Volume 35, Issue 6, June 2008
Index of content:
- Imaging Scientific Session: Room 332
- X‐Ray Imaging: Radiography and Vascular Imaging
35(2008); http://dx.doi.org/10.1118/1.2962360View Description Hide Description
Purpose: To measure the impact of thermal and multiplication‐excess noise and the instrumentation‐noise equivalent exposure (INEE) of a new solid‐state x‐ray image intensifier (SSXII) based on electron‐multiplying CCDs (EMCCDs). Method and Materials: The SSXII consists of a 350μm thick CsI(Tl) phosphor grown on a fiber‐optic plate optically coupled to a Peltier cooled EMCCD camera with a fiber‐optic input window via a 4:1 minifying fiber‐optic taper. The EMCCD provides signal multiplication gain prior to the read‐out electronics, thereby eliminating this primary noise source, effectively allowing for instrumentation‐noise free operation even at very low x‐ray exposures. Two additional potential noise sources, thermal and multiplication‐excess noise, were investigated. Thermal noise was determined using a linear fit of dark‐signal variance versus integration time at a constant readout rate. Multiplication‐excess noise was determined using signal variance normalized to gain versus signal plots. The gradient of a linear fit to this data at 1x gain as compared to higher gain values (up to 14x) was used to determine the excess noise factor (ENF). Overall noise‐performance was then determined using the INEE. Results:Thermal noise was measured to be < 2% of the maximal signal (12‐bits) with an EMCCD gain of 275x and an integration time of 30 ms, and is considered an upper bound under typical clinical operating conditions, indicating thermal noise has a negligible impact on overall image quality. The ENF was unity for the range of gains tested, suggesting the inherent excess noise of the electronic‐multiplication process has little effect when imaging secondary quanta from an x‐ray converting phosphor. An INEE of < 0.2μR further demonstrates the SSXII's capability to operate instrumentation‐noise free at very low x‐ray exposures. Conclusion: The SSXII provides quantum‐noise‐limited performance well below typical fluoroscopic exposure levels with negligible thermal and multiplication‐excess‐noise.
(Support: NIH Grants R01‐NS43924, R01‐EB002873, Toshiba Medical Systems Corporation).
MO‐D‐332‐02: A Formulism Towards Clinically Relevant Radiographic Imaging Metrics in the Presence of Patient Build‐Up, Scatter and Attenuation: CMTF, CNPS, and CDQE35(2008); http://dx.doi.org/10.1118/1.2962361View Description Hide Description
Purpose: We present a formulism to extend conventional radiographic metrics: modulation transfer function(MTF),noise power spectrum (NPS) and detective quantum efficiency (DQE), to clinically equivalent analogues: cMTF, cNPS and cDQE, that include patient specific influences of scatter and beam attenuation. We also propose a clinical interpretation of the above metrics: clinical resolution index (CRI), clinical noise index (CNI) and clinical contrast index (CCI), in which a clinical confidence level of resolution, noise and contrast is predicted for a given anatomical structure and geometry. Method and Materials: The formulation and validation of our clinical metrics were conducted on an Elekta iViewGT portal imager. We used a bar‐pattern to evaluate MTF as a function of spatial frequency (f), build‐up (b) and scatter depth (s) within 30 cm thickness of water to obtain cMTF(f,b,s). Uniform flat‐fields through 30 cm thickness of water containing varying attenuating media indicated by mass thickness (δt), a reasonable approximation for Compton limited megavoltage imaging, were used to measure cNPS(f,δt). Finally, cDQE(f,b,s,δt) was computed and normalized by the exit fluence associated with the attenuated fields. Clinical confidence indices for delineating anatomical structures (CRI, CNI and CCI) were obtained by expressing cMTF, cNPS and cDQE as a function of radiographic object size (w = 0.5/f), along with calibrated charts providing the transformation from anatomical size or form to the radiographic size marker (w). Results: Major differences were observed between conventional MTF, NPS and DQE measurements and our clinical equivalents (cMTF, cNPS and cDQE) that reflected the significant influence of patient attenuation and scatter. CRI, CNI and CCI curves were established for several selected anatomical sites that included bony as well as soft tissuecontrast.Conclusion: The clinically equivalent imaging metrics described above provide a significant link between conventional radiographic metrics defined for idealized systems and realistic clinical image quality.
