Magnetic resonance imaging: bioeffects and safety concerns

Magnetic resonance imaging (MRI) is the state-of-the-art noninvasive imaging modality in clinical diagnosis. During MRI examination, the patient is exposed to three different forms of electromagnetic radiation: (i) a static magnetic field, (ii) gradient magnetic fields, and (iii) radiofrequency (RF) fields. Each of these may cause significant adverse bioeffects if applied at sufficiently high exposure levels. This article describes in some detail the areas of health concern for both the patient and the health practitioner with respect to the use of clinical MRI, in addition to describing the potential bioeffects of electromagnetic radiations used in this sophisticated imaging modality.     

Nuclear magnetic resonance imaging: a tool for microscopists?

Nuclear magnetic resonance (NMR) imaging is revolutionizing the field of noninvasive diagnosis because of its excellent resolution and inherent high soft-tissue contrast. It is also feasible to use NMR imaging in the microscopic milieu. However, application at this level is handicapped by several technical and theoretical limitations. Foremost among these is the difficulty of obtaining sufficient signal from voxels of microscopic dimensions in sufficiently short time scales to make the technique practical. Other limitations include the effects of Brownian motion and the inherent frequency dispersion over each voxel. These constraints limit three-dimensional resolution to 1-10 micron. Even within these limits, the nondestructive nature of the technique and its unique sensitivity to the state of water within cells and tissues promise to make it a valuable tool for future microscopists.     

Principles of magnetic resonance imaging

This article aims to provide an educational document of magnetic resonance imaging principles for applied biomedical users of the technology. Basic principles are illustrated using simple experimental models on a preclinical imaging system.     

Functional magnetic resonance imaging: imaging techniques and contrast mechanisms.

Functional magnetic resonance imaging (fMRI) is a widely used technique for generating images or maps of human brain activity. The applications of the technique are widespread in cognitive neuroscience and it is hoped they will eventually extend into clinical practice. The activation signal measured with fMRI is predicated on indirectly measuring changes in the concentration of deoxyhaemoglobin which arise from an increase in blood oxygenation in the vicinity of neuronal firing. The exact mechanisms of this blood oxygenation level dependent (BOLD) contrast are highly complex. The signal measured is dependent on both the underlying physiological events and the imaging physics. BOLD contrast, although sensitive, is not a quantifiable measure of neuronal activity. A number of different imaging techniques and parameters can be used for fMRI, the choice of which depends on the particular requirements of each functional imaging experiment. The high-speed MRI technique, echo-planar imaging provides the basis for most fMRI experiments. The problems inherent to this method and the ways in which these may be overcome are particularly important in the move towards performing functional studies on higher field MRI systems. Future developments in techniques and hardware are also likely to enhance the measurement of brain activity using MRI.     

A review of the current use of magnetic resonance imaging in pregnancy and safety implications for the fetus

This paper presents an overview of the application of and risks of exposure to Magnetic Resonance Imaging (MRI) in pregnancy. It reviews the risks to the fetus by considering the hazards in terms of the three main components of an MRI system. These are the static magnetic field, the time-varying magnetic gradient fields and the pulsed radio frequency fields. The hazards discussed are biological effects, miscarriage, heating effects and acoustic noise exposure. This paper also presents a survey of MRI sites within the United Kingdom to ascertain the extent of MRI usage in pregnancy. To validate the situation of MRI in pregnancy a survey was sent to 352 MR units throughout the United Kingdom. The questions were grouped to assess (a) maternal MRI diagnosis (b) fetal MRI and (c) work practices for pregnant MRI staff. The results showed that 91% of sites were imaging pregnant women in need of diagnosis in the second and third trimester. This paper highlights that MRI can add information for fetal central nervous system abnormalities identified by ultrasound screening, however within the UK direct fetal imaging was only performed in 8% of sites. This paper indicates the need for research to be undertaken for specific MRI clinical conditions. It also advises that risk assessment for pregnant staff working in MRI is performed, and that there is a clear need for further research into the effects of MRI in pregnancy as there is a need for clear authoritive advice.     

Plaque imaging and characterization using magnetic resonance imaging: towards molecular assessment

Identification of high-risk atherosclerotic lesions prone to rupture and thrombosis may greatly decrease the morbidity and mortality associated with atherosclerosis. High-resolution magnetic resonance imaging (MRI) has recently emerged as one of the most promising techniques for the non-invasive study of atherothrombotic disease, as it can characterize plaque composition and monitor its progression. The development of MRI contrast agents that specifically target components of the atherosclerotic plaque may enable non-invasive detection of high-risk lesions. This review discusses the use of high-resolution MRI for plaque detection and characterization and the potentials of "Molecular Imaging" using a variety of molecules present in atherosclerotic plaques that may serve as targets for specific contrast agents to allow the identification of high-risk atherosclerotic lesions in-vivo. Ultimately, such agents may enable treatment of "high-risk" patients prior to lesion progression and occurrence of complications.     

