B. Gino Fallone

Characterization, prediction, and correction of geometric distortion in 3 T MR images

The work presented herein describes our methods and results for predicting, measuring and correcting geometric distortions in a 3 T clinical magnetic resonance (MR) scanner for the purpose of image guidance in radiation treatment planning. Geometric inaccuracies due to both inhomogeneities in the background field and nonlinearities in the applied gradients were easily visualized on the MR images of a regularly structured three-dimensional (3D) grid phantom. From a computed tomography scan, the locations of just under 10 000 control points within the phantom were accurately determined in three dimensions using a MATLAB-based computer program. MR distortion was then determined by measuring the corresponding locations of the control points when the phantom was imaged using the MR scanner. Using a reversed gradient method, distortions due to gradient nonlinearities were separated from distortions due to inhomogeneities in the background B0 field. Because the various sources of machine-related distortions can be individually characterized, distortions present in other imaging sequences (for which 3D distortion cannot accurately be measured using phantom methods) can be predicted negating the need for individual distortion calculation for a variety of other imaging sequences. Distortions were found to be primarily caused by gradient nonlinearities and maximum image distortions were reported to be less than those previously found by other researchers at 1.5 T. Finally, the image slices were corrected for distortion in order to provide geometrically accurate phantom images.

A TCP-NTCP estimation module using DVHs and known radiobiological models and parameter sets

Radiotherapy treatment plan evaluation relies on an implicit estimation of the tumor control probability (TCP) and normal tissue complication probability (NTCP) arising from a given dose distribution. A potential application of radiobiological modeling to radiotherapy is the ranking of treatment plans via a more explicit determination of TCP and NTCP values. Although the limited predictive capabilities of current radiobiological models prevent their use as a primary evaluative tool, radiobiological modeling predictions may still be a valuable complement to clinical experience. A convenient computational module has been developed for estimating the TCP and the NTCP arising from a dose distribution calculated by a treatment planning system, and characterized by differential (frequency) dose‐volume histograms (DDVHs). The radiobiological models included in the module are sigmoidal dose response and Critical Volume NTCP models, a Poisson TCP model, and a TCP model incorporating radiobiological parameters describing linear‐quadratic cell kill and repopulation. A number of sets of parameter values for the different models have been gathered in databases. The estimated parameters characterize the radiation response of several different normal tissues and tumor types. The system also allows input and storage of parameters by the user, which is particularly useful because of the rapidly increasing number of parameter estimates available in the literature. Potential applications of the system include the following: comparing radiobiological predictions of outcome for different treatment plans or types of treatment; comparing the number of observed outcomes for a cohort of patient DVHs to the predicted number of outcomes based on different models/parameter sets; and testing of the sensitivity of model predictions to uncertainties in the parameter values. The module thus helps to amalgamate and make more accessible current radiobiological modeling knowledge, and may serve as a useful aid in the prospective and retrospective analysis of radiotherapy treatment plans. PACS number: 87.53.Tf

A two-step scheme for distortion rectification of magnetic resonance images

The aim of this work is to demonstrate a complete, robust, and time-efficient method for distortion correction of magnetic resonance (MR) images. It is well known that MR images suffer from both machine-related spatial distortions [gradient nonlinearity and main field (B0) inhomogeneity] and patient-related spatial distortions (susceptibility and chemical shift artifacts), and growing interest in the area of MR-based radiotherapy treatment planning has put new requirements on the geometric accuracy of such images. The authors present a two-step method that combines a phantom-based reverse gradient technique for measurement of gradient nonlinearities and a patient-based phase difference mapping technique for measurement of B0 inhomogeneities, susceptibility, and chemical shift distortions. The phase difference mapping technique adds only minutes to the total patient scan time and can be used to correct a variety of images of the same patient and anatomy. The technique was tested on several different phantoms, each designed to isolate one type of distortion. The mean distortion was reduced to 0.2 +/- 0.1 mm in both gradient echo and spin echo images of a grid phantom. For the more difficult case of a highly distorted echo planar image, residual distortion was reduced to subvoxel dimensions. As a final step, the technique was implemented on patient images. The current technique is effective, time efficient, and robust and provides promise for preparing distortion-rectified MR images for use in MR-based treatment planning.

A continuous penalty function method for inverse treatment planning

Conventional inverse treatment planning attempts to calculate dose distributions that may not be feasible given the specified dose levels to various anatomical structures. A technique for inverse treatment planning has been developed that uses only target dose levels which are easily selectable to be feasible. A nonlinear constrained minimization problem is formulated to reflect the goal of sparing critical organs as much as possible while delivering a certain target dose within specified uniformity. The objective function is the squared dose delivered to critical organs. The constraints require the delivery of certain target dose within specified uniformity and non-negative pencil beam weights. A continuous penalty function method is introduced as a method for solving the large-scale constrained minimization problem. The performance of the continuous penalty function method is optimized by numerical investigation of few numerical integration schemes and a pair of weighting functions which influence the utility of the method. Clinical examples are presented that illustrate several features of the technique. The properties of the continuous penalty function method suggest that it may be a viable alternative to conventional inverse treatment planning.

Investigation of a 3D system distortion correction method for MR images

Interest has been growing in recent years in the development of radiation treatment planning (RTP) techniques based solely on magnetic resonance (MR) images. However, it is recognized that MR images suffer from scanner‐related and object‐induced distortions that may lead to an incorrect placement of anatomical structures. This subsequently may result in reduced accuracy in delivering treatment dose fractions in RTP. To accomplish the accurate representation of anatomical targets required by RTP, distortions must be mapped and the images rectified before being used in the clinical process. In this work, we investigate a novel, phantom‐based method that determines and corrects for 3D system‐related distortions. The algorithm consists of two key components: an adaptive control point identification and registration tool and an iterative method that finds the best estimate of 3D distortion. It was found that the 3D distortions were successfully mapped to within the voxel resolution of the raw data for a 260×260×240mm3 volume. PACS numbers: 87.61.‐c, 87.53.Tf, 87.53.Xd, 87.56.‐v, 87.56.Fc, 87.62.+n

Assessment of a commercially available automatic deformable registration system.

