Volume 23, Issue 2, February 1996
Index of content:
23(1996); http://dx.doi.org/10.1118/1.597703View Description Hide Description
Computed radiography(CR) has shown promise in digital mammographic screening due to its good low spatial frequency MTF and its relatively wide exposure latitude. The CRimage format has not gained acceptance clinically because of reduced high spatial frequency resolution as compared to film‐screen images. X‐ray capillary optics, aligned between the breast and CR phosphor imaging plate, will capture primary x‐ray photons almost exclusively. Due to the very small angle of acceptance, scatteredphotons angled more than about 1.6×10−3 radians from primary trajectory will not be accepted at the capillary optic entrance. The virtual elimination of detected scatter means almost 100% of the possible primary contrast should be visible in the image. In addition, the image can be magnified without focal spot blurring. Effective resolution of CRimages can be increased by a factor equal to that magnification. Clinical implementation of future capillary optics are expected to be either in the form of a large, stationary, post‐patient optic that accepts primary from the entire breast or a fan‐shaped optic that is scanned across the breast. Measurements of a test capillary optic showed a reduction of scatter fraction to 0.018. Images of a lucite contrast detail phantom revealed a corresponding increase in imagecontrast when compared to anti‐scatter grid and no grid methods. Spectral transmission measurements using a high‐purity germanium detector showed good primary transmission (45%–50%) in the mammographic energy range. The MTF measurements of both stationary and scanned capillary optics showed improvement at the 5% MTF level to 8.4 mm−1 for scanned optics and 9.2 mm−1 for stationary optics representing a 68% and 84% respective increase over the CRMTF without magnification or capillary optics.
23(1996); http://dx.doi.org/10.1118/1.597704View Description Hide Description
In this paper we examine the few‐view reconstruction problem as it applies to imaging vascular trees. A fully automated reconstruction algorithm is described that circumvents the traditional ‘‘correspondence problem,’’ using only notions of consistency and connectivity. It is assumed that the vascular tree is a connected structure and that its centerlines have been identified in three or more images. The first of three steps in the procedure involves generating a connected structure that represents the multiplicity of solutions that are consistent with any two (different) projections. The second step assigns to each branch in this structure a measure of agreement based on its relationship with one or more additional views of the vasculature. The problem then becomes one of propagating this information, via connectivity relationships and consistency checks, throughout the above structure to distinguish between the branches comprising the imaged structure and the accompanying artifacts. In this paper we present the theory and methodology of the technique, while in a companion paper we address the issue of validation via simulations and experiments. Together, these papers shed some light on why ambiguities arise and often lead to errors in the few‐view reconstruction problem. Strategies to handle these errors are described and results are presented that demonstrate the ability to obtain adequate reconstructions with as few as three distinct views.
Dual‐slice spiral versus single‐slice spiral scanning: Comparison of the physical performance of two computed tomography scanners23(1996); http://dx.doi.org/10.1118/1.597705View Description Hide Description
In this paper we deal with two types of spiral scanners; one is a single‐slice spiral scanner, while the other employs dual‐slice technology into spiral scanning. Physical performance parameters, including imagenoise,contrast resolution, spatial resolution (transversal and longitudinal), and radiation exposure are measured. Computer simulations based on two interpolation methods (180° and 360° linear interpolation) are also used in evaluating the slice‐sensitivity profile (SSP) and noise. The results show that the noise behaves in the same way for both types of scanners. The noise change, relative to that of the standard scan with the same scanning parameters, depends solely on the interpolation algorithm. Table speed and scanner geometry (either single slice or dual slice) have no effect on the noise value. For the given table speed, as well as individual detector collimation (slice width) the dual‐slice scan results in better longitudinal resolution (SSP) compared to a single‐slice scan if the scan is obtained with nonoverlapping slices (pitch greater than 2). This is because the dual‐slice scan obtains twice the number of nonoverlapped projections for the same length, which reduces the degradation of the slice profile by using more densely arranged projections (in the longitudinal direction) for the interpolation. In the dual‐slice scanner the workable scan rate is extended up to pitch 4 compared to a pitch of 2 for the single‐slice scanner. Therefore, the dual‐slice spiral scanner is preferred in applications requiring an increased scan rate with comparative image quality.
23(1996); http://dx.doi.org/10.1118/1.597706View Description Hide Description
Helical Computed Tomography (HCT) has become the method of choice for many routine clinical studies. The advantages of HCT include the capability of scanning a complete anatomical volume in a single breath hold, the capability of generating images at any desired location, and the improved patient throughput. However, these advantages come at the expense of some image quality compromises. This is mainly caused by the fact that the projection set is inherently incomplete and inconsistent, due to the constant patient translation during the data acquisition process. In this paper, we will briefly review the research work performed in this area and present a more general approach to the problem. We give two specific examples of the general approach and compare the performance of one of the examples with one of the best methods available today.
23(1996); http://dx.doi.org/10.1118/1.597794View Description Hide Description
In fluoroscopic portal imaging systems, a metal plate is bonded to a phosphor screen and together these act as the primary x‐ray sensor. The light from the screen is collected and imaged by a lens on the target of a video camera. The demagnification (M) between the large area of the phosphor being imaged and the small active area of the video camera results in poor optical coupling between the screen and the video camera. Consequently x‐ray quantum noise is small compared to other noise sources. By reducing the demagnification, the light from the screen is collected more efficiently, so we were able to increase the x‐ray quantum noise relative to other noise sources and thus unambiguously identify it. The noise power spectrum was measured as a function of M to determine the relationship between the x‐ray quantum noise, shot noise, and amplifiernoise. It was found by extrapolation to clinical demagnifications that the amplifiernoise dominates x‐ray quantum noise at all spatial frequencies, but the shot noise was less than the x‐ray quantum noise at low spatial frequencies. For low spatial frequencies, this implies that a secondary quantum sink can be avoided. If amplifiernoise could be sufficiently reduced, x‐ray quantum limited images could be obtained in clinical systems at low spatial frequencies.
