Volume 33, Issue 6, June 2006
Index of content:
- Imaging Scientific Session: Room 330 A
MO‐D‐330A‐01: Retrospective Sorting of 4D CT Into Breathing Phases Based On Imaging Analysis of a Fixed‐Geometry Fiducial33(2006); http://dx.doi.org/10.1118/1.2241403View Description Hide Description
Purpose: To evaluate a novel fiducial method for retrospective sorting of four‐dimensional computed tomography (4D‐CT). Breathing‐induced motion and deformation of internal anatomy confounds planning and delivery of radiotherapy. Patient‐specific assessment of respiratory motion using 4D‐CT is becoming more clinically accepted as it depicts discrete sampling of the internal anatomy throughout the respiratory cycle. Various strategies for sorting the images exist. External strategies rely on integration of a motion sensor with the CT device but may lack robustness with respect to sensor location. Internal strategies based on image analysis alone may not be robust enough. This work proposes a hybrid retrospective sorting method based on an easily positioned fiducial device that does not require additional hardware and is usable with any multi‐slice or helical CT scanner. An image‐processing algorithm automatically extracts the breathing phase of each slice by fiducial position and sorts images into phases accordingly. Method and Materials: A 50 cm rod‐like device covering the entire field‐of‐view is placed along the patient, providing a well distinguishable fiducial position in each CT slice without impacting image quality. Image analysis determines the fiducial centroid position in each slice with sub‐mm accuracy, allowing phase‐based binning of the image slices according to breathing phase. To validate the method, a motion phantom with the rod affixed was subject to a cine 8‐slice CT scan with 2.5 mm slice thickness. Images were sorted with the fiducial method and compared with images sorted by a commercial 4D‐CT system. Results: Phase‐sorted images of the phantom were reconstructed using the fiducial method. Image quality was comparable to those reconstructed by the commercial 4D‐CT system. Conclusions:Image analysis of a rod fiducial allows retrospective sorting of 4D CT according to breathing phase. This method does not require additional hardware, interfacing with the CT scanner, or manual interaction with the images.
33(2006); http://dx.doi.org/10.1118/1.2241404View Description Hide Description
Purpose: During diagnostic x‐ray CTimaging procedures or image guided radiotherapy,image quality will be degraded if target organs move during the data acquisition. This can be caused by patients' occasional motion or by intrinsic motion like cardiac and respiratory motion. The inconsistency in the projection data is the major reason for the image quality degradation. We present and validate a method to improve the consistency of the projections using a novel Fan‐beam Data Consistency Condition (FDCC) such that the image quality can be improved. Method and Materials: Computer‐simulated dynamic phantoms are generated and projection data are acquired from these dynamic phantoms. Using the FDCC, individual projection data from one view of fan‐beam projections can be estimated from filtering all the other projection data acquired from different view angles. Then those projections contaminated by motion are re‐estimated using the FDCC, resulting in a corrected sinogram. A standard Fan‐beam Filtered Back Projection (FBP) reconstruction algorithm is then used to reconstructimages from the corrected sinograms. Motion artifacts can be alleviated using this procedure. Results:Images are reconstructed from both the original sinogram where projections are contaminated by motion and the corrected sinogram after applying the FDCC. Strong motion artifacts are observed in the imagesreconstructed from the contaminated sinogram while improvement can be found in the reconstructed images using the corrected sinogram.Conclusions: A novel method using the new FDCC is proposed to combat the motion artifacts due to the temporal inconsistency in the projection data. Numerical simulations were conducted to demonstrate the potential of this correction scheme to mitigate motion artifacts. Thus, the preliminary numerical results indicate that the FDCC has potential use in combating both cardiac and respiratory motion in CTimaging.
