Jainil P. Shah

Initial In Vivo Quantification of Tc-99m Sestamibi Uptake as a Function of Tissue Type in Healthy Breasts Using Dedicated Breast SPECT-CT

A pilot study is underway to quantify in vivo the uptake and distribution of Tc-99m Sestamibi in subjects without previous history of breast cancer using a dedicated SPECT-CT breast imaging system. Subjects undergoing diagnostic parathyroid imaging studies were consented and imaged as part of this IRB-approved breast imaging study. For each of the seven subjects, one randomly selected breast was imaged prone-pendant using the dedicated, compact breast SPECT-CT system underneath the shielded patient support. Iteratively reconstructed and attenuation and/or scatter corrected images were coregistered; CT images were segmented into glandular and fatty tissue by three different methods; the average concentration of Sestamibi was determined from the SPECT data using the CT-based segmentation and previously established quantification techniques. Very minor differences between the segmentation methods were observed, and the results indicate an average image-based in vivo Sestamibi concentration of 0.10 ± 0.16 μCi/mL with no preferential uptake by glandular or fatty tissues.

Three dimensional dose distribution comparison of simple and complex acquisition trajectories in dedicated breast CT

PURPOSE A novel breast CT system capable of arbitrary 3D trajectories has been developed to address cone beam sampling insufficiency as well as to image further into the patients chest wall. The purpose of this study was to characterize any trajectory-related differences in 3D x-ray dose distribution in a pendant target when imaged with different orbits. METHODS Two acquisition trajectories were evaluated: circular azimuthal (no-tilt) and sinusoidal (saddle) orbit with ±15° tilts around a pendant breast, using Monte Carlo simulations as well as physical measurements. Simulations were performed with tungsten (W) filtration of a W-anode source; the simulated source flux was normalized to the measured exposure of a W-anode source. A water-filled cylindrical phantom was divided into 1 cm(3) voxels, and the cumulative energy deposited was tracked in each voxel. Energy deposited per voxel was converted to dose, yielding the 3D distributed dose volumes. Additionally, three cylindrical phantoms of different diameters (10, 12.5, and 15 cm) and an anthropomorphic breast phantom, initially filled with water (mimicking pure fibroglandular tissue) and then with a 75% methanol-25% water mixture (mimicking 50-50 fibroglandular-adipose tissues), were used to simulate the pendant breast geometry and scanned on the physical system. Ionization chamber calibrated radiochromic film was used to determine the dose delivered in a 2D plane through the center of the volume for a fully 3D CT scan using the different orbits. RESULTS Measured experimental results for the same exposure indicated that the mean dose measured throughout the central slice for different diameters ranged from 3.93 to 5.28 mGy, with the lowest average dose measured on the largest cylinder with water mimicking a homogeneously fibroglandular breast. These results align well with the cylinder phantom Monte Carlo studies which also showed a marginal difference in dose delivered by a saddle trajectory in the central slice. Regardless of phantom material or filled fluid density, dose delivered by the saddle scan was negligibly different than the simple circular, no-tilt scans. The average dose measured in the breast phantom was marginally higher for saddle than the circular no tilt scan at 3.82 and 3.87 mGy, respectively. CONCLUSIONS Not only does nontraditional 3D-trajectory CT scanning yield more complete sampling of the breast volume but also has comparable dose deposition throughout the breast and anterior chest volume, as verified by Monte Carlo simulation and physical measurements.

Initial evaluation of a newly developed high resolution CT imager for dedicated breast CT

A new, high resolution 40x30cm2 area CsI-TFT based CT imager having 127μm pixel pitch was developed for fully-3D breast CT imaging as part of a SPECT-CT system. The imager has two narrow edges suited for pendant breast CT imaging close to the chest wall. The scintillator thickness of 600 microns provides >90% absorption for the 36keV mean x-ray energy of the cone beam source. The 2D MTF is ˜7.5% at the 3.9 lp/mm Nyquist frequency. The imager has excellent linearity over the full dynamic range. The imager is mounted on the CT device and initial tomographic imaging of geometric and breast phantoms demonstrate the reliable and robust imaging capabilities of this device for breast CT.