MO‐D‐332‐03: Scatter Rejection Properties and Low‐Contrast Performance of a Scan Equalization Digital Radiography (SEDR) System: Initial Experience with Chest Phantom Images35(2008); http://dx.doi.org/10.1118/1.2962362View Description Hide Description
Purpose: To investigate scatter rejection properties and low‐contrast performance of a flat‐panel based SEDR system for chest imaging.Method and Materials: A prototype SEDR system was developed to improve the image quality especially in heavily attenuated regions. Slot scan imaging geometry was used to reduce x‐ray scatter without attenuating primary x‐rays. Regional beam width modulation was used to equalize the x‐ray exposures at the detector input for more uniform image signal‐to‐noise ratios. A low‐dose chest phantom (pre‐scan) image was acquired with the slot‐scan technique to determine the equalization factors for SEDR imaging. A steel bar was placed in front of the chest phantom to measure the scatter component and scatter‐to‐primary ratios (SPRs) across the phantom image. Two images acquired with the same techniques were subtracted from each other for measuring the noise levels. SPRs, SNRs, and contrast‐to‐noise ratios (CNRs) of the SEDR images were measured and compared with those of the slot‐scan images and full‐field images acquired with and without anti‐scatter grid. Results: The SEDR technique resulted in lower SPRs in heavily attenuated regions like retrocardum and mediastinum than the slot‐scan and full‐field techniques. They resulted in similar SPRs in lungs as the slot‐scan technique. Both the SEDR and slot‐scan methods produced better SNRs than the anti‐scatter grid method in full‐field imaging. The SEDR technique resulted in the best CNRs, followed in order by the slot‐scan and anti‐scatter grid techniques. The improvement of CNR was more pronounced in heavily attenuated regions. Conclusion: The SEDR technique can effectively reject scatter without having to attenuate the primary x‐rays. Furthermore, it can improve image SNRs and CNRs by regionally compensating for x‐ray attenuation by patient's anatomy.
Acknowledgement: This work was supported in part by grants CA104759 and CA124585 from NIH‐NCI, a grant EB00117 from NIH‐NIBIB, and a subcontract from NIST‐ATP.
MO‐D‐332‐04: Implementation of Variable Temporal Filtering in a High‐Resolution, Region‐Of‐Interest, High‐Sensitivity, Micro‐Angiographic Fluoroscope (HSMAF) Detector35(2008); http://dx.doi.org/10.1118/1.2962363View Description Hide Description
Purpose: Temporal filtering with operator‐selectable weighting factor has been implemented in a LabVIEW‐based Graphical User Interface (GUI) for use with a new high‐resolution, High‐Sensitivity, Micro‐Angiographic Fluoroscope (HSMAF) in order to enhance visualization during neuro‐interventional procedures. Method and Materials: The detector which is based on a CsI(Tl) phosphor, a light image intensifier, and a fiber‐optic taper coupled to a charge‐coupleddevice(CCD) camera, provides the angiographer with real‐time display of high‐resolution region‐of‐interest (ROI) images. The high sensitivity of the HSMAF enables the detector to work in fast frame‐rate, low‐dose x‐ray procedures such as fluoroscopy and roadmapping which are subject to noise. The variable temporal filtering is able to reduce the imagenoise and improve visualization of details of small structures such as stent struts. The selectable weighting factor which can be set before or even during acquisition, offers a fine adjustment tool for the compromise between motion blurring and noise reduction. The temporal filter weighting‐factor can be varied between 1 and 12, typically; however, in extremely noisy cases and small object motion it can go up to a higher value. Results: Variable temporal filtering and its resulting noise reduction, have significantly improved image quality and the visualization capability of the HSMAF system. The variable weighting factor gave the flexibility to control the amount of noise smoothing, even during the fluoroscopic procedure. Despite the increased processing time needed for temporal filtering, the GUI can still maintain the 30 fps display rate during fluoroscopy and roadmapping. Conclusion: The ability of variable noise reduction during roadmapping and fluoroscopy with the HSMAF provides angiographers and interventionalists with a capability of enchanced visualization of small vessel and endovascular device details, such as stent‐struts, hence making diagnoses and image guided interventions more accurate.
(Support from NIH Grants R01NS43924, R01EB002873 and Toshiba Medical Systems Corporation).
MO‐D‐332‐05: Low Dose Myocardial CT Perfusion Measurements Using Prior Image Constrained Compressed Sensing (PICCS)35(2008); http://dx.doi.org/10.1118/1.2962364View Description Hide Description
Purpose: To describe a method of low dose CT myocardial perfusion imaging based on an angularly under‐sampled projection acquisition and PICCS image reconstruction, and evaluate its performance in a preliminary porcine study. Method and Materials: Recently an extension to compressed sensing has been proposed in which a prior image is utilized as a constraint in the image reconstruction (i.e. Prior Image Constrained Compressed Sensing — PICCS). In this case the prior image is reconstructed with a short scan about 3 seconds before contrast arrives. Using the PICCS algorithm significant under‐sampling of the subsequent time frames is feasible, which would yield a direct dose savings if a pulsed x‐ray tube is used. In order to demonstrate the potential dose savings, a contrast enhanced porcine scan was performed on a 64 slice MDCT system. Results: Three regions‐of‐interest were identified in the myocardium adjacent to the left ventricle. The ability of the under‐sampled PICCS acquisitions to portray the temporal enhancement dynamics seen in full‐dose, fully‐sampled dynamic CTimaging was assessed by comparing quantitative perfusion parameters derived from time‐density curves. For dose reduction factors from 4.3 to 12.8, the average error in the time to peak ranged from −3% to 2.5%, and the average error in upslope parameter ranged from −14% to 7.5%. Conclusion: The results of a single porcine study indicate the potential for significant dose reduction (e.g. an order of magnitude) using a gated pulsed acquisition with respect to myocardial perfusion measurements.
