Visualization of cerebral and vascular abnormalities by NMR imaging. The effects of imaging parameters on contrast

Imaging of Uveal Melanoma—Current Standard and Methods in Development

Simple Summary

Uveal melanoma is the most prevalent intraocular tumor in adults, derived from melanocytes; the liver is the most common site of its metastases. Due to troublesome tumor localization, different imaging techniques are utilized in diagnostics, i.e., fundus imaging (FI), ultrasonography (US), optical coherence tomography (OCT), single-photon emission computed tomography (SPECT), positron emission tomography/computed tomography (PET/CT), magnetic resonance imaging (MRI), fundus fluorescein angiography (FFA), indocyanine green angiography (ICGA), or fundus autofluorescence (FAF). Specialists eagerly use these techniques, but sometimes the precision and quality of the obtained images are imperfect, raising diagnostic doubts and prompting the search for new ones. In addition to analyzing the currently utilized methods, this review also introduces experimental techniques that may be adapted to clinical practice in the future. Moreover, we raise the topic and present a perspective for personalized medicine in uveal melanoma treatment.

Abstract

Uveal melanoma is the most common primary intraocular malignancy in adults, characterized by an insidious onset and poor prognosis strongly associated with tumor size and the presence of distant metastases, most commonly in the liver. Contrary to most tumor identification, a biopsy followed by a pathological exam is used only in certain cases. Therefore, an early and noninvasive diagnosis is essential to enhance patients’ chances for early treatment. We reviewed imaging modalities currently used in the diagnostics of uveal melanoma, including fundus imaging, ultrasonography (US), optical coherence tomography (OCT), single-photon emission computed tomography (SPECT), fundus fluorescein angiography (FFA), indocyanine green angiography (ICGA), fundus autofluorescence (FAF), as well as positron emission tomography/computed tomography (PET/CT) or magnetic resonance imaging (MRI). The principle of imaging techniques is briefly explained, along with their role in the diagnostic process and a summary of their advantages and limitations. Further, the experimental data and the advancements in imaging modalities are explained. We describe UM imaging innovations, show their current usage and development, and explain the possibilities of utilizing such modalities to diagnose uveal melanoma in the future.

Early development of arterial spin labeling to measure regional brain blood flow by MRI

Two major avenues of work converged in the late 1980's and early 1990's to give rise to brain perfusion MRI. The development of anatomical brain MRI quickly had as a major goal the generation of angiograms using tricks to label flowing blood in macroscopic vessels. These ideas were aimed at getting information about microcirculatory flow as well. Over the same time course the development of in vivo magnetic resonance spectroscopy had as its primary goal the assessment of tissue function and in particular, tissue energetics. For this the measurement of the delivery of water to tissue was critical for assessing tissue oxygenation and viability. The measurement of the washin/washout of "freely" diffusible tracers by spectroscopic based techniques pointed the way for quantitative approaches to measure regional blood flow by MRI. These two avenues came together in the development of arterial spin labeling (ASL) MRI techniques to measure regional cerebral blood flow. The early use of ASL to measure brain activation to help verify BOLD fMRI led to a rapid development of ASL based perfusion MRI. Today development and applications of regional brain blood flow measurements with ASL continues to be a major area of activity.

Review. The Agfa Mayneord lecture: MRI of short and ultrashort T₂ and T₂* components of tissues, fluids and materials using clinical systems

A variety of techniques are now available to directly or indirectly detect signal from tissues, fluids and materials that have short, ultrashort or supershort T₂ or T₂* components. There are also methods of developing image contrast between tissues and fluids in the short T₂ or T₂* range that can provide visualisation of anatomy, which has not been previously seen with MRI. Magnetisation transfer methods can now be applied to previously invisible tissues, providing indirect access to supershort T₂ components. Particular methods have been developed to target susceptibility effects and quantify them after correcting for anatomical distortion. Specific methods have also been developed to image the effects of magnetic iron oxide particles with positive contrast. Major advances have been made in techniques designed to correct for loss of signal and gross image distortion near metal. These methods are likely to substantially increase the range of application for MRI.