MO‐D‐330A‐03: Correction of Streaking Artifacts in CT Images and Its Influence On Monte Carlo Dose Calculations33(2006); http://dx.doi.org/10.1118/1.2241405View Description Hide Description
Purpose: To quantify the impact of streaking artifacts in CTimages due to metal implants in patients on Monte Carlo dose calculation and to determine the impact of their correction. Method and Materials: For CT artifact correction a method of interpolation of missing data in sinograms was developed. Three contrast phantoms were constructed containing two steel cylinders that produced streaking artifacts. CT scans of the phantoms were obtained and the images were corrected for the artifacts. Three sets of Monte Carlo dose calculations (MCDC) using EGSnrc/DOSXYZnrc code were performed. Dose was calculated on: (1) the original CTimage, (2) the CT artifact corrected image, and (3), the exact phantom geometry. The dose distributions of the original CTimages and the CT artifact corrected images were then compared to the dose calculated on the exact geometry. Results: A calibration point for metal had to be added to the default EGSnrc CTcalibration curve to improve dose calculation results. Additional improvement in dose calculation results and in image quality was noted after the artifact correction was done. MCDC without adding the extra calibration point and without correction for streaking artifacts was found to lead to large dose errors. The error in dose calculations performed with the default calibration was found to be 25% in the original CTimages. The error improved greatly when the CTimages were corrected for artifacts and when the extended calibration was used; the error decreased then to less than 2 %. Conclusion: This work proves that the correction of streaking artifacts is important for MCDC; it significantly decreases dose calculation error and it improves image quality. The work also suggests that for MCTP an additional calibration point for a metallic material should be added to the default CTcalibration curve.
33(2006); http://dx.doi.org/10.1118/1.2241406View Description Hide Description
Purpose: We aim to devise new weighting schemes to improve the signalto‐noise ratio (SNR) in imagesreconstructed from data acquired in reduced fan‐beam scans by fully utilizing the redundant information. Method and Materials: Recently, we have developed a backprojection‐filtration (BPF) algorithm, which can reconstruct ROI‐images from the transversely truncated data in a reduced‐scan with a scanning angular range less than that in a short‐scan. However, the measured data in a practical reduced scan contain some redundant information. In this work, we devised two weighting schemes to appropriately incorporate the redundant data. Applying the weighting schemes to the BPF algorithm, we derived two new algorithms, derivative of weighted BPF (WD‐BPF) and weighted‐derivative BPF (DW‐BPF) algorithms. Both of these two algorithms can be used to improve the SNR in reconstructed images.Results: The ROI‐images are reconstructed from the truncated projection data in a short‐scan and a reduced‐scan, respectively. For the reduced‐scan, some shading artifacts appear in the imagesreconstructed by use of the WD‐BPF algorithm. This is caused by the numerical error in the derivative of the discontinuous weighting function. In contrast, artifact‐free images can be obtained by use of the DW‐BPF algorithm. For the short‐scan, both algorithms can obtain artifact‐free images, because the weighting function is smooth. The results in the noise study show that imagenoise obtained by use of WD‐BPF and DW‐BPF algorithms are similar within the ROI. Conclusion: We propose two weighting schemes for handling the data redundancy in reduced‐scan fan‐beam CT. Our results demonstrate that the proposed two algorithms can utilize the redundant data to improve the signal‐noise ratio. Moreover, the noise properties of these two algorithms are similar to each other.
33(2006); http://dx.doi.org/10.1118/1.2241407View Description Hide Description
We evaluated Feldkamp artifacts, which are specific to cone‐beam CT, in phantom and clinical studies using the 256 multi‐detector‐row CT (256MDCT), and compared the reconstruction accuracy of axial and helical scans.
Imagenoise, slice sensitivity profile (SSP) and artifacts with the 256MDCT were evaluated using a phantom and the results were compared to those with a 64MDCT. We also examined chest and abdomen scans produced with the 256MDCT in volunteers.
For the axial scan, Feldkamp artifacts were visualized as high‐frequency streak‐like artifacts that are oriented horizontally at the edge of the scan region in the phantom study. Similar results were obtained with the volunteers in soft‐tissue regions near either bony structures or air pockets. Feldkamp artifacts with the 256MDCT can lead to misdiagnosis if not correctly identified and minimized via helical scanning. Imagenoise was less for axial than helical scans, while SSP was better with helical than axial scans.
Feldkamp artifacts observed in the 256MDCT images, however, did not generally affect the interpretation of images. The 256MDCT promises more accurate diagnosis, and will provide volumetric cine images of wider cranio‐caudal coverage, enabling new applications of CT.
