Experts Column
Dose: Walter Huda |
MRI: David Pickens |
Professional: Jerry Allison
IMRT: Ivan Brezovich |
Quantitative Imaging: Ehsan Samei
Dose: Walter Huda, PhD, FAAPM
With CT doses climbing higher, what techniques might the industry employ to reduce radiation dose without sacrificing image quality? Do we expect more improvements to come from detector design, or reconstruction algorithms?
In the US, population radiation doses from medical imaging are now in excess of 3 mSv per year, with half of this exposure a result of CT imaging. Accordingly, the medical imaging community is very interested in finding ways to reduces CT doses but without sacrificing diagnostic performance. It is unlikely that any significant improvements will be achieved by improving the medical imaging chain (e.g. radiation detectors), since CT is a quantum limited imaging modality and current CT detectors do an excellent job of detecting x-rays, with detection efficiencies in excess of 90%.
One promising avenue is the introduction of improved image reconstruction algorithms that could reduce the amount of noise (mottle) in images without sacrificing other aspects of image quality (contrast, resolution, artifacts etc.). In the past few years, there has been an explosion of interest in Iterative Reconstruction algorithms that offer the potential to reduce CT artifacts (streak), whilst also reducing image noise. Commercial products are now available from GE (ASIR) and Siemens (IRIS), and are undergoing clinical evaluation.
In PET and SPECT, Iterative Reconstruction algorithms are now the norm, and have displaced Filtered Back Projection for image reconstruction. In CT, the main limitation is the need to 100 times the computing power needed for FBP, but it does seem likely that IR techniques will take over in the foreseeable future. Whether IR methods will also result in sizeable dose reductions is a matter of scientific investigation, which should become evident in the next few years.
MRI: David Pickens, PhD, FAAPM
What are the latest trends in the field of Magnetic Resonance Imaging?
Magnetic resonance imaging (MRI) is simultaneously a mature and an evolving technology. While MRI has been available commercially for 25 or more years and is a mainstay of clinical imaging, ongoing developments provide new ways to image patients. Recently, parallel imaging has been incorporated into most high-performance systems because of the resulting improvements in speed and image quality. Ongoing developments in this area include new coil, software, and system designs with vendors offering 32 channel coils and researchers working on 128 channel or more systems.
Other recent developments are advances in fusing images from positron emission tomography (PET) and MRI. Papers in the literature demonstrate the value of these fused images, leading at least one vendor to design and introduce a commercial fully integrated system that solves the problems associated with moving patients between imaging devices.
Other advancements include the increasing use of magnetic resonance spectroscopy (MRS) for clinical applications in both diagnosis and in monitoring therapy for cancer, made possible by improved hardware, pulse sequences, and analysis software.
Lastly, much work is being done to develop techniques at high fields, those above the FDA limits of 4T ranging up to 11.7T. A number of systems are installed at research institutions to evaluate these high field imagers for mostly head, but increasingly, for body applications as body resonators become available. A large amount of work remains to be done for these systems to arrive in the clinic due to many technical problems to be overcome. However, high and very high field systems are being used in preclinical systems to develop new methods for evaluating cancer therapy, among many applications. These pre-clinical applications will most likely drive the developments for human imaging at high fields in the future.
Professional: Jerry D. Allison, PhD, FAAPM
What professional developments are on the horizon for AAPM members?
The advent of CAMPEP accredited training, CAMPEP accredited residencies and Maintenance of Certification will likely improve the initial qualifications and career long growth of board certified medical physicists. These transitions may also improve our standing with our physician colleagues since they will better understand the requisite training and experience necessary to be a board certified medical physicist. Regardless of these changes in our profession, the KEY element to keeping our profession and our professional careers healthy is active participation in the professional, educational and scientific activities of our Association and our Chapter. I encourage all medical physicists to find a way to give back to our profession.
IMRT: Ivan A. Brezovich, PhD, FAAPM
IMRT moving toward Stereotactic Radio Surgery (SRS)
Linac-based small field radiotherapy for cranial lesions, at its time a major improvement over whole brain irradiation, dates back to the 1980s. Its practicality for the treatment of multiple, irregularly shaped lesions, however, is limited by the need to reposition the patient between lesions and to apply multiple "shots" to elongated targets that are difficult to treat with circular collimators. For similar reasons, the GammaKnife loses its practicality as targets become highly irregular and their number increases beyond three to five.
High-definition MLCs with 2.5 mm leaf width, sub-millimeter precision of MLC leaf and couch positioning, and dose rates up to 2,400 MU/min now make it feasible to treat multiple isolated metastatic lesions expeditiously, with only few gantry rotations. Cone-beam CT scans taken immediately before treatment may eliminate the need for invasive head frames. However, before this promising modality becomes clinical routine, a number of issues have to be addressed.
MLCs and treatment planning systems were originally developed for relatively large single targets, and physicists do not routinely commission these devices for the small radiation fields required by SRS. Repositioning patients on the treatment couch also becomes very critical. Even a small rotational misalignment of the head translates into considerable displacements of targets that are located far off isocenter. Extensive research is now under way on how to efficiently measure small-beam parameters, test or modify treatment planning systems for the small fields, and how to accurately reproduce the treatment planning geometry. Integral doses and organs at risk become of heightened concern as the number of beams increases. While inter-fraction reproducibility may be achievable with current technology using cone beam CT scans preceding treatment and subsequent 6-dimensional couch adjustments, non-invasive immobilization requires further refinement and verification to assure intra-fraction stability.
Quantitative Imaging: Ehsan Samei, PhD, DABR, FAAPM, FSPIE
As many in this readership would attest, in its first century (1895-1995), medical imaging has been primarily developed as a qualitative technique. Imaging devices have often been seen as "cameras" with which one can take "pictures" of the interior of the human body. Radiology is correspondingly developed as a subspecialty focused on trying to make sense of what the images exhibit in the context of other related clinical data. The latter, understanding the meaning of the image data in the context of other clinical data, has always been an objective from which radiology has drawn its relevance and significance.
The second century of medical imaging (1995-) has witnessed notable leaps in the advancement of the imaging technologies enabling images to become more and more robust and reproducible. A corresponding reduction in variability across all the components of imaging systems has provided an opportunity to extract more quantitative information from image data in such a way that the information can contribute to clinical care in a more quantitative way. A quantitative approach to imaging enables one to characterize a medical condition in more definitive ways than what can be afforded by a more conventional interpretive qualitative approach. This offers unprecedented opportunities to quantitatively characterize disease conditions, a cornerstone of the practice of evidence-based medicine. More specifically, quantitative imaging enables monitoring the progress of a disease or a treatment regimen across time, making it possible to identify or optimize treatment techniques towards more efficacious treatment that are evidence-based and patient-specific.
These worthy goals are only possible if imaging is performed in such a way that the quantitative information can be most precisely extracted from the image data. This is not necessarily the case currently as imaging systems are still primarily designed and used to provide the best interpretive quality and not necessarily the best quantitative quality. To orient the imaging practice towards quantitative ends, one needs to have relevant figures of merit -- one cannot improve something that cannot be measured. Researchers currently pursue quantitative imaging focused on identifying figures of merit that are explicitly directed towards quantitative precision. Implicit in quantification is characterization of specific imaging tasks. Current work focuses on precision in the estimation of pathological properties in CT, MRI, and PET, designing imaging protocols that can render least variability for clinically-relevant quantitative tasks. The goal is to make imaging precise enough to be used as a predictor of therapeutic interventions.