Cardiovascular applications of magnetic resonance imaging

Magnetic resonance (MR) imaging is a unique imaging modality that is gaining rapid acceptance for a variety of medical indications. Diagnostic information is obtained noninvasively, without the potential hazards of ionizing radiation. The spatial resolution and anatomic detail of MR imaging rival those of other currently available imaging methods. By gating to an electrocardiographic signal cardiac imaging is possible. Since March 1983 the authors have had experience with cardiac MR imaging in both animals and humans. Cardiac anatomy is well shown by this technique, which allows detection and characterization of intracardiac masses, congenital heart disease and anomalies of the great vessels. Myocardial infarction has been detected in both animals and humans without the use of contrast agents, and acute cardiac transplant rejection has been visualized in an animal model. Limitations of MR imaging primarily have been lengthy imaging times and the sensitivity of the images to motion. With further investigation and experience this technique may become useful for studying a wide variety of cardiovascular disorders.     

Birdcage volume coils and magnetic resonance imaging: a simple experiment for students

Background

This article explains some simple experiments that can be used in undergraduate or graduate physics or biomedical engineering laboratory classes to learn how birdcage volume radiofrequency (RF) coils and magnetic resonance imaging (MRI) work. For a clear picture, and to do any quantitative MRI analysis, acquiring images with a high signal-to-noise ratio (SNR) is required. With a given MRI system at a given field strength, the only means to change the SNR using hardware is to change the RF coil used to collect the image. RF coils can be designed in many different ways including birdcage volume RF coil designs. The choice of RF coil to give the best SNR for any MRI study is based on the sample being imaged.

Results

The data collected in the simple experiments show that the SNR varies as inverse diameter for the birdcage volume RF coils used in these experiments. The experiments were easily performed by a high school student, an undergraduate student, and a graduate student, in less than 3 h, the time typically allotted for a university laboratory course.

Conclusions

The article describes experiments that students in undergraduate or graduate laboratories can perform to observe how birdcage volume RF coils influence MRI measurements. It is designed for students interested in pursuing careers in the imaging field.

     

Definition of a brain reference line in magnetic resonance imaging: the chiasmato-commisural line

Proposes and defines a cephalic reference plane that joins the chiamatic notch to the posterior commissure, easily shown on a mid-sagittal cut using magnetic resonance. The horizontal cuts obtained prove to be very close to those oriented according to the lateral fissure, and orthogonal the great axis of the brainstem.     

Visual documentation of ovine pituitary gland development with magnetic resonance imaging following zeranol treatment

The objective of the current study was to determine whether magnetic resonance imaging (MRI) could be successfully utilized to document the effect of an oestrogenic anabolic agent on pituitary gland growth. The experimental animals consisted of two 1/2 sibling Suffolk wethers (castrated rams), which received either no implant (control, n = 1) or a 24 mg zeranol implant at day 0 and day 42 (zeranol; n = 1). Animals were anaesthetized with propofol and supported with oxygen during the MRI procedure. A mobile MRI unit with a 0.5 tesla (T), superconducting magnet was used to obtain 3 mm thick, non-contrast enhanced, T1-weighted (TR 500-600, TE25) sagittal, transverse and dorsal images of the pituitary gland. Sagittal images were recorded only when the mesencephalic aqueduct and infundibulum were distinctly visible in the same image. Pituitary glands were imaged at 14-day intervals for 70 days to determine if and when the anabolic effects of zeranol on pituitary gland growth could be visualized using MRI techniques. Three separate measurements of the pituitary gland dimensions made with the on-screen cursor were averaged to calculate pituitary gland dimensions and volume. A computer-assisted image analysis system and laser film images were used to determine pituitary gland area. Increases in pituitary gland volume for control and zeranol-treated animals were evident within 14 days, and by the end of the 70-day study, the increase in pituitary volume for the zeranol-treated animal was three times greater than that of the control animal. Overall, our results indicate that MRI technology can be successfully used to document the development of the pituitary gland in vivo. Application of knowledge gained from this novel approach to study the growth, development and function of endocrine glands over time, and within the same animal, will enhance human and animal endocrine diagnostic procedures.