MO‐D‐330A‐06: Development of the X‐Ray Detector with Sequential Readout Circuits for Multidetector‐Row Computed Tomography33(2006); http://dx.doi.org/10.1118/1.2241408View Description Hide Description
Purpose: To develop a low‐cost X‐ray detector with sequential readout circuits, to realize enough low noise for multidetector‐row computed tomography(MDCT), and to evaluate image quality. Method and Materials: We have developed an X‐ray detector that has a MOS‐switch for each pixel, connects many pixels of a common column with the electric readout circuit, and outputs the signals of these pixels from one circuit by turning on lines of switches in order. It has fewer readout circuits than a conventional MDCT detector, but new design is necessary to realize enough low noise for MDCT. First, to make the required noise specific, we simulated the relation of the detectornoise and imagenoise (simulation(A)). Second, to consider how to realize it, we simulated the detectornoise with the circuit noise model (simulation(B)). Third, we constructed the detector in order to evaluate its noise. Last, we developed a test CTsystem with these detectors to evaluate imagenoise with phantoms. Results: The result of the simulation(A) indicated that detectornoise had to be less than about 10‐k rms electrons, and we found to be able to achieve it by optimizing the circuit parameters of the low pass filter and the data line as a result of the simulation(B). We constructed the detectors with these parameters to evaluate these noise, and it turned out that it was about 10.5‐k rms electrons and the required noise was achieved. Moreover, the result to evaluate the noise from images with phantoms indicated that the main was X‐ray quantum noise and the detectornoise was low enough to be ignored when the object was a cylindrical water‐filled phantom less than about 30 cm in diameter and the slice thickness of the images was 0.625 mm. Conclusion: We developed a low‐noise X‐ray detector with sequential readout circuits for MDCT.
33(2006); http://dx.doi.org/10.1118/1.2241409View Description Hide Description
Purpose: Most tomographic imaging systems available today use a single x‐ray source and multiple projection images are obtained by rotating the x‐ray source around the object. Therefore the data acquisition rate is limited by the gantry rotation speed, which is approaching the physical limit. We proposed to develop a novel stationary scanning x‐ray imaging system based on carbon nanotubefield emission x‐ray (FEX) technology. Instead of a single x‐ray source the proposed system is based on a multi‐pixel FEX source. The new scanner promises a dramatically faster data acquisition rate by reducing or totally eliminating the mechanical motion. Method and Materials: We have constructed a prototype stationary scanning x‐ray imaging system with an array of 9 individually addressable x‐ray source pixels, each of which can produce a different projection image of the object. The core of this novel x‐ray imagingtechnology is a gated carbon nanotubefield emissioncathode array. By programming the gate voltage of the cathode array, the multi‐pixel x‐ray source can generate an electronically triggered scanning x‐ray beam and produce multiple projection images from different viewing angles without mechanical motion. A Hamamatsu C7921 flat panel x‐ray sensor was used to collect all 9 projection images.Results:Tomosynthesisimages of a mouse and a standard breast‐imaging phantom (Stereotactic Needle‐biopsy Tissue Equivalent Phantom, Nuclear Associates, NY) using the prototype stationary scanning x‐ray imaging system are acquired. Tomosynthesisreconstructions were applied to the breast phantom. The slice imagesreconstructed using an iterative reconstruction algorithm clearly show the internal structures of the breast‐imaging phantom at different depths. Conclusion: We have developed a stationary scanning x‐ray imaging system using a carbon nanotube based multi‐pixel FEX source. The mechanical motion free approach can lead to a faster and simplified tomographic imaging system.
Conflict of Interest: Research partially supported by Xintek Inc.
33(2006); http://dx.doi.org/10.1118/1.2241410View Description Hide Description
Purpose: Implementing Quantitative Computed Tomography (QCT) on Multi‐Slice Computed Tomography (MSCT) scanners requires investigating the effects of axial vs. helical scan modes and protocol parameter variations on quantitative data. While previous work in this area focused on single‐slice axial techniques, technological developments in Computed Tomography(CT) justify more complex assessment. Method and Materials: All scans were obtained using two phantoms designed for bone mineral (BM) densitometry: a reference phantom (three different density cores) and a QA torso phantom. Both phantoms have known properties and are required for long‐term quantitative BM density assessment. The scan acquisition parameters that were varied included kV, mA, rotation speed, pitch, image thickness, detector configuration, reconstruction algorithm, table height, and tube temperature. To assess long‐term scanner drift, the QA Torso phantom was scanned multiple times over three months on each of seven MSCT scanners (five GE Lightspeed‐ 16s, one GE Lightspeed Qx/i, and one GE Lightspeed‐Plus). The daily variability of the individual MSCT scanners and scanner‐to‐scanner variability was determined by coefficient of variation (mean/variance) from the QA Torso phantom data sets over time. All data were collected and analyzed in Hounsfield Units (HU) to provide insight about variations upstream of the actual BM density analysis through commercial software.Results: This study found no significant difference (p > 0.05) in mean HU between phantom images obtained using axial and helical scan mode, or when varying most of the other scan acquisition parameters. However, varying kV and reconstruction algorithm did result in significant (p<0.0001) quantitative shifts. Preliminary data indicated daily variability of 0.8% – 1.9% and scanner‐to‐scanner variability of 1.4%.Conclusion: MSCT systems can be optimized for use in determining the BM density of a vertebral body, provided very careful control of scan acquisition protocol is observed.