Sensitivity analysis for liver iron measurement through neutron stimulated emission computed tomography: a Monte Carlo study in GEANT4.

Neutron stimulated emission computed tomography (NSECT) is being developed as a non-invasive imaging modality to detect and quantify iron overload in the human liver. NSECT uses gamma photons emitted by the inelastic interaction between monochromatic fast neutrons and iron nuclei in the body to detect and quantify the disease. Previous simulated and physical experiments with phantoms have shown that NSECT has the potential to accurately diagnose iron overload with reasonable levels of radiation dose. In this work, we describe the results of a simulation study conducted to determine the sensitivity of the NSECT system for hepatic iron quantification in patients of different sizes. A GEANT4 simulation of the NSECT system was developed with a human liver and two torso sizes corresponding to small and large patients. The iron concentration in the liver ranged between 0.5 and 20 mg g(-1), corresponding to clinically reported iron levels in iron-overloaded patients. High-purity germanium gamma detectors were simulated to detect the emitted gamma spectra, which were background corrected using suitable water phantoms and analyzed to determine the minimum detectable level (MDL) of iron and the sensitivity of the NSECT system. These analyses indicate that for a small patient (torso major axis = 30 cm) the MDL is 0.5 mg g(-1) and sensitivity is ∼13 ± 2 Fe counts/mg/mSv and for a large patient (torso major axis = 40 cm) the values are 1 mg g(-1) and ∼5 ± 1 Fe counts/mg/mSv, respectively. The results demonstrate that the MDL for both patient sizes lies within the clinically significant range for human iron overload.

Design of a nested SPECT-CT system with fully suspended CT sub-system for dedicated breast imaging

A fully suspended, stand-alone cone beam CT system capable of complex trajectories, in addition to a simple circular trajectory, has previously been developed and shown to minimize cone beam sampling insufficiencies and have better sampling close to the chest wall for pendant breast CT imaging. A hybrid SPECT-CT system with SPECT capable of complex 3D trajectories has already been implemented and is currently in use. Here, the individual systems are redesigned into one hybrid system where each individual component is capable of traversing independent, arbitrary trajectories around a pendant breast and anterior chest wall in a common field of view. The integration also involves key hardware upgrades: a new high resolution 40x30cm2 flat panel CT imager with an 8mm bezel on two sides for closer chest wall access, a new x-ray source, and a unique tilting mechanism to enable the spherical trajectories for CT. A novel method to tilt the CT gantry about a 3D center of rotation is developed and included in the new gantry, while preserving the fully-3D SPECT system nested within the larger CT gantry. The flexibility of the integrated system is illustrated.

Detailed Characterization of 2D and 3D Scatter-to-Primary Ratios of Various Breast Geometries Using a Dedicated CT Mammotomography System.

With a dedicated breast CT system using a quasi-monochromatic x-ray source and flat-panel digital detector, the 2D and 3D scatter to primary ratios (SPR) of various geometric phantoms having different densities were characterized in detail. Projections were acquired using geometric and anthropomorphic breast phantoms. Each phantom was filled with 700ml of 5 different water-methanol concentrations to simulate effective boundary densities of breast compositions from 100% glandular (1.0g/cm3) to 100% fat (0.79g/cm3). Projections were acquired with and without a beam stop array. For each projection, 2D scatter was determined by cubic spline interpolating the values behind the shadow of each beam stop through the object. Scatter-corrected projections were obtained by subtracting the scatter, and the 2D SPRs were obtained as a ratio of the scatter to scatter-corrected projections. Additionally the (un)corrected data were individually iteratively reconstructed. The (un)corrected 3D volumes were subsequently subtracted, and the 3D SPRs obtained from the ratio of the scatter volume-to-scatter-corrected (or primary) volume. Results show that the 2D SPR values peak in the center of the volumes, and were overall highest for the simulated 100% glandular composition. Consequently, scatter corrected reconstructions have visibly reduced cupping regardless of the phantom geometry, as well as more accurate linear attenuation coefficients. The corresponding 3D SPRs have increased central density, which reduces radially. Not surprisingly, for both 2D and 3D SPRs there was a dependency on both phantom geometry and object density on the measured SPR values, with geometry dominating for 3D SPRs. Overall, these results indicate the need for scatter correction given different geometries and breast densities that will be encountered with 3D cone beam breast CT.