35(2008); http://dx.doi.org/10.1118/1.2962365View Description Hide Description
Purpose: A novel Fourier‐based iterative acquisition and reconstruction method, termed Equally‐Sloped Tomography (EST), has been developed for reconstructing an image from an undersampled number of projections. We review this method and present the first series of comparative experiments and simulations quantifying the image quality and the dose reduction through this method. Method and Materials: Using the MOBY whole body mouse phantom and the Zubal whole body human phantom, CT simulations of EST vs. filtered back projection (FBP) and iterative algebraic methods were performed as function of number of projections and flux. The reconstruction image quality was quantified using SNR and CNR of contoured organs,modulation transfer function(MTF), and Fourier ring correlation (FRC). The simulations were experimentally verified using a cone‐beam x‐ray microCT scanner. A microfluidic resolution phantom and a MTF phantom were manufactured to quantify the experimental image quality. Using the microCT phantoms, the resolution, SNR, and CNR were tabulated as a function of flux and number of projections. Using this data, a dose reduction factor was calculated by determining the flux and number of projections that resulted in an equivalent SNR and resolution for EST relative to conventional reconstructions.Results: For the same number of projections and flux, all simulated and experimental reconstructions via EST resulted in better or equivalent SNR, FRC, and resolution relative to FBP. The dose reduction factor based on the SNR and resolution equivalence criteria was 35–45% for reconstructions with the number of projections above the Nyquist criteria and 40–50% dose reduction when the number of projections is undersampled. Conclusion: The results indicate that EST provides an effective method for dose reduction in CT and warrants further investigation. Conflict of Interest: Research sponsored by TomoSoft Technologies corporation.
MO‐D‐332‐07: Update On the Development of a New Dual Detector (Micro‐Angiographic Fluoroscope/Flat Panel) C‐Arm Mounted System for Endovascular Image Guided Interventions (EIGI)35(2008); http://dx.doi.org/10.1118/1.2962366View Description Hide Description
Purpose: To develop a dual detector C‐arm unit, capable of high‐resolution microangiography and fluoroscopy, and Region‐of‐Interest Cone‐Beam CT (ROI‐CBCT). Method and Materials: The Microangiographic Fluoroscope (MAF) (1024×1024×12bits, 35μm pixels, 4 cm field‐of‐view, FOV) was attached with a specially designed holder to a standard C‐arm Flat‐Panel (FP) system. The MAF consists of a 300 μm CsI input phosphor coupled to a dual stage GEN2 micro‐channel plate light image intensifier (LII), followed by a minifying fiber‐optic taper coupled to a 30 fps CCDcamera. The LII has a large variable gain allowing usage for very low (fluoroscopic) exposures while maintaining very good image quality. The holder allows facile placing of the new detector into the FP FOV when use is required or parking when not. The source‐to‐image distance and the orientation of this detector are selected using the same controls as for the standard C‐arm unit. A special switch attached onto the holder allows automatic collimation of the x‐ray beam to the active area of the MAF. The new system was tested in multiple experiments involving phantoms and animals for reliability and capability to perform EIGI procedures and also for dual detector ROI‐CBCT. Results: The new system is being used routinely for EIGI fluoroscopic guidance and microangiography in our research lab. The design allows a variable SID between 69 and 104 cm. During rotational angiography and dual detector ROI‐CBCT, 194 projections are acquired, one every degree. Interventional devices such as endovascular stents placed in the animals and phantoms were reconstructed with great accuracy, and virtually without artifacts. Conclusion: Implementation of such a high‐resolution imager on a clinical system could bring substantial benefits for the treatment of cerebrovascular disease and also potentially increase the motivation to develop improved and more effective endovascular devices.
(Funding: NIH Grants R01 EB002873, NS43924).