33(2006); http://dx.doi.org/10.1118/1.2241411View Description Hide Description
Purpose: Different fanbeam reconstruction algorithms are being evaluated and the noise performances of these algorithms are compared at equivalent MTF.Method and Materials: The fanbeam reconstruction algorithms under comparison are FBP with Parkers smooth weighting (PFBP), LCFBP, DFBP, reconstruction algorithm by Noo et.al and exact reconstruction algorithm by Kudo et.al . The MTF from the five different algorithms were plotted and compared. In order to establish the basis for an unbiased comparison of the noise variance between different algorithms, we established the condition of equivalent spatial resolution. To achieve this, a window function was applied to the ramp filtering kernel before back‐projection for PFBP, LCFBP and Kudo's algorithm. A homogenous phantom was numerically simulated and Poisson noise with N0= 2e5 was added to the projection data. The images were reconstructed from projection data with and without the Poisson noise added. These images were then subtracted from each other to result in a subtracted or pure noise image. The variance in these noise images over five different ROIs was subsequently compared. FBP with Parkers smooth weighting was chosen as the gold‐standard and percentage decrease in variance in the imagesreconstructed using other four algorithms with respect to that of PFBP was tabulated. Results: The results showed that the new reconstruction algorithms had better noise performance than state‐of‐the‐art reconstruction algorithm (PFBP) after establishing the condition of equivalent spatial resolution. DFBP and exact reconstruction algorithm by Noo performed much better than the other three algorithms. Equal weighting scheme utilized definitely improved the noise performance over smooth weighting. DFBP showed a decrease of variance by about 23 % compared to PFBP. Conclusion: The reduction in noise variance theoretically leads to a radiation dose reduction by about 23 %. This will be of significant importance especially in pediatric imaging.
Joint Imaging‐History Scientific Session: MRI and Multi‐Modality Imaging
33(2006); http://dx.doi.org/10.1118/1.2241493View Description Hide Description
The history of MRI is now a little over 30 years old. It's impact on the scientific, and in particular medical, community has been immense, and shows no sign of slowing. From the first demonstration in 1973 of the spatial encoding of proton density information in the nuclear magnetic resonance signal, via the application of pulsed magnetic field gradients, a succession of technological developments now makes MRI the modality of choice for a wide range of cross sectional biomedical imaging applications. This talk will offer some insights into the origins of MRI, the rapid development of spin echo and gradient recalled echo, 2D and 3D, imaging and the birth of faster and faster imaging involving multiple echoes (RARE and echo planar imaging). Additionally, interpretation of MRI‐accessible contrasts emerged, offering specific insights into physiology via NMR relaxation time constants and such MRI “flavors” as diffusion weighted imaging. In parallel, MRI‐specific contrast media (“magnetic dyes”) were introduced in the 1980's offering both “enhancement” of MRI signal as well as the opportunity to study dynamic processes (such as bolus tracer passage, with concomitant estimation of perfusion and, later, microvascular permeability). The 1990's saw the continued clinical adoption of MRI as well as the commitment to physiologically‐specific imaging and the introduction of blood oxygenation level dependent (BOLD) contrast and the birth of “functional magnetic resonance imaging, fMRI”, for the spatial mapping of brain functional organization. More recently yet further acceleration in image acquisition speed has been offered by the introduction of multiple (parallel) receiver coil elements and the adoption of the principles of sensitivity encoding. Presently, acceleration factors of ×2 are routine, ×4 are commonplace and ×9 or more are in development. The future offers the possibility of massively parallel acquisitions with effectively single‐shot temporal resolution of the order of milliseconds. While the contrast resolution of MRI is transitioning from anatomy through physiology towards biochemical processes, the speed of MRI is accelerating from static to real‐time, hurdling successive physical and physiological boundaries (such, as involuntary motion, breath‐hold, cardiac cycle and ultimately towards the speed of neuronal processes). The overall vision of this talk is to convey the ongoing development of MRI towards both increasing spatial and temporal resolution as well as towards increasingly specific biological interpretation.