Characterization of X-ray scattering for various phantoms and clinical breast geometries using breast CT on a dedicated hybrid system

OBJECTIVE The purpose of this study was to utilize a dedicated breast CT system using a 2D beam stop array to physically evaluate the scatter to primary ratios (SPRs) of different geometric phantoms and prospectively acquired clinical patient data. METHODS Including clinically unrealizable compositions of 100% glandular and 100% fat, projection images were acquired using three geometrically different phantoms filled with fluids simulating breast tissue. The beam stop array method was used for measuring scatter in projection space, and creating the scatter corrected primary images. 2D SPRs were calculated. Additionally, a new figure of merit, the 3D normalized scatter contribution (NSC) volumes were calculated. RESULTS The 2D SPR values (0.52-1.10) were primarily dependent on phantom geometry; a secondary dependence was due to their uniform density; 2D SPRs were low frequency and smoothly varying in the uniformly filled phantoms. SPRs of clinical patient data followed similar trends as phantoms, but with noticeable deviations and high frequency components due to the heterogeneous distribution of glandular tissue. The maximum measured patient 2D SPRs were all <0.6, even for the largest diameter breast. These results demonstrate modest scatter components with changing object geometries and densities; the 3D NSC volumes with higher frequency components help visualize scatter distribution throughout the reconstructed image volumes. Furthermore, the SPRs in the heterogeneous clinical breast cases were underestimated by the equivalent density, uniformly filled phantoms. CONCLUSIONS These results provide guidance on the use of uniformly distributed density and differently shaped phantoms when considering simulations. They also clearly demonstrate that results from patients can vary considerably from 2D SPRs of uniformly simulated phantoms.

SU‐E‐l‐01: Investigating the Dependence of 2D and 3D Scatter‐To‐Primary Ratios on Breast Density in Clinical Breast CT

PURPOSE To characterize 2D and 3D scatter-to-primary ratios (SPR) and investigate the dependence of SPR on breast density based on clinicalpatient imaging on our dedicated SPECT-CT mammotomography system. METHODS As a part of an on-going IRB approved protocol, 7 consented women underwent a breast imaging study with our SPECT-CT system. Using a quasi-monochromatic x-ray cone beam and flat panel detector, 240 projections were obtained. 6 beam stop array (BSA) projections were also obtained over 40 degree intervals. The CT data were scatter corrected using a phantom-validated algorithm based on the BSA technique. For each projection, 2D scatter fluence was determined behind each beam stop shadow through the patients breast, and cubic spline interpolated throughout the thresholded breast region. 2D SPRs were calculated as the average of 6 measured ratios of scatter projections to scatter corrected (primary) projections. Angular cubic spline interpolation was performed to obtain the remaining 234 azimuthal BSA projections for reconstruction correction. Corrected and uncorrected projection images were individually reconstructed and 3D SPRs calculated as the ratio of difference between primary and un-corrected volumes to primary volumes. Histograms of the reconstructed patient data-sets yielded distinct peaks representing linear attenuation coefficients of glandular+skin and fatty tissue. Percentage glandular and fatty tissue was estimated by fitting a double Gaussian to the histogram and integrating area under the curve. RESULTS Only 4 CT data sets devoid of motion and truncation artifacts were suitable for scatter correction. Preliminary results show that 2D SPR values peak at the center of breast volumes and were greatest (∼50%) for the most glandular breast, whereas 3D SPR values remained fairly constant throughout the breast and did not obviously correlate with density, consistent with prior phantom results. CONCLUSIONS 2D SPR values are greater in breasts with higher glandular tissue composition, whereas 3D SPR values appear independent of breast composition. This work is funded by the National Cancer Institute of the National Institutes of Health grant RO1-CA096821, with partial support (JHP) from NIH T32-EB001040. MPT is the inventor of this CT technology, and is named as an inventor on the patent for this technology awarded to Duke University. If this technology becomes commercially successful, MPT and Duke could benefit financially.