35(2008); http://dx.doi.org/10.1118/1.2962367View Description Hide Description
Purpose: To investigate various design rules in the coronary arterial tree system by using cone‐beam CTimages of swine heart.Method and Materials: The left anterior descending (LAD), left circumflex (LCX), and right coronary arteries (RCA) were casted with a radio‐opaque polymer. Cone‐beam CT was used to generate volumetric data. Vessel and myocardium segmentation, centerline extraction, tree tracking procedure were implemented to determine vessel branch dimension and geometry. Diameters and lengths of each detected branch were computed where the procedure was validated by using phantoms with known dimensions. The data from the arterial tree was combined with that from the myocardial tissue where each tissue voxel was assigned to its corresponding arterial branch. The sum of the distal coronary arterial branch lengths and volumes were correlated to the regional myocardial mass. Results: The correlation of the logarithm of the total distal arterial lengths (L) to the logarithm of the regional myocardial mass (M) were , and for the LAD, LCX and RCA, respectively. The correlation of the logarithm of the total distal arterial lumen volumes (V) to logarithm of the regional myocardial mass were , and for the LAD, LCX and RCA, respectively. Conclusion: The implemented image processing procedures successfully extracted information from a large 3D dataset of the coronary arterial tree to reveal the coronary artery design rules and provide prognostic indications in the form of arterial tree parameters and anatomical area at risk.
MO‐D‐332‐09: Cardiac Function Measurements On An Interventional C‐Arm System with Isotropic Spatial Resolution and High Temporal Resolution Using Prior Image Constrained Compressed Sensing (PICCS)35(2008); http://dx.doi.org/10.1118/1.2962368View Description Hide Description
Purpose: To achieve three dimensional isotropic dynamic cardiacCTimaging with high temporal resolution for evaluation of cardiac function with a slowly rotating C‐arm system. In this work we propose an acquisition and image reconstruction framework which enables simultaneously high spatial resolution and high temporal resolution. Method and Materials: A recently introduced extension to compressed sensing in which a prior image is used as a constraint in the reconstruction has enabled this application. This new algorithm is referred to as Prior Image Constrained Compressed Sensing (PICCS). An in‐vivo animal experiment (e.g. a beagle model) was conducted using an interventional C‐arm system. The imaging protocol was as follows: contrast was injected, the contrast equilibrated, breathing was suspended for ∼14 seconds during which time 420 equally spaced projections were acquired. This data set was used to reconstruct a fully sampled blurred image volume using the conventional FDK algorithm (e.g. the prior image). Then the data set was retrospectively gated into 19 phases according to the recorded ECG signal (heart rate ∼ 95bpm) and images were reconstructed with the PICCS algorithm. Results:Cardiac MR was used as the gold standard due to its high temporal resolution. The same short‐axis slice was selected from the PICCS‐CT data set and the MR data set. Manual contouring on the peak systolic and peak diastolic frames was performed to assess the ejection fraction contribution from this single plane. The calculated ejection fractions with PICCS‐CT agreed well with the MR results. Conclusion: We have demonstrated the ability to use a slowly rotating interventional C‐arm system in order to make measurements of cardiac function. The new technique provides high isotropic spatial resolution (∼0.5 mm) along with high temporal resolution (∼ 33 ms). The evaluation of cardiac function demonstrated agreement with single slice cardiac MR.
- Dosimetry, Radiation Protection, and Quality Control I
35(2008); http://dx.doi.org/10.1118/1.2962400View Description Hide Description
Purpose: To numerically generate radiographic x‐ray spectra that can be conveniently employed in radiation transport simulations or other radiation detection applications. Method and Materials: Based initially on the Tucker, et al model, we developed and evaluated a new code, DXS (Diagnostic X‐Ray Spectra), to numerically generate spectra for tungsten‐target x‐ray tubes spanning the radiographic energy range. The model parameters in our code were adjusted by comparison with corresponding MCNP5 simulated spectra; we modified the semi‐analytical formulation for the characteristic x‐ray production, a caveat of Tucker's model, by incorporating a factor that better accounts for the dependence of the K‐peaks on the tube potential. Parametric fitting functions are used to model the self‐attenuation in the target and attenuation due to inherent and added filtration (aluminum,beryllium,copper,tantalum are the options implemented in the code), as well as for the tungsten mass stopping power and the Thomson‐Whiddington constant. Comparison with Monte Carlo simulated and published measured spectra were used to validate the new code. Results: Normalized to unit area DXS code‐generated spectra for several tube potentials from 50 to 140 kVp agree well, less than 2% relative difference in nearly all energy bins (2 keV), with corresponding MCNP5 simulated spectra for similar tube parameters. Few exceptions are noted and may be attributed to either poorer statistics in the low and high energy tails of the spectrum, or to insufficient accuracy of the numerical computations for the steepest part of the spectra at high accelerating potentials. Good agreement is seen between the DXS and Bhat et al measured spectra. Conclusion: The DXS code generates the spectra, according to user specified input parameters (tube potential, anode angle, filtratation) and energy intervals, and augments them into any discretized energy group structure. Hence, the code can be of great benefit in radiation transport simulations.
