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"The major advances over the last few years have related to the development of MRI for functional and interventional use as well as improvements in its traditional morphological applications." --Dr. H Satishchandra, Head-Department of Radiodiagnosis & Imaging, BMC&H
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Dr. H Satishchandra (MD, FICR), Professor & Head, Department of Radiodiagnosis & Imaging, Bangalore Medical College & Research Institute, Government of Karnataka, Bangalore
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Dr. H Satishchandra, MD, FICR, is the Professor and Head of the Department of Radiodiagnosis & Imaging at Bangalore Medical College & Research Institute, Government of Karnataka, Bangalore. Dr. Satishchandra is also the immediate Past National President of the Indian Radiological & Imaging Association (IRIA). He recently chaired the organizing committee of the 61st National Conference of IRIA.
Magnetic Resonance Imaging (MRI) is a noninvasive method of mapping the internal structure of the body. It completely avoids the use of ionizing radiations and appears to be non-hazardous. MRI has now rapidly progressed from being a technique with great potential to one, which has become the primary, and often the only, diagnostic method required for many clinical problems. Technical advances have included improvements in spatial resolution, contrast and in particular, speed of imaging. In fact, the major advances over the last few years have related to the development of MRI for functional and interventional use as well as improvements in its traditional morphological applications.
NEW AND EMERGING TECHNOLOGY
New Instrumentation
Many new engineering and technological advances integrated in the design of up-to-date MR systems. For example, below are some of the new technologies:
- Magnet Design:The overall size of a magnet needed to produce a high (1.5T) field suitable to whole-body imaging has been dramatically reduced. This has consequences for patient friendliness as well as sitting requirements. There have been several developments which may influence the choice of field strength. Higher field whole-body research systems operating at 3T and above 8T have been installed. Several of the major MR system providers are manufacturing very high field systems for clinical use.
- Gradient Subsystem:One of the major factors influencing image acquisition speed is the functionality of the gradient subsystem. Shielded gradient coil sets have been introduced, which minimize the production of artifacts due to eddy currents. Such advancements have facilitated the introduction of echo-planar imaging into clinical practice.
- Radiofrequency (RF) Subsystem: Surface RF coils are used to improve the signal-to-noise ratio returned from a localized anatomical region under study. The simultaneous use of multiple surface coils or phased-array surface coils is now widespread and enables the acquisition of images covering a large anatomical area (e.g., the whole spinal column) which have the benefit of a high signal–noise ratio. Perhaps more important, in terms of patient throughput, is the advent of SENSE and SMASH technology. The techniques which have arisen from this technology utilize the spatial information present in RF coil arrays of specific geometry to reduce the number of phase-encode steps necessary and hence the time taken to acquire a complete image over a given area.
- Computing Power: This has enabled real-time image reconstruction without noticeable delay following data acquisition, which is of particular importance when hundreds of images (e.g. tissue perfusion studies) or thousands of images (e.g., blood-oxygen level dependent fMRI studies) are acquired during a single patient scan episode. Digital data processing speeds will become even more critical if ultra-high resolution imaging at very high field strengths (=3T) with acquisition matrices of at least 2048 x 2048 becomes widespread.
Fast Sequences and New Contrast Mechanisms
There are several reasons for minimizing the time taken to acquire an image dataset. These include: the potential improvement in patient throughput; minimizing any stress experienced by the patient remaining in the confines of the magnet; reducing the chances of image degradation due to voluntary or involuntary patient movement and the ability to monitor changes in imaging over short time-spans.
Echo-Planar Imaging
Echo-planar imaging (EPI) uses a train of gradient echoes after a single 90° RF excitation and is one of the fastest techniques available. Single-Shot EPI has led to a plethora of clinical studies using techniques such as full-tensor diffusion and arterial perfusion imaging (using both endogenous and exogenous contrast mechanisms). In addition, snapshot EPI has the ability to visualize fluid flow patterns. Other areas where EPI has proved particularly useful include rapid imaging of the heart and major vessels and imaging of the liver.
Single-Shot Fast Spin Echo
This is currently being studied in many clinical situations including pediatric neuroradiology where it is hoped the ultra-fast methods may reduce the sedation/general anesthesia rate.
Spectroscopy
- Proton Spectroscopy: The brain has been the most extensively studied organ to date by proton MRS. This technique seems to be particularly useful for the diagnosis and staging of prostate cancer.
- Phosphorus Spectroscopy: Phosphorus spectroscopy has been used to evaluate the metabolism of a variety of tissues. This information has been utilized to study the neonatal brain. A number of applications have been investigated, including tumor response to treatment, changes in liver metabolism in patients with cancer and more recently the evaluation of myocardial pathology. Furthermore, some basic research into the effects of metabolism on performance in top athletes has shown promising results as a functional physiological marker.
Interventional MRI
Clinical MRI is mainly used as a diagnostic imaging modality, but it is perhaps no great surprise to find that it is increasingly being applied to guide various forms of intervention. Several studies have shown that MR can be used to aid biopsy guidance and to monitor laser, radiofrequency and cryoablation therapies. Major advantages of MR-guided intervention include the lack of exposure to ionizing radiation to both staff and patients.
Conclusion
In the first 20 years of clinical practice, magnetic resonance imaging has moved from being a curiosity to being the technique of first choice in a variety of diseases.
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