23(1996); http://dx.doi.org/10.1118/1.597708View Description Hide Description
Some U.S. hospitals double‐load x‐ray cassettes for certain procedures. Loading two films in the same cassette for portable emergency room (ER), intensive care unit (ICU), or operating room radiographs provides both the referring clinicians and the radiologists with immediate images. Our study demonstrates a cost increase of 15%, an increase in air kerma for a chest x ray from 0.12 to 0.35 mGy (12–35 mrad), slight differences in optical density, imagecontrast, and spatial resolution under double‐loading conditions. Our study shows that double loading cassettes may improve patient care by economically expediting the communication of radiographic findings. The decision to double load portable ICU or ER cassettes must be based on a balance of factors.
23(1996); http://dx.doi.org/10.1118/1.597711View Description Hide Description
23(1996); http://dx.doi.org/10.1118/1.597796View Description Hide Description
A noninvasive assay for the spatial distribution of the percentage cellularity in human bone marrow is presented. Twelve individuals were studied using two magnetic resonance imaging techniques: (1) fast spin echoimaging with frequency selective presaturation, and (2) three‐point chemical shiftimaging. The data were compared to results obtained using a previously validated stimulated echo spectroscopic method. The results of this study demonstrate that a measure of the percentage cellularity in bone marrow is possible using magnetic resonance imaging techniques provided that high‐quality water or lipid suppression is achieved across the region of interest. Since the method is applicable to bone marrow at any anatomic location, it may prove useful in dosimetric calculations during and after a course of internal or external beam radiotherapy.
Correction of motion artifacts in linogram and projection reconstruction MRI using geometry and consistency constraints23(1996); http://dx.doi.org/10.1118/1.597713View Description Hide Description
Motion results in various artifacts such as blurring and streaks in clinical imaging of subjects based on reconstruction from projections. We model subject motion‐induced artifacts due to scaling, translational and rotational motion. A correction algorithm based on the Ludwig–Helgason consistency conditions is derived here. These conditions are satisfied whenever the projection data are consistent. We apply the algorithm to simulated data collected on linogram (LR) and projection reconstruction (PR) geometries, and to real PR geometry data, in magnetic resonance imaging(MRI). The results show that motion‐induced in‐plane, interview artifacts can be reduced with application of the algorithm. The algorithm is general enough to be applied to certain other cases arising in tomographic imaging.
Transmission imaging of large attenuators using a slant hole collimator on a three‐headed SPECT system23(1996); http://dx.doi.org/10.1118/1.597715View Description Hide Description
By combining conjugate views, truncation‐free attenuation profiles of patients can be obtained by using slant hole collimators on three‐headed SPECT systems. The alterations in reconstruction algorithms necessary for use with slant hole collimators and potential image artifacts are discussed. Based on an evaluation of the size of objects that can be imaged without truncation and the size of the overlap region in the conjugate views, a 15° slant angle was determined to be optimal. Studies with a 30° slant hole collimator verified the ability of slant hole transmission imaging to provide accurate, truncation‐free attenuation maps of a 56 cm lateral width phantom. The center of rotation was determined to be dependent on the slant angle and radius of rotation of the slant collimator. These studies also demonstrated that the spatial resolution in the transaxial plane of the attenuation maps depends on radius of rotation of the slant hole collimator, but does not depend on the radius of rotation of an uncollimated transmission source. A multiline transmission source was investigated for use with estimating the attenuation map in Tc‐99m labeled sestamibi perfusion imaging.
23(1996); http://dx.doi.org/10.1118/1.597718View Description Hide Description
The delayed‐gamma neutron activation facility at Brookhaven National Laboratory was originally calibrated using an anthropomorphic hollow phantom filled with solutions containing predetermined amounts of Ca. However, 99% of the total Ca in the human body is not homogeneously distributed but contained within the skeleton. Recently, an artificial skeleton was designed, constructed, and placed in a bottle phantom to better represent the Ca distribution in the human body. Neutron activation measurements of an anthropomorphic and a bottle (with no skeleton) phantom demonstrate that the difference in size and shape between the two phantoms changes the total body calcium results by less than 1%. To test the artificial skeleton, two small polyethylene jerry‐can phantoms were made, one with a femur from a cadaver and one with an artificial bone in exactly the same geometry. The femur was ashed following the neutron activation measurements for chemical analysis of Ca. Results indicate that the artificial bone closely simulates the real bone in neutronactivation analysis and provides accurate calibration for Ca measurements. Therefore, the calibration of the delayed‐gamma neutron activation system is now based on the new bottle phantom containing an artificial skeleton. This change has improved the accuracy of measurement for total body calcium. Also, the simple geometry of this phantom and the artificial skeleton allows us to simulate the neutron activation process using a Monte Carlo code, which enables us to calibrate the system for human subjects larger and smaller than the phantoms used as standards.
23(1996); http://dx.doi.org/10.1118/1.597721View Description Hide Description
An evaluation of moderating materials is presented for boronneutron captured therapy. PbF2 was considered and its performance was compared to that of Al2O3 and AlF3. PbF2 is found quite effective as a moderator for medical epithermal neutron beams. (AIP)