MO‐E‐332‐02: Skin Doses in Interventional Radiology Procedures Associated with Oncology Diagnosis and Treatment — Are There Reviewable Sentinel Events?35(2008); http://dx.doi.org/10.1118/1.2962401View Description Hide Description
Purpose: Fluoroscopic examinations with cumulative dose exceeding 15 Gy to a single field is now considered as a ‘reviewable sentinel event’ according to Joint Commission standards. Guidance from the FDA suggests that the potential for injury be recorded in the patient's record for cumulative absorbed dose of 1 Gy or more. The purpose of this study was to estimate the peak radiation skindoses for interventional radiology procedures performed at a high patient volume cancer center. Method and Materials: A single‐center, IRB‐approved retrospective study was performed using data from an oncologic interventional radiology section. Peak skindoses were estimated from consecutive procedures performed during 2006 in three different fluoroscopic suites equipped for these studies. Of 6598 consecutive procedures, 3966 (60%) had dose‐area‐product (DAP) measurements recorded and were included in the study. Results: The mean estimated peak skindose was 0.19 Gy (range 4.95 microGy to 8.65 Gy) with a maximum individual skindose of 8.65 Gy. No procedures resulted in skindoses >15 Gy and over 95% of the procedures resulted in skindoses <1 Gy. Procedures with specific instances of skindoses >1 Gy included: embolization, biliary drain/stent, IVC filter, nephrostomy, arteriogram, abscess catheters, foreign body retrieval, catheter change, cholecystostomy, and gastronomy tube check. Embolizations, and biliary drain/stent procedures were most likely to result in skindoses >1 Gy. Significant variations in skindose were noted for various instances of the same procedure (e.g. range 0.6 mGy to 8.65 Gy for hepatic embolizations). Conclusion: Even when potential errors in methodology are considered, it is unlikely that any typical case performed in an oncologic interventional radiology practice would exceed the Joint Commission ‘reviewable sentinel event’ level of 15 Gy. Identifying procedures that could have peak skindoses greater than 1 Gy can be useful for informed consent and clinical followup.
35(2008); http://dx.doi.org/10.1118/1.2962402View Description Hide Description
Purpose:Charge coupled devices (CCDs) are being increasingly used in radiation therapy.CCDcameras are becoming the tool of choice for applications such as two‐dimensional dosimetry of scintillator sheets or to read hundreds of miniature scintillation detectors arranged in arrays. However, CCDs are sensitive to stray radiation. This effect induces transient noise. Radiation‐induced noise strongly alters the image and therefore limits its quantitative analysis. The purpose of this work is to characterize the radiation‐induced noise and to develop filtration algorithms to restore image quality. Method and Materials: Two models of CCDcameras (Andor Luca and Apogee U2000) were used for measurements in linac environments. Images were acquired with and without radiation. The structure of the transient noise was first characterized. Then, three methods of noise filtration were compared: median filtering of a time series of identical images, uniform median filtering of single images and an adaptive filter with hard‐switching mechanism. Results: The intensity distribution of noisy pixels was similar in both cameras. However, the spatial distribution of the noise was different: the average noise cluster size was 1.2±0.6 and 3.2±2.7 pixels for the U2000 and the Luca respectively. The median of a time series of image resulted in the best filtration and minimal image distortion. For applications where time series is impractical, an adaptive filter must be used to reduce image distortion. We have implemented a modification to the switch filter in order to handle non‐isolated group of noisy pixels. Conclusion: We have characterized the transient noise produced in CCDcameras by scattered radiation from a linac and have developed an efficient filtration scheme to remove this noise and restore image quality. Use of our filtration scheme allows detailed quantitative analysis of an image even when subjected to scattered radiation.
Supported by the NCI (1R01CA120198‐01A2).