1. To understand the origins and background of today's MRI.
2. To understand the rapid development of MRI acquisition speed.
3. To understand the use of tailored MRI to derive physiologically specific imagecontrast.
4. To anticipate the continued acceleration of both imaging speed and biological application of MRI.
33(2006); http://dx.doi.org/10.1118/1.2241494View Description Hide Description
Purpose: Approximately 170,000 new cases of brain metastases and 17,000 cases of primary braincancer are diagnosed in the United States each year. Stereotactic Radiotherapy (SRT) is fast becoming the method of choice for treatment of non‐superficial brain lesions. SRT treatment plans of malignant and metastatic braintumors typically incorporate a 2 cm margin to account for microscopic tumor spread, however, distant and/or recurrent tumors sometimes occur. Our hypothesis is that paths of elevated water diffusion may provide a preferred route for transport or migration of cancercells through an unknown mechanism. If our hypothesis is correct then future SRT treatment would be modified to provide elongated treatment margins along the paths of elevated water diffusion leading from the primary tumor site; thereby reducing the incidence of recurrence and improving clinical outcomes. Method and Materials: MR diffusiontensorimaging datasets were acquired in patient subjects treated with SRT of malignant and/or metastatic braintumors. DTI was performed using an EPI sequence on a 1.5T clinical GE scanner with 20 serial axial images of voxel dimensions 0.976×0.976×6 mm; TR 10s; TE 89.4 ms; 25 diffusion gradient directions plus 3 reference (b=0) scans. Following SRT, patients were given repeated MRI follow‐ups at regular intervals to identify early tumor recurrence. When recurrent tumors were detected, DTIstudio and FSL software was used to compute paths of preferred water diffusion through the primary tumor site and the site of recurrence. Results: There exists an apparent correlation between patterns of recurrence in the brain and paths of elevated diffusion leading from the primary braintumor.Conclusions: Our preliminary results on a small number of patient datasets suggest that this hypothesis is correct and further investigation is warranted. Future work will employ a more sophisticated fiber analysis on additional patient images and verification with animal models.
TU‐C‐330A‐03: Sulci Density Map to Aid in the Use of Apparent Diffusion Coefficient for Therapy Evaluation33(2006); http://dx.doi.org/10.1118/1.2241496View Description Hide Description
Purpose: To quantify the density and spatial variation of sulci in the human brain so as to more accurately calculate Apparent Diffusion Coefficients (ADCs) for use in radiotherapy evaluation. Methods and Materials: ADCs are calculated for Volumes Of Interest (VOI) using Diffusion Weighted Magnetic Resonance Imaging (DWMRI).
In the brain, the sulci are the narrow fissures separating convolutions and are filled with CerbroSpinal Fluid (CSF). Since CSF is free fluid, a VOI that contains a high density of sulci should, in principal, have a higher ADC.
Using ImageJ ( http://rsb.info.nih.gov/ij/ ), we have analyzed sagittal T1 weighted MRimages of a number of patients. The images sets were acquired on one of two different GEMRI machines: 1.5T and 3.0T field strength. We have plotted the normalized pixel standard deviation, as a function of distance from a medial point. We have concentrated on the cerebral hemispheres, superior to the corpus callosum so as to focus on sulci and avoid voxel dissimilarities due to variations in other brainanatomy.Results: Relative to the 25% most medial slices, we see a decrease in the normalized standard deviation of pixel intensity of 13.7% ± 5.6% (SD) of the next 25% of the slices. The most lateral 50% of the slices (25% left and right) had about the same normalized standard deviation as the most medial (increase of 1.0% ± 7.1% (SD)). These results are consistent with the fact that near the periphery of each brain hemisphere, there are more sulci. Conclusion: Sulci density, as measured by normalized standard deviation of pixel intensity, has a substantial variation across the hemispheres of the brain. This fact should be considered when assessing variations in ADCs used for radiotherapy evaluation.
