 volunteers. The CT images were segmented into regions representing adipose, fibroglandular and skin tissues, and then deformed using a finite-element algorithm to simulate the effects of compression in mammography. The mean volume, VBD and thickness of the compressed breast for these deformed images were respectively 558 cm(3), 23.6% and 62 mm. The displaced CT images were then used to generate simulated digital mammograms, considering the effects of the polychromatic x-ray spectrum, the primary and scattered energy transmitted through the breast, the anti-scatter grid and the detector efficiency. The simulated mammograms were analyzed with the VBD algorithm and compared with the deformed CT volumes. With the Rh/Rh anode filter, the root mean square difference between the VBD from CT and from the algorithm was 2.6 PVBD, and a linear regression between the two gave a slope of 0.992 with an intercept of -1.4 PVBD and a correlation with R(2) = 0.963. The results with the Mo/Mo and Mo/Rh anode/filter were similar.

Noise power properties of a cone-beam CT system for breast cancer detection

The noise power properties of a cone-beam computed tomography (CT) system dedicated for breast cancer detection were investigated. Uniform polyethylene cylinders of various diameters were scanned under different system acquisition conditions. Noise power spectra were calculated from difference data generated by subtraction between two identical scans. Multidimensional noise power spectra (NPS) were used as the metric to evaluate the noise properties of the breast CT (bCT) under different system acquisition and reconstruction conditions. A comprehensive investigation of the noise properties was performed in regard to system acquisition parameters including kVp, mA, number of cone-beam projection images used, cone angle, and object size. The influence on reconstruction parameters including interpolation method, reconstruction filter, field of view, matrix size, and slice thickness were also studied. Under certain conditions, the zero-dimensional NPS (image variance) was used as a quantitative index to compare the influence from different scan parameters, especially the radiation dose. If the total scan dose is changed by linearly changing the total number of projection images while the dose per frame is kept constant, the noise power has a linear relationship with the reciprocal of the total dose. If the total scan dose is changed by linearly changing the dose per frame while the total number of projection images is kept constant, the noise power has a quadratic relationship with the reciprocal of the total dose. With the same amount of total dose, using fewer projection images results in lower image noise power in the CT image. Quantitative results from this noise power analysis provide guidance for the bCT system operation, optimization, and data reconstruction.

PET characteristics of a dedicated breast PET/CT scanner prototype

A dedicated breast PET/CT system has been constructed at our institution, with the goal of having increased spatial resolution and sensitivity compared to whole-body systems. The purpose of this work is to describe the design and the performance characteristics of the PET component of this device. Average spatial resolution of a line source in warm background using maximum a posteriori (MAP) reconstruction was 2.5 mm, while the average spatial resolution of a phantom containing point sources using filtered back projection (FBP) was 3.27 mm. A sensitivity profile was computed with a point source translated across the axial field of view (FOV) and a peak sensitivity of 1.64% was measured at the center of the FOV. The average energy resolution determined on a per-crystal basis was 25%. The characteristic dead time for the front-end electronics and data acquisition (DAQ) was determined to be 145 ns and 3.6 micros, respectively. With no activity outside the FOV, a peak noise-equivalent count rate of 18.6 kcps was achieved at 318 microCi (11.766 MBq) in a cylindrical phantom of diameter 75 mm. After the effects of exposing PET detectors to x-ray flux were evaluated and ameliorated, a combined PET/CT scan was performed. The percentage standard deviations of uniformity along axial and transaxial directions were 3.7% and 2.8%, respectively. The impact of the increased reconstructed spatial resolution compared to typical whole-body PET scanners is currently being assessed in a clinical trial.

High-resolution 18F-FDG PET with MRI for monitoring response to treatment in rheumatoid arthritis