MO‐E‐332‐04: Comparison of Dose‐Area‐Product (DAP) and Fluoroscopy Time Between a Mobile and a Fixed C‐Arm Unit for Electrophysiology (EP) Procedures35(2008); http://dx.doi.org/10.1118/1.2962403View Description Hide Description
Purpose: To determine if there is a difference in dose‐area‐product (DAP) or fluoroscopy time between electrophysiology (EP) procedures performed using a mobile and a fixed C‐arm fluoroscopic unit. Method and Materials: DAP and fluoroscopy‐time data was logged for 800 EP procedures performed using a mobile C‐arm unit from 2003 to 2005. In early 2006, a fixed C‐arm unit was installed and the same data logged for over 200 procedures. The procedures were sorted into five categories: 1. electrophysiology studies, 2. radiofrequency ablations, 3. pacemaker and implantable‐cardiac‐defibrillator implants, 4. biventricular interventions, and 5. lead changes. For each category, the distributions of DAP and time were compared and the average, median and range of values determined. Results: The median fluoroscopy time more than doubled for all procedure categories when using the fixed unit. Median DAP increased significantly for procedures 3 and 4, but remained nearly the same for procedures 1, 2 and 5. In all cases, the procedures were successfully completed without evidence of compromising patient care for either unit. However, the cardiologists were much more conservative in their use of fluoroscopy for the mobile unit due to its heat loading limitations and, also, they worked quicker because of their impression that the older mobile C‐arm gave more radiation dose to themselves and the patient. In addition, fluoroscopy at 15 frames per second was used on the fixed unit versus 7 frames per second on the mobile unit. Conclusion: Although the DAP and fluoroscopy time generally was higher for the fixed installation, it should not be concluded that the mobile unit is to be preferred. Rather, these results point out the importance of physician training and dose monitoring, not only to track patient radiation risk, but also to provide physician feedback.
(*Support: NIH Grant R01‐NS43924).
MO‐E‐332‐05: The Effect of Copper Beam Filtration On the Transmission of Scattered X‐Rays Through a Typical Lead Barrier35(2008); http://dx.doi.org/10.1118/1.2962404View Description Hide Description
Purpose: Standard use of copper beam filtration in modern cardiac catheterization and angiography systems can substantially change the x‐ray beam quality and ultimately have an impact on scattered x‐ray transmission through shielded barriers. A study was performed to investigate these effects. Method and Materials: Scatter was measured using broad‐beam geometry at 50 and 100 cm from the center of an 8″‐thick Lucite phantom with an 1800 cc ionization chamber. A Siemens Axiom Artis system was employed using 60, 81, 102 and 125 kVp with filtrations of 0, 0.1, 0.2, 0.3, 0.6 and 0.9mm of copper. The ionization chamber was wrapped in 1/16″‐thick lead to determine the transmission of scattered x‐rays through a typical shielded barrier. Results: Transmission at 125 kVp varied from 1.7×10−3 to 3.0×10−3 for beams with 0 to 0.9 mm of Cu, respectively. Similarly, transmission varied at 102 kVp from 1.2×10−3 to 2.2×10−3 and at 81 kVp from 7.1×10−4 to 9×10−4. Transmission at 60 kVp was about 7×10−4 for beams with 0 to 0.3 mm of Cu. Transmissions without Cu filtration at 125 and 102 kVp were measured to be about half the theoretical value reported by Simpkin and Dixon, which could result from variations in lead thickness. At 60 and 81 kVp transmissions without Cu filtration are more than an order of magnitude higher than that reported at 70 kVp by Simpkin and Dixon. Conclusion: Additional copper filtration can increase the barrier transmission up to a factor of two over unfiltered beams. However, it is unlikely that this amount of increase in transmission will significantly modify a shielding calculation. Further investigation is needed to determine the changes in typical workloads and scattered air kerma at different angles for these systems to understand the combined effect of these changes on barrier shielding calculations.
35(2008); http://dx.doi.org/10.1118/1.2962405View Description Hide Description
Purpose: To create a series of EPID Monte Carlo dose computation kernels which accounts for observed machine‐to‐machine variations in EPID response. Method and Materials:Field size response of aS500 and aS1000 imagers are measured for several Varian Cl21‐series machines that were dosimetrically matched in a water phantom. Deviations in imager response are attributed to differences in back‐scattering materials beneath the imaging panels. Mono‐energetic convolution kernels with various backscatter thicknesses are simultaneously created by sub‐dividing a thick back‐scattering slab into multiple sub‐slabs and using the EGSnrc LATCH bit to score sub‐slab kernel contributions. Energy‐binned particle fluence incident upon the detector convolved with the imager‐specific kernels are used to compute the EPIDimage.Imager‐specific kernels are determined by matching computed and measured EPID field‐size response, using the number of sub‐slabs as a free parameter. Final kernels are used for Monte Carlo‐based pre‐treatment and in‐treatment EPID dose computations. Results: The EPIDimagers on dosimetrically matched accelerators are found to differ. Most, but not all of the deviations appear to be correlated with the imager mounting arm type. The imager‐specific kernels matched the field‐size response for each imager within 1%, and resulted in dosimetric agreement between measured and computed images for pre‐treatment dosimetric verification of IMRT fields. Conclusion: Dosimetric differences between portal imagers on matched accelerators can be accounted for by using computation kernels with differing amounts of back‐scattering materials. Kernels for multiple different back‐scattering thickness can be efficiently calculated. Resultant imager‐specific kernels may be useful for efficient pre‐treatment and in‐treatment Monte Carlo‐based EPID dose computations. Conflict of Interest: This work was funded in part by Varian Medical Systems.