Neutron time-of-flight spectroscopy for depth-resolved quantification through NSECT

With advances in detector technology, gamma-ray detectors are now capable of reporting both time of arrival of a photon and its energy. Although the gamma energies detected from the inelastic scattering of neutrons with elemental nuclei in the tissue of interest are being exploited in Neutron Stimulation Emission Computed Tomography (NSECT) to detect different elemental disorders, the timing information is largely ignored. Here we present a technique to utilize the time of arrival of gamma photons at a detector to locate focal liver lesions in diseases such as hemochromatosis and liver cancer. A GEANT4 simulation of 5-MeV neutrons was used to irradiate a liver phantom with multiple lesions with different iron concentrations. The time of arrival of gamma photons from neutron-56Fe inelastic scatter was recorded using a 360 degree, 100% efficient detection system and used to locate the lesions in the beam path. The resulting spectra were resolved in nanosecond time bins (corresponding to the expected arrival time of inelastic-scatter gamma photons from the lesion) and clearly demonstrated the ability to localize the focal liver lesions through neutron time-of-flight (TOF) spectroscopy. The preliminary results showed errors of only 10–20% in lesion position, demonstrating the strong potential of the technique.

Development of Fully-3D CT in a Hybrid SPECT-CT Breast Imaging System

This work describes initial measurements with the CT subsystem of the assembled, fully-3D, hybrid SPECT-CT system for dedicated breast imaging. The hybrid system, designed for clinical breast imaging, consists of fully-flexible SPECT and CT subsystems, with each capable of 3D acquisition motions. The SPECT subsystem employs a 16 × 20i¾?cm2 CZT detector with 2.5i¾?mm pixellation, is capable of viewing into the chest wall in addition to imaging the complete breast volume, and has been extensively reported elsewhere. The polar tilting capability of the CT subsystem has marked improvement in volumetric sampling while eliminating cone beam artifacts due to the fully-3D acquisitions. The CT subsystem can also view into the chest wall, while delivering <5i¾?mGy total dose, compared with a simple circular orbit breast CT. The CT subsystem consists of a 0.4i¾?mm focal spot x-ray tube with a rotating 14i¾? W-anode angle, and a 40i¾?×i¾?30i¾?cm2 CsITl flat panel imager having 127 micron pixellation and 8.0i¾?mm bezel edge, placed on opposing ends of the completely suspended gantry. A linear stage mechanism is used to tilt the suspended CT gantry up to ±15i¾? in the polar directions about the 3D center of rotation; the SPECT system is nestled inside the suspended CT gantry, oriented perpendicular to the CT source-detector pair. Both subsystems rest on an azimuthal rotation stage enabling truncated spherical trajectories independently for each. Several simple and more complex 3D trajectories were implemented and characterized for the CT subsystem. Imaging results demonstrate that additional off-axis projection views of various geometric phantoms and intact cadaveric breast, facilitated by the polar tilting yield more complete breast-volume sampling and markedly improved iteratively reconstructed images, especially compared to simple circular orbit data. This is the first implementation of a hybrid SPECT-CT system with fully-3D positioning for the two subsystems, and could have various applications in diagnostic breast imaging.