State of the Art MRI: A Technology Update(2004)

drgmscn · 发表于 2008-8-4 17:01:10

by Geoffrey Clarke PhD, Andres Rahal MD

With significant advances in detector technology, new MR applications continue to be developed and adopted into clinical practice at an impressive rate.

MRI has become a mature and important clinical imaging procedure. MRI is not only distinguished by the flexibility of the image contrast between tissues that it allows, but also by the range of anatomical and physiological studies that can be undertaken with this technology. Current advancements in MRI technology leverage progress made in recent years in superconductivity, digital signal processing, networking electronics, and image visualization. Despite its relative maturity, MRI technology is still very dynamic and new applications are being developed and adopted into clinical practice at an impressive rate. Those innovations, which have resulted in significant recent advancements in newer clinical MRI products, are discussed below.

ADVANCES IN DETECTOR TECHNOLOGY

Magnets . MRI systems currently being offered for clinical applications in North America have static magnetic fields ranging from 0.2T to 3T. Magnets continue to be offered as both open and tubular systems. For both categories, the use of superconducting magnets has allowed the available field strength to increase. Open magnet systems have field strengths as high as 0.7T, while the fastest growing sector of the market are MRI systems based on 3T superconducting magnets. The current 3T magnet offerings also include self-shielding, which allows them to be housed in a room about the same size as 1.5T superconducting MRI systems required in the early 1990s. Presently, the main limitation of the 3T systems is the relatively limited selection of radio-frequency (RF) coils available compared to their 1.5T counterparts. Additional savings in space and in maintenance cost are secondary to no longer needing a large outer liquid nitrogen core to maintain the magnet at absolute zero temperatures, with the adoption of cryogen gas recycling devices that permit recirculation of the evaporated helium.

Phased array coils and parallel imaging . Phased array coil technology was originally developed to improve the intensity uniformity of MR images obtained using surface coils, while preserving their inherent gain of signal-to-noise ratio (SNR). Recently, new methods for encoding the MRI signal are being adopted that fall under the generic name of parallel imaging. ¹ Parallel imaging methods use the unique spatial perspective of the signal that comes from individual coils, along with the known sensitivity profiles of the surface coil elements within the array (Figure 1). This strategy allows a reduction in the amount of time required to obtain the MR image up to a factor related to the number of independent coil channels within the array. Multiple RF channels are required to process these data independently, and in principle, an eight-channel coil would be able to image eight times as fast. However, practical considerations limit image acceleration to values below the maximum allowed by theory.

The clinical impact of this parallel imaging will be considerable for the higher field (3T) MRI systems. The use of parallel imaging technology can not only reduce scan time but also reduce the number of RF pulses required to form an image. This will be important to limit heating of the patient to regulatory guidelines, particularly for body imaging at 3T. When parallel imaging is employed, image uniformity and SNR both also are compromised as scan times are reduced. Innovative phased array coil designs with up to 10 channels have been developed to accommodate parallel imaging methods at higher magnet fields. The newest MRI systems are being offered with the receiver system designed to have the capacity for a highly scalable number of individual RF channels. The maximum number of RF channels that can practically be incorporated into the design of clinical MRI systems is currently a matter of considerable engineering controversy.

Digital signal processing . The development of very fast-switching gradient coils over the past few years and the trend to move to magnets operating at higher frequencies have led to requirements for faster signal digitization. Advances in digital signal processing (DSP) methods have allowed the introduction of a new class of methods for encoding data in the time domain. Many of these gain encoding efficiencies by exploiting a priori knowledge of the character of the MRI signal. Coupled with hardware advances made in the speed of analog-to-digital converters, the application of DSP methods has allowed the MRI signal to be recorded at higher frequencies and with greater fidelity.

USABILITY ISSUES

Operator consoles continue to develop in a windowed, mouse-driven graphical user interface (GUI) environment. The operator's familiarity with the user interface should improve as many of the major MRI systems manufacturers migrate from UNIX-based platforms to systems using Windows? operating systems. The use of higher field magnets requires even greater diligence on the part of the technologist to ensure that patients with inappropriate implants are not scanned using MRI. The greater possibility of inadvertently causing patient burns during RF excitation at high 3T requires that the operator choose protocols that have reductions in the flip angle or number of RF pulses. ²

Image processing and display . Algorithms for processing MR angiographic data continue to develop and provide magnetic resonance angiography (MRA) displays with fewer artifacts than the simple maximum intensity projection algorithm. Current packages also include automatic rendering of MRA data sets. Other image processing packages that continue to improve are those used for obtaining semiquantitative measures of tissue perfusion using first-pass contrast-enhanced methods and semiautomated methods for defining ventricular myocardium and following the motion of the ventricular wall through the cardiac cycle. The development of surface rendering algorithms for a number of applications, notably virtual angioscopy and virtual colonography, also continues.