MO‐E‐332‐07: Instrumentation Noise Equivalent Exposure (INEE) for Routine Quality Assurance: INEE Measurements On a Clinical Flat Panel Detector35(2008); http://dx.doi.org/10.1118/1.2962406View Description Hide Description
Purpose: To measure the instrumentation‐noise equivalent exposure (INEE) of a clinical digital imagingsystem under various modes of operation. Method and Materials: The INEE is defined as the exposure at which the quantum‐noise equals the instrumentation‐noise and is measured from the plot of pixel gray‐level‐value variance versus detector entrance exposure. The intercept of such a plot represents the instrumentation‐noise in gray‐level values and the slope provides the conversion factor from these arbitrary units to equivalent exposure. Sequences of 90 flat‐field images in DA and DSA modes were acquired at 1 frame‐per‐second using a Varian PaxScan 2020 flat panel detector, both with image processing enabled and disabled. Image receptor input exposure was measured using an ionization chamber and effects of uncertainties in exposure calibration on the resulting INEE were investigated. Results: The INEE was observed to vary depending on the mode used and was measured to be 0.9±0.2 and 44±6 μR in DA and DSA modes, respectively, with image processing disabled. Exposure calibration error will result in additional proportionate INEE error. With image processing enabled the variance was highly nonlinear with exposure in DA mode indicating a need to have access to linear, unprocessed data. The difference in integration capacitance between DA and DSA modes, 0.5 and 4 pF respectively, helps explain the change in the INEE between these modes. Conclusion: INEE measurements were done on a clinical digital image receptor and were found to be dependent on the operational mode used. The importance of disabling image processing during INEE measurement was demonstrated. Due to the ease in gathering and analyzing the necessary data, we found the INEE to be a useful quality assurance tool for assessing a system's instrumentation‐noise in terms of a clinically relevant measure of exposure. (Support: NIH Grants R01‐ NS43924, R01‐EB002873, Toshiba Medical Systems Corporation).
- Magnetic Resonance Imaging
35(2008); http://dx.doi.org/10.1118/1.2962592View Description Hide Description
Purpose: To establish a criterion for distinguishing viable and necrotic tumor cells using the initial relative slope of the Gd‐DTPA DCE‐MRI and to apply this criterion to optimize the specificity of tumor hypoxia imaging based on PET.Method and Materials: Three nude rats with Dunning R3327‐AT prostate adenocarcinoma xenografts were imaged by dynamic MRI following tail vein injection of gadopentetate dimeglumine, with imaging parameters of 1‐mm slice thickness, 0.5‐mm spacing, and 0.13mm × 0.13mm voxel size. The time‐intensity curve of each voxel was obtained at 1‐min interval to 25‐min post‐injection. Histologic (standard H&E stain and high‐power microscopy) examination was performed of 8‐μm thick slices parallel to the DCE‐MRI imaging axis and the necrotic regions were identified. The initial relative slope, where I0 and I(t) are MR image intensity before injection and at time t, was calculated for different times and compared with the pathologically defined necrotic region to determine the threshold that best distinguishes viable and necrotic cells. This criterion was then used to study the specificity of FMISO PET with the same xenografts scanned with DCE‐MRI and PET.Results: The optimal criterion for identifying viable and necrotic tumor regions was that a DCE‐MRI voxel was necrotic if at 2 minutes after contrast injection. When this criterion was applied to the PETimages of three xenografts, necrotic region was found to have a wide range of image intensities, which is inconsistent with the hypothesis that high image intensity exclusively identifies hypoxic viable tumor. Among the 319 hypoxic voxels determined from the three animals' PET, only 75% corresponded to viable cells. Conclusion: A criterion was established to identify necrotic/viable tumor cells using DCE‐MRI. Using PET alone for hypoxia imaging is problematic because the specificity might be compromised by necrotic regions with high PETimage intensity.