In recent years, a number of approaches have been applied to the problem of deformable registration validation. However, the challenge of assessing a commercial deformable registration system – in particular, an automatic registration system in which the deformable transformation is not readily accessible – has not been addressed. Using a collection of novel and established methods, we have developed a comprehensive, four‐component protocol for the validation of automatic deformable image registration systems over a range of IGRT applications. The protocol, which was applied to the Reveal‐MVS system, initially consists of a phantom study for determination of the systems general tendencies, while relative comparison of different registration settings is achieved through postregistration similarity measure evaluation. Synthetic transformations and contour‐based metrics are used for absolute verification of the systems intra‐modality and inter‐modality capabilities, respectively. Results suggest that the commercial system is more apt to account for global deformations than local variations when performing deformable image registration. Although the protocol was used to assess the capabilities of the Reveal‐MVS system, it can readily be applied to other commercial systems. The protocol is by no means static or definitive, and can be further expanded to investigate other potential deformable registration applications. PACS numbers: 87.19.xj, 87.56.Da, 87.57.nj

Exploiting the chemical shift displacement effect in the detection of glutamate and glutamine (Glx) with PRESS.

A PRESS (Point RESolved Spectroscopy) sequence for the improved detection of the C2 protons of Glx (glutamate and glutamine) at approximately 3.75ppm is presented in this work. It is shown that for spins like the C2 protons of Glx which are involved solely in weak coupling interactions, the chemical shift displacement effect can be turned to advantage by exploiting PRESS refocusing pulses with bandwidths less than the chemical shift difference between the target spins and the spins to which they are weakly coupled. The narrow-bandwidth PRESS sequence allows refocusing of the J-coupling evolution of the target protons in the voxel of interest independently of echo time yielding signal equivalent to that which can be obtained with a one-pulse acquire sequence (assuming ideal pulses and ignoring T2 relaxation). The total echo time of PRESS was set long enough for the decay of macromolecule signal and the two echo times were empirically optimized so that the Glx signal at 3.75ppm suffered minimal contamination from myo-inositol. The efficacy of the method was verified on phantom solutions of Glx and on brain in vivo.

Design and Optimization of a Novel Bored Biplanar Permanent-Magnet Assembly for Hybrid Magnetic Resonance Imaging Systems

We present a novel design for a biplanar permanent-magnet assembly suitable for use in hybrid magnetic resonance imaging (MRI) systems. The key feature of our design is a large cylindrical hole that is longitudinally bored through the entire magnet assembly. The presence of the bore permits the potential inclusion of additional peripheral devices within or near the magnet structure that may benefit from being placed along, or oriented parallel to, the main magnetic field. In particular, the magnet assembly can be considered for use in an integrated system consisting of a 6 MV medical linear accelerator (linac) coupled to an MRI system for state-of-the-art real-time image-guided adaptive radiotherapy. We use magnetic field calculations based on the finite-element method (FEM) to quantify the detrimental effects of the bore on the field homogeneity in the imaging volume for pole-piece designs found commonly in industry. We then demonstrate that shape optimization of the pole pieces yields novel pole designs that lead to suitable levels of field homogeneity. We examined the resultant magnetic field within the bore for the optimized design and found that it has maximum field homogeneity.

Clinical Evaluation of Normalized Metal Artifact Reduction in kVCT Using MVCT Prior Images (MVCT-NMAR) for Radiation Therapy Treatment Planning

PURPOSE To evaluate the metal artifacts in diagnostic kilovoltage computed tomography (kVCT) images of patients that are corrected by use of a normalized metal artifact reduction (NMAR) method with megavoltage CT (MVCT) prior images: MVCT-NMAR. METHODS AND MATERIALS MVCT-NMAR was applied to images from 5 patients: 3 with dual hip prostheses, 1 with a single hip prosthesis, and 1 with dental fillings. The corrected images were evaluated for visualization of tissue structures and their interfaces and for radiation therapy dose calculations. They were compared against the corresponding images corrected by the commercial orthopedic metal artifact reduction algorithm in a Phillips CT scanner. RESULTS The use of MVCT images for correcting kVCT images in the MVCT-NMAR technique greatly reduces metal artifacts, avoids secondary artifacts, and makes patient images more useful for correct dose calculation in radiation therapy. These improvements are significant, provided the MVCT and kVCT images are correctly registered. The remaining and the secondary artifacts (soft tissue blurring, eroded bones, false bones or air pockets, CT number cupping within the metal) present in orthopedic metal artifact reduction corrected images are removed in the MVCT-NMAR corrected images. A large dose reduction was possible outside the planning target volume (eg, 59.2 Gy to 52.5 Gy in pubic bone) when these MVCT-NMAR corrected images were used in TomoTherapy treatment plans without directional blocks for a prostate cancer patient. CONCLUSIONS The use of MVCT-NMAR corrected images in radiation therapy treatment planning could improve the treatment plan quality for patients with metallic implants.

T2 determination of the J-coupled methyl protons of lipids: In vivo ilustration with tibial bone marrow at 3 T

To demonstrate how J‐coupling modulations of the CH3 lipid resonance can be minimized enabling a representative T2 to be measured.