33(2006); http://dx.doi.org/10.1118/1.2241497View Description Hide Description
Purpose: in vitro 1H MRS studies have suggested that Gln/Glu ratio is useful in detecting an early stage of malignant transformation. Due in part to the technical difficulty, in vivo detection of well‐separated Gln and Glu of human tumors has not been reported. A recent study proposed that a standard STEAM sequence with optimized TE/TM (80/50 ms) can be used to simultaneously detect Gln and Glu peaks around 2.4 ppm with virtually no spectral overlap at 4T. In this study, we report preliminary results of the application of this technique for braintumors.Method and Materials: 1H MRS of eight patients with braintumors were acquired with a quadrature head coil in a 4T system. Four patients had biopsies within two weeks of their respective MRS study, and the remainder had biopsies prior to their scans. A standard STEAM sequence was used with TE=80ms, TM=50 ms, TR = 2–3 seconds, spatial resolution = 4–12 cm3, and acquisition time = 15–30 minutes. All data sets were processed using LCModel. Results: In addition to providing “typical” characteristics of NAA, Cho, and Cr for braintumors, one of the striking observations is consistent and remarkable increase in Gln concentration (water as reference) and Gln/Cr ratio for all cases studied; while decrease in Glu concentration (water as reference) but wide variability in Glu/Cr ratio (0–3 times of corresponding contralateral control). The results also demonstrate occurrences of opposite changes between Gln and Glu content for braintumors, illustrating the importance of simultaneously detecting Gln and Glu for the study of tumor metabolism. The consistent and remarkable increases in Gln content suggest direct Gln involvement in tumor metabolism, in agreement with previous biochemical analysis. Conclusion: In vivo Glu, Gln can provide useful information to help diagnosis of braintumors.
TU‐C‐330A‐05: Optimization of Outer Volume Suppression for Improved Prostate MR Spectroscopic Imaging33(2006); http://dx.doi.org/10.1118/1.2241498View Description Hide Description
Purpose: To adapt a new MR Spectroscopy (MRS) technique employing non‐cuboidal voxels, called conformal voxel MRS (CV‐MRS), for use in prostate spectroscopicimaging in order to reduce contamination of spectra by lipid signal surrounding the prostate.
Method and Materials: CV‐MRS uses twenty or more spatial saturation (SS) pulses, placed around the prostate, to reduce the lipid signal affecting the spectra within the prostate. A water/oil phantom was designed to simulate the prostate and surrounding lipid signal. Use of the new CV‐MRS technique reduced the lipid signal contamination by 84% as compared to standard cuboidal voxel MRS. To further reduce the lipid contamination, the routinely used 90 degree flip angle used for each SS pulse was modified to take into account the regrowth of lipid signal with its short T1relaxation time.Results: Contrary to our expectations, resulting spectra from the optimized approach actually showed an increase in lipid contamination by 10%. We tracked the problem down to overlapping SS pulses. Using a simulated 3D model, we found that 68% of the volume we were trying to saturate experienced multiple overlapping SS pulses, with some regions being saturated 7 or more times. Regions of the volume experiencing an even number of SS pulses were found to increase the lipid contamination signal by 88% to 200%. Conversely, regions experiencing an odd number of SS pulses had a reduction in lipid contamination of 55%. Conclusion: Changing the ordering of the SS pulses, such that the overlapping pulses occur later in the train of 20 SS pulses reduced the problem of lipid signal from those overlapping volumes. In summary, we have developed an improved outer volume saturation technique which reduces lipid contamination problems in prostate MR spectroscopicimaging.
33(2006); http://dx.doi.org/10.1118/1.2241499View Description Hide Description
Purpose: In order to develop radiation treatment planning based on a displacement probability function for lungtumors we developed a dynamic MRI protocol to image the internal human respiratory lung motion during a several minute period that simulates the duration of a radiation treatment, evaluated the long time displacement probability distribution function (PDF) of pulmonary vessels as surrogate tumors, and assessed its reproducibility with repeat imaging. Methods and Materials: TrueFISP (fast imaging with steady‐state precession) sequence was adapted to acquire real time MR images of human lungs during 5 minute scans in both sagittal and coronal planes. A total of 26 pulmonary vessels from different regions (upper, middle and lower) in 3 healthy subjects were examined. Motion profile and displacement PDF of each tracked pulmonary vessel were evaluated. Experiments were repeated after 2‐3 weeks to test the reproducibility. Results: Motion profiles and displacement PDF of the same subject showed similarity, but great variation between different subjects. Displacement PDFs varied tremendously but tended to stabilize during the 5‐minute scan, and were reproduced reasonably well to various degrees in the repeated experiments after a subject specific stabilizing time (270s, 120s, 200s for Subject 1, 2, 3 respectively). Conclusions: Experiments for the first time using ultra‐fast real time MRI in extended time scans produce stable and reproducible displacement PDF of internal pulmonary structures, weakly depending upon different individual breathing patterns. This methodology is being investigated in a clinical trial at our institution to determine in a larger scale whether the reproducibility of motion is statistically significant and whether patients with lungtumor exhibit similar predictable breathing characteristics.