Significant advances have been made in determining the statistical character of functional MRI (fMRI) using blood-oxygen-level-dependent contrast mechanisms. These developments have been primarily aimed at the scientific investigation of normal and abnormal brain function. Although this method has not yet had a significant impact on clinical practice, as the clinical niches for fMRI become better defined, they may become more widely used. A recent study suggested that fMRI may offer a more economical approach to the assessment of language lateralization than the standard Wada test. ³

Software for the coregistration of MRI data with images from other radiological modalities continues to be developed. This technology is particularly important for stereotactical surgical and radiosurgical treatment planning. Image display technology used in MRI is similar to that used for other radiological devices that produce direct digital images. These technologies have been previously described in this journal. ⁴

Data management advances . Many, if not most, of the current MRI systems are DICOM 3-compliant, although the specific degree of their compliance is still somewhat variable. Most manufacturers also allow image data to be written to portable media, most often CD-ROM. The interfaces to the portable media writing systems are, for the most part, rudimentary, with data often written in a simple list mode and cataloging functions dependent on the capabilities of the DICOM reader software used by the referent. Compatibility issues between CD-ROM writers and the MRI system's main computer may create problems for service engineers. It is anticipated that the incorporation of writable DVD technology will be available in the next year or so, which will be useful as the average size of clinical image data sets is increasing rapidly.

Workflow improvements . The development of clinical imaging protocols at different sites is now facilitated by software that allows sharing of established imaging protocols over the Internet between different clinical sites; this technology improves patient throughput since the sequence of images for each individual patient can be planned in advance on a separate computer, before the patient goes in the scanner. This saves time from the scanning session with the patient and helps to consistently retain the best imaging parameters across all patients.

Other new software . Recent commercial adoption of spiral scanning methods gives the radiologist yet another approach to image acceleration that complements the gains obtainable with parallel imaging. Spiral scan methods oversample the center of k-space by following a radial trajectory and are purported to be less sensitive to motion artifacts.

With high-speed imaging and stronger magnetic fields employed in open MRI systems, progress continues toward developing MR image-guided surgery protocols. In addition, advanced MRA methods, particularly using first-pass contrast-enhanced MRA, are gaining wider acceptance. The relatively short time required for acquiring echo-planar images has propelled the development of diffusion tensor imaging, especially in the brain, where it can be used to assess neuronal fiber tracks.

NEW CLINICAL APPLICATIONS

It is anticipated that the development of magnetic resonance spectroscopy (MRS) for the 3T products will boost development and clinical evaluation of new examination protocols, particularly with regard to the evaluation of prostate and ovarian cancer. Higher field strengths produce not only greater SNR for the MRS study, but also improved definition of the spectral lines obtained from the intracellular chemical components under investigation.

Y6 j+ J9 Q, D, f$ d

Figure 1. Parallel imaging with multiple radio frequency (RF) coils in a phased-array can be analogous to acquiring multiple projections of the x-ray beam in computed tomography. Each RF coil receives signals from different parts of the body and this spatial information inherent in the signal from each coil can be processed to produce significant improvements in imaging speed.

Central nervous system . The use of perfusion and diffusion imaging is becoming a clinical standard in addition to regular MR imaging to help guide the early treatment of stroke. Furthermore, improvements have been observed in MR spectroscopy for the characterization of brain tumors and the follow-up of their treatment.

Musculoskeletal imaging . A recently published study of trends in knee imaging with MRI from 1991 to 1995 reported that the annual rate of MRI knee studies increased by 140% in that time period. ⁵ There is no reason to assume that this trend has abated over the last decade. Indeed, there are at least three different models of dedicated extremity scanners on the market with field strengths ranging from 0.2T to 1T. Studies are benefiting from improved methods for visualizing cartilage, tendons, and ligaments using very short time-to-echo (TE) methods that are enabled by the fast gradient systems that are now commonplace on modern high-end MRI systems. ⁶ Utilization of these contrast mechanisms is also being extended to brain and abdominal organ imaging. The incorporation of parallel imaging methods and the use of high-field MRI in the development of this method will increase the overall use of these techniques.