Eur J Nucl Med Mol Imaging (2010) 37:1047 DOI 10.1007/s00259-009-1364-x IMAGE OF THE MONTH High-resolution 18 F-FDG PET with MRI for monitoring response to treatment in rheumatoid arthritis Abhijit J. Chaudhari & Spencer L. Bowen & George W. Burkett & Nathan J. Packard & Felipe Godinez & Anand A. Joshi & Stanley M. Naguwa & David K. Shelton & John C. Hunter & John M. Boone & Michael H. Buonocore & Ramsey D. Badawi Received: 20 November 2009 / Accepted: 10 December 2009 / Published online: 30 January 2010 # The Author(s) 2010. This article is published with open access at Springerlink.com Molecular imaging can potentially provide means for mon- itoring response to therapy in rheumatoid arthritis (RA) early in the course of disease [1].Quantitative measurements of RA disease activity made in the wrist by whole-body PET scanners, however, have inadequate accuracy because of limited spatial resolution [2]. A high-resolution PET/CT scanner for imaging extremities has been built at our insti- tution [3]. In conjunction with a clinical MRI scanner, high- resolution PET/MR images can be obtained for the wrist. The CT image is used for PET/MR image coregistration. A 57-year-old female with established RA was stable until a recent clinical flare-up in the right wrist. Clinical exami- nation revealed synovitis, swelling, and diminished range of motion. The patient also had a history of osteoarthritis (OA). An extremity 18 F-FDG PET/CT scan immediately following MRI at baseline was performed on this patient. Tumor necrosis factor alpha (TNF-α) inhibitor (etanercept) therapy was then initiated as a part of the patient’s standard of care. The patient was re-scanned 5 weeks after starting treatment. The figure shows high-resolution 18 F-FDG PET images (pseudocolor) overlaid on pre-contrast MRI images (gray This work was funded by the NIH grants UL1-RR024146, R01CA129561, R01EB002138 and the UC Davis Imaging Research Center. A. J. Chaudhari (*) : S. L. Bowen : G. W. Burkett : N. J. Packard : F. Godinez : D. K. Shelton : J. C. Hunter : J. M. Boone : M. H. Buonocore : R. D. Badawi Department of Radiology, UC Davis Medical Center, Sacramento, CA, USA e-mail: ajchaudhari@ucdavis.edu A. A. Joshi Department of Neurology, UCLA School of Medicine, Los Angeles, CA, USA S. M. Naguwa Department of Internal Medicine, UC Davis Medical Center, Sacramento, CA, USA scale) at baseline (left column) and 5 weeks (right column). Significant reduction in PET signal (suggesting reduced inflammation) in the synovium and at sites of erosions (white arrows) is visible. The green arrow shows inflammation due to OA. Physician examination at 3 months confirmed that this patient responded positively to etanercept. This case illustrates the potential of high-resolution PET with MRI for quantitative visualization of early response to therapy in RA. Open Access This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which per- mits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited. References 1. Brenner W. 18F-FDG PET in rheumatoid arthritis: there still is a long way to go. J Nucl Med. 2004;45(6):927–9. 2. Beckers C, Ribbens C, Andre B, Marcelis S, Kaye O, Mathy L, et al. Assessment of disease activity in rheumatoid arthritis with (18)F-FDG PET. J Nucl Med. 2004;45(6):956–64. 3. Bowen SL, Wu Y, Chaudhari AJ, Fu L, Packard NJ, Burkett GW, et al. Initial characterization of a dedicated breast PET/CT scanner during human imaging. J Nucl Med. 2009;50(9):1401–8.

Assessment of image quality in soft tissue and bone visualization tasks for a dedicated extremity cone-beam CT system

AbstractObjectiveTo assess visualization tasks using cone-beam CT (CBCT) compared to multi-detector CT (MDCT) for musculoskeletal extremity imaging.MethodsTen cadaveric hands and ten knees were examined using a dedicated CBCT prototype and a clinical multi-detector CT using nominal protocols (80kVp-108mAs for CBCT; 120kVp- 300mAs for MDCT). Soft tissue and bone visualization tasks were assessed by four radiologists using five-point satisfaction (for CBCT and MDCT individually) and five-point preference (side-by-side CBCT versus MDCT image quality comparison) rating tests. Ratings were analyzed using Kruskal–Wallis and Wilcoxon signed-rank tests, and observer agreement was assessed using the Kappa-statistic.ResultsKnee CBCT images were rated “excellent” or “good” (median scores 5 and 4) for “bone” and “soft tissue” visualization tasks. Hand CBCT images were rated “excellent” or “adequate” (median scores 5 and 3) for “bone” and “soft tissue” visualization tasks. Preference tests rated CBCT equivalent or superior to MDCT for bone visualization and favoured the MDCT for soft tissue visualization tasks. Intraobserver agreement for CBCT satisfaction tests was fair to almost perfect (κ ~ 0.26–0.92), and interobserver agreement was fair to moderate (κ ~ 0.27–0.54).ConclusionCBCT provided excellent image quality for bone visualization and adequate image quality for soft tissue visualization tasks.Key Points• CBCT provided adequate image quality for diagnostic tasks in extremity imaging. • CBCT images were “excellent” for “bone” and “good/adequate” for “soft tissue” visualization tasks. • CBCT image quality was equivalent/superior to MDCT for bone visualization tasks.