33(2006); http://dx.doi.org/10.1118/1.2241500View Description Hide Description
Purpose: To demonstrate the feasibility of a new imaging technique consisting in simultaneous Magnetic ResonanceMammography and Electrical Impedance Scanning of the breast which does not require theoretically fat suppression and injection of paramagnetic contrast agents. Method and Materials: A theoretical formulation of the expected signal was developed and verified by the computer simulation demonstrating the distortion of the magnetic field caused by the injected currents. Two conducting breast phantoms were designed using breast tissue and tumor equivalent materials: (a) a soap phantom with a soap and salt solution as a cancer surrogate and (b) an agar phantom with a piece of fat‐free hotdog as a cancer surrogate. The stabilization paddles in a Symphony Breast Biopsy Array were modified to include Faraday shield electrodes fed by a variable frequency power source. The phantoms were placed between the modified paddles and imaged with a Siemens Magnetom Symphony Maestro Class 1.5 Tesla system with the current densities reaching 4.5 A/m2 and frequencies ranging from 200 Hz to 1000 Hz. Gradient re‐phased, spin echo, and echo‐planar sequences were tested to maximize the expected output signal. The images were subjected to statistical analysis to determine a set of parameters which produce detectable signal with minimal injected currents.Results: The expected signal was observed in agreement with the simulation. The agar phantom proved to be more stable and showed consistency in the imaging results. A large number of variables, including imagingsequence parameters, experimental setup parameters, and phantom quality, requires a more thorough analysis of the proposed technique. Conclusions: We have demonstrated the feasibility of simultaneous magnetic resonance and electrical impedanceimaging that has the potential to revolutionize currentMagnetic ResonanceMammography. A significant effort should be put into optimization of imaging parameters at minimum current without compromising patient safety or signal quality.
33(2006); http://dx.doi.org/10.1118/1.2241501View Description Hide Description
Purpose: To assess the performance of the induction motor of a rotating‐anode x‐ray tube in the magnetic fringe field of a clinical MRIscanner. The x‐ray tube must be placed in the fringe field near the entrance of an MRIscanner so that a hybrid x‐ray/MRI system for use in percutaneous aortic valve replacement in aortic stenosis patients can be constructed. Method and Materials: A standard rotating‐anode x‐ray tube insert was placed into the fringe field of a 1.5 T unshielded research MRIscanner. The induction motor in the x‐ray tube was aligned so that the magnetic field lines were in the plane of its stator core. The induction motor of the x‐ray tube was placed in magnetic fields ranging from 0 to 500 G. The magnetic fields were measured with a Model 4048 Gauss meter. The rotation speed of the anode was measured using a strobe light. The power consumed by the motor during operation in the fringe field was measured separately using a PLM‐1 power meter. The fringe fields of an actively shielded clinical 1.5 T scanner were measured with the Gauss meter and compared to the fields applied to the motor. Results: The anode rotation speed dropped from 3437 +/− 6.8 rpm to 2744 +/− 5.3 rpm when the magnetic fringe field was increased from 0 to 400 G. The average power consumed by the motor increased from 70.5 +/− 0.4 W to 78.2 +/− 0.1 W when the fringe field was increased from 0 to 500 G. Conclusion: This work indicates the feasibility of safely operating an x‐ray tube induction motor in the fringe field of an MRIscanner.Power consumption did not significantly increase and anode rotation speed did not fall below 3000 rpm until a fringe field exceeding 300 G was applied.