Imaging in cancer . MRI continues to be a key method for establishing the staging of cancer over CT in specific sites of the body including the uterus and bladder, prostate, ovaries, and head and neck cancer. Contrast-enhanced MRI can reduce the number of biopsies in women with abnormal mammograms; and in difficult cases it can reveal residual cancer and help in treatment planning

MR angiography . The increased speed of newer MRI systems, coupled with improved resolution and software processing methods, allows impressive angiographic imaging results through all of the human body. The quality of modern MRA images is such that they are currently accepted by physicians for guidance of clinical decisions, saving many times the need of performing invasive procedures on patients to perform the same diagnosis, and permit better planning of such procedures to perform invasive treatment of the abnormality.

Cardiac MRI . Recent small-scale studies have demonstrated that cardiac magnetic resonance (CMR) can produce images of myocardial perfusion that compare favorably with those obtained using positron emission tomography and single photon emission computed tomography. ⁷ Heavily T1-weighted images that depict the late enhancement of myocardial scar tissue can be produced with MRI that have significantly increased temporal and spatial resolution compared with nuclear medicine methods. Coronary angiography with MRI continues to develop in competition with similar advances using modern CT methods. Currently, neither approach is at a stage that it can supplant x-ray angiography performed in the catheterization laboratory. An intriguing new development is the use of MRI to image the walls of blood vessels. The aim of these projects is to use the high spatial resolution and the dynamic soft tissue contrast of MRI to characterize vulnerable atherosclerotic plaques. ⁸

ECONOMIC ISSUES

Despite the high initial capital cost, the number of MRI scanners in the United States continues to grow at a steady rate. Current estimates put the number of installed MRI systems at well over 6,000. Like personal computer technology, the costs of MRI systems decrease inversely to the rate of growth of their capabilities. For instance, the cost of a 3T MRI system is about the same as a top-of-the-line 1.5T system 5 years ago. This trend toward using the most advanced technologies from a number of sectors also results in the systems becoming obsolete in 5 to 8 years unless the system manufacturers provide an affordable upgrade path to allow their customers access to the latest computer, magnet, receiver, and gradient technologies.

A major constraint to the rate of diffusion of MRI technology is the shortage of both trained technologists and radiologists to produce and interpret MRI studies. MRI is a multifaceted technology and the complexity increases as new applications are developed. The programs that accredit the education of radiologists and technologists are slowly adapting to this reality, but are still behind the curve compared to the economic demand and requirements for highly trained personnel in the MRI clinic.

Geoffrey C. Clarke, PhD, is a medical physicist and associate professor of radiology at the University of Texas Health Science Center in San Antonio.

Andres Rahal, MD, is a radiologist from Colombia who is currently working on his PhD in radiological sciences at the University of Texas Health Science Center in San Antonio.

References:
5 a+ O7 G! m q( [

Sodickson DK, McKenzie CA. A generalized approach to parallel magnetic resonance imaging. Med Phys. 2001;28:1629-1643. r2 [- E& g, Y+ ?" j `2 |" p" |
Schaefer DJ, Felmlee JP. Radio-frequency safety in MR examinations. RSNA Special Cross-Specialty Categorical Course in Diagnostic Radiology: Practical MR Safety Considerations for Physicians, Physicists and Technologists. Oak Brook, Ill: RSNA; 2001:111-121. % ^" U' E) t) a6 h+ S8 h/ j
Medina LS, Aguirre E, Bernal B, Altman NR. Functional imaging versus Wada test for the evaluation of language lateralization: cost analysis. Radiology. 2004;230:49-54. ; t4 N5 e. ^. I! }9 _* r
Maidment A, Seibert JA, Flynn MJ. Technical advances in DR and CR, Part 1. Imaging Economics. 2004;17(3):45-51. . {) P8 c4 b4 C7 r& c
Solomon DH, Katz JN, Carrino JA, et al. Trends in knee magnetic resonance imaging. Med Care. 2003;41:687-92. 9 `" D R t* s% \2 d3 w8 l9 F% h
Robson MD, Gatehouse PD, Bydder M, Bydder GM. Magnetic resonance: an introduction to ultrashort TE (UTE) imaging. J Comput Assist Tomogr. 2003;27:825-46. 2 H, J$ z/ \: d3 \8 h* n! L
Ibrahim T, Nekolla SG, Schreiber K, et al. Assessment of coronary flow reserve: comparison between contrast-enhanced magnetic resonance imaging and positron emission tomography. J Am Coll Cardiol. 2002;39:864-870. 4 F7 G; S* v. g" w6 b+ \+ O
Yuan C, Mitsumori LM, Ferguson MS, et al. In vivo accuracy of multispectral magnetic resonance for identifying lipid-rich necrotic cores and intraplaque hemorrhage in advanced human carotid plaques. Circulation. 2001;104:2051-2056.

账号		自动登录	找回密码
密码			马上注册