TU‐D‐332‐02: Abdominal Fat Quantification Accuracy Using Water Saturated B‐SSFP MRI Sequences: A Verification Study35(2008); http://dx.doi.org/10.1118/1.2962593View Description Hide Description
Purpose: To verify the fat quantification accuracy of abdominal fat quantification results on images acquired with a Water Saturated b‐SSFP (WS b‐SSFP) MRI Sequence. Method and Materials: 10 healthy (5 female and 5 male) volunteers underwent both WS T1W TSE and WS b‐SSFP scans with breath hold on a 1.5 T clinical MR scanner. For the WS T1W TSE sequence, the parameters were as follows: TR/TE/flip angle = 500 ms/5 ms/90°, turbo factor = 7 and readout bandwidth = 128 KHz. For the WS 3D b‐SSFP sequences, the imaging parameters were as follows: TR/TE/flip angle = 2.9 ms/1.2 ms/55°, ETL = 128 with 12 dummy echoes, and readout bandwidth = 200 KHz. For WS T1W TSE sequence, 6 axial slices centered at L2‐L3 level were acquired in two consecutive expired breath‐holds, with 14 seconds each. Since WS b‐SSFP is a much faster scan, 8 slices (with slice 2–7 imaged at the same positions of the 6 TSE slices) were obtained in 11 seconds in one breath‐hold. For each subject, fat volumes from six common slices of WS TSE and WS b‐SSFP were measured and summed. Total abdominal fat (TAF), intra‐abdominal fat (IAF), and subcutaneous abdominal fat (SAF) volumes from images of both scans were measured and compared. Results: The TAF, IAF, and SAF mean differences of the fat volume measured from images of the two scans are only 0.34%, −0.25%, and 0.91%, respectively. The Bland‐Altman plots show that all the measurement differences between WS b‐SSFP and WS TSE are within the 95% of difference. Conclusion: The two methods are the same in terms of the measurement accuracy for human abdominal imaging. Therefore, WS b‐SSFP MRI can be used for abdominal fat quantification to reduce imaging and post‐processing difficulties and ultimately reduce research cost in abdominal fat distribution studies.
35(2008); http://dx.doi.org/10.1118/1.2962594View Description Hide Description
Purpose: The goal of this research is to develop a novel method for quantifying relative magnetic moment of any small object appearing in MRI images. This method may be useful in evaluating the magnetic characteristics of localized microbleeds in the brain or nanoparticles of contrast agents. We have successfully developed a 2D version applied only to long narrow cylindrical objects [1–2]. A key feature of this method is in the extraction of the desired information from MRI data without any a priori information. Method and Materials: The complex signal from a spherical object in a concentric spherical region is given in . The signal contains three unknowns: effective proton density, volume of the object, and magnetic moment. The center of the object can be determined by minimizing the aggregate signal from the spherical region. The magnetic moment can be derived from the composite complex signals of three concentric spheres. Whether the object is paramagnetic or diamagnetic (relative to water) can be determined from the imaginary part of the complex signal. Both the thermal noise and the discrete voxels were included in simulations and were studied by the error propagation method. Results: The center of the spherical object was determined within 0.3 voxel from its true center. With a proper choice of the echo time, the magnetic moment of the object can be determined within 5% of the actual magnetic moment. This result has been confirmed by both simulations and error propagation analysis. Conclusion: Our preliminary study demonstrates the feasibility of the new method that can be used to accurately quantify the magnetic moment of a voxel wide object such as microbleeds or implanted nanoparticles without any a priori information.
TU‐D‐332‐04: Modulating Mn2+ Efflux with SEA0400, Using Cardiac Manganese‐Enhanced MRI (MEMRI) T1‐Mapping in a Murine Model35(2008); http://dx.doi.org/10.1118/1.2962595View Description Hide Description
Purpose: is an important regulator of contractile function in the heart. Efflux mechanisms of the intracellular concentration are regulated by the exchanger (NCX) and plasma membrane‐ATPase (PMCA). During myocardial ischemic‐reperfusion intracellular overloads via the reverse mode of the NCX, exacerbating myocardial injuries. Protocols that selectively inhibit this exchanger have shown potential therapeutic effects. Cardiac manganese‐enhanced MRI (MEMRI) can be implemented to quantify concentration in vivo, where has be sugested as a surrogate marker for . This study introduces a potential technique to study cardiac efflux by inhibiting the NCX using SEA0400. Method and Materials: Male C57Bl/6 mice (6–13 weeks) were separated into two groups to study the rate of efflux; a control group and a group treated with SEA0400. Both groups were infused with a single dose of 190±2 nmoles/g BW . The SEA0400 group were injected with 50 mg/kg SEA0400 one hour post‐ infusion. Images were acquired on a horizontal 7.0 T Bruker BioSpec MRI spectrometer equipped with a micro imaging gradient. T1‐maps were acquired pre‐ infusion and at various time points post‐ infusion using an ECG‐gated, flow‐compensated Look‐Locker MRI pulse sequence. The change in relaxivity, ΔR1, in the left ventricular free wall (LV Wall), was calculated at different time points post‐infusion. Results: In the LV Wall 50% of the signal enhancement is attenuated within ∼3–4 hours post‐ infusion. SEA0400 demonstrates the effectiveness of reducing the rate of efflux. At a SEA0400 dose of 50 mg/kg the efflux half‐life was approximately two times longer than the control group. Conclusion: This T1‐mapping technique can be used to quantify efflux rates from the myocardium. By using a NCX inhibiting agent this technique can potentially be employed to interrogate individual efflux mechanisms and rates in vivo.