33(2006); http://dx.doi.org/10.1118/1.2241502View Description Hide Description
Purpose: To investigate the feasibility of performing longitudinal image‐based measurements using CT and MR to estimate contrast agent concentrations in organs and tissues in vivo. Method and Materials: A CT and MR contrast agent (200 mg/kg of iodine and 16 mg/kg of gadolinium encapsulated in liposomes) was administered intravenously to a 2 kg New Zealand White rabbit. At 5 minutes, 24, 48, 72, 96, 120 and 168 hours following contrast injection, the rabbit was imaged in CT (120kV, 200mA) and in MR (3D FSPGR, TR/TE= 9.8/4.3). 1mL of blood was collected from the same rabbit at each of the above times. The rabbit liver and spleen were harvested at the study end point (168 hours). The blood and tissues samples were then analyzed using high performance liquid chromatography (HPLC) to measure iodine content and inductively coupled plasma atomic emission spectrometry (ICP‐AES) to measure gadolinium content. Results: The differential blood CT attenuation vs. plasma iodine concentration correlation was well approximated with a linear fit (R2=0.9), while the differential blood MR signal intensity vs. plasma gadolinium concentration correlation was found to be nonlinear. These correlations were used to estimate the iodine and gadolinium content in the liver and the spleen. Using the CT correlation, the liver and the spleen iodine content were estimated to be 70% and 60% of the extracted amounts, respectively. The MR‐based method did not yield satisfactory gadolinium content estimates. Conclusion: This study attempted to correlate CT attenuation and MR signal increases to local iodine and gadolinium concentrations, respectively. In CT, the linear correlation obtained with blood data allowed for estimation of iodine content in the liver and spleen to 60–70% accuracy. In MR, although the presence of the contrast agent could be detected visually over a 7‐day period, additional effort is required to achieve reliable agent concentration estimations.
Image Segmentation and CAD
33(2006); http://dx.doi.org/10.1118/1.2241546View Description Hide Description
Purpose: To develop a computerized lesion detection method for DCE‐MRI breast images using the fuzzy c‐means clustering algorithm. Method: Contrast‐enhanced MR imaging is increasingly being incorporated into procedures for the screening of women at high risk of developing breast cancer. Such screening programs may potentially benefit from computer prompts that indicate potential lesion sites. In addition, analysis of other enhancing regions in the breast may reduce the number of false detections. Thus, we are developing an automated computerized lesion detection method based on the fuzzy c‐means clustering algorithm. The proposed method consists of four stages: (1) Breast volume segmentation based on a volume growing method; (2) Fuzzy c‐means clusteringanalysis on voxel‐based kinetics within the 4D breast image data (3D over time); (3) Voxel‐by‐voxel membership assignment to the most‐enhancing categories; and (4) Connectivity & size criteria for eliminating some false‐positive detections. Methods were evaluated by calculating detection sensitivity for malignant lesions, detection sensitivity for all lesions, and number of false‐positive detections per breast volume for output from the most‐enhancing kinetic categories. Results: Our preliminary studies are based on 20 MRI cases including 21 lesions (9 biopsy‐proven malignant cases, 5 biopsy‐proven benign cases; 6 cases without pathological proof). Based on computer‐identified regions from the most enhancing membership category, the proposed method correctly detected 16 lesions, including all nine malignant ones. In addition, most of the benign cases fell into either the most‐enhancing or second‐most‐enhancing categories. Preliminary results yielded, on average, 9 false‐positive detections per breast volume, which will subsequently be input to the classifier stage that exams morphological and kinetic characteristics for false positive reduction. Conclusion: The preliminary results with our FCM‐based computerized MRI lesion detection method are promising for potential use in breast cancer screening. Conflict of Interest: M.L.G. is a shareholder in R2 Technology, Inc.
33(2006); http://dx.doi.org/10.1118/1.2241547View Description Hide Description
Search involves locating lesions in images under conditions of uncertainty regarding the number and locations of lesions that may be present. A mathematical model of search is presented that applies to situations, as in the free‐response paradigm, where on each image the number of normal regions that could be mistaken for lesions is unknown and the number of observer generated localizations of suspected regions (marks) is unpredictable. The search model is based on a two‐stage descriptive model proposed in the literature according to which at the first stage the observer uses mainly peripheral vision to identify likely lesion candidates, and at the second stage the observer decides whether or not to report the candidates. The mathematical search model regards the unpredictable numbers of lesion and non‐lesion localizations as random variables and models them via appropriate statistical distributions. The model has three parameters quantifying the perceived lesion signal‐to‐noise ratio, the observer's expertise at rejecting non‐lesion locations and the observer' expertise at finding lesions. A figure‐of‐merit quantifying the observer's search performance is described, as well as ROC and FROC curves predicted by the search model. Finally, we describe a preliminary method for estimating the parameters of the search model from free‐response data.