Quantitative Analysis of Magnetic Resonance Imaging Susceptibility Artifacts Caused by Neurosurgical Biomaterials: Comparison of 0.5, 1.5, and 3.0 Tesla Magnetic Fields

Magnetic resonance (MR) imaging is an important diagnostic tool for neurosurgical diseases but susceptibility artifacts caused by biomaterial instrumentation frequently causes difficulty in visualizing postoperative changes. The susceptibility artifacts caused by neurosurgical biomaterials were compared quantitatively by 0.5, 1.5, and 3.0 Tesla MR imaging. MR imaging of uniform size and shape of pieces ceramic (zirconia), pure titanium, titanium alloy, and cobalt-based alloy was performed at 0.5, 1.5, and 3.0 Tesla. A linear region of interest was defined across the center of the biomaterial in the transverse direction, and the susceptibility artifact diameter was calculated. Susceptibility artifacts developed around all biomaterials at all magnetic field strengths. The artifact diameters caused by pure titanium, titanium alloy, and cobalt-based alloy increased in the order of 0.5, 1.5, to 3.0 Tesla magnetic fields. The artifact diameter of ceramic was not influenced by magnetic field strength, and was the smallest of all biomaterials at all magnetic field strengths. The artifacts caused by biomaterials except ceramic increase with the magnetic field strength. Ceramic instrumentation will minimize artifacts in all magnetic fields.

Quantification of susceptibility artifacts produced on high-field magnetic resonance images by various biomaterials used for neurosurgical implants. Technical note

Although various biomaterials such as ceramics or titanium alloy are widely used in neurosurgery, the susceptibility artifacts that appear around these materials cause problems when a magnetic resonance (MR) imager is used to assess lesions after surgery. The purpose of the present study was to quantify the susceptibility artifacts produced by various biomaterials used for neurosurgical implants. Using a 3-tesla MR imaging unit, we obtained MR images of various biomaterials, including six types of ceramics, a cobalt-based alloy (Elgiloy), pure titanium, a titanium alloy, and stainless steel. All implants shared a uniform size and shape. In each image, a linear region of interest was defined across the center of the biomaterial in the transverse direction, and the diameter of the susceptibility artifact was calculated. The ceramics produced a considerably smaller artifact diameter than those produced by other biomaterials. Among the types of ceramics, zirconia was found to produce the smallest artifact diameter. Among the remaining biomaterials, the diameters of the artifacts decreased in order from that associated with stainless steel to those associated with cobalt-based alloys, pure titanium, and titanium alloy. Little difference was observed between the artifact diameters associated with pure titanium and titanium alloy. Ceramics are the most suitable biomaterials for minimizing artifacts in high-field MR imaging.

"Willed action": a functional MRI study of the human prefrontal cortex during a sensorimotor task

Functional MRI (fMRI) was used to examine human brain activity within the dorsolateral prefrontal cortex during a sensorimotor task that had been proposed to require selection between several responses, a cognitive concept termed "willed action" in a positron emission tomography (PET) study by Frith et al. [Frith, C. D., Friston, K., Liddle, P. F. & Frackowiak, R. S. J. (1991) Proc. R. Soc. London Ser. B 244, 241-246]. We repeated their sensorimotor task, in which the subject chooses to move either of two fingers after a stimulus, by fMRI experiments in a 2.1-T imaging spectrometer. Echo-planar images were acquired from four coronal slices in the prefrontal cortex from nine healthy subjects. Slices were 5 mm thick, centers separated by 7 mm, with nominal in-plane spatial resolution of 9.6 x 5.0 mm2 for mean data. Our mean results are in agreement with the PET results in that we saw similar bilateral activations. The present results are compared with our previously published fMRI study of a verbal fluency task, which had also been proposed by Frith et al. to elicit a "willed action" response. We find a clear separation of activation foci in the left dorsolateral prefrontal cortex for the sensorimotor (Brodmann area 46) and verbal fluency (Brodmann area 45) tasks. Hence, assigning a particular activated region to "willed action" is not supported by the fMRI data when examined closely because identical regions are not activated with different modalities. Similar modality linked activations can be observed in the original PET study but the greater resolution of the fMRI data makes the modality linkages more definite.

Magnetic resonance angiography

Magnetic resonance angiography (MRA) permits the non-invasive visualization of blood flow through the effects of moving spins on the magnetic resonance signal. MRA techniques can be divided into two main classifications depending upon the primary effect responsible for contrast in the image. Angiograms can be produced using either the time-of-flight (TOF) or phase contrast (PC) methods. Each method has particular advantages and limitations as an angiographic imaging technique and these are reflected in their respective applications. This review article is intended to outline the scientific and technical development of MRA from its basis in the earliest in vitro nuclear magnetic resonance (NMR) experiments, through the implementation of in vivo angiographic techniques on whole body MRI systems, to the recent rapid expansion in MRA acquisition and processing techniques.

Negative MRI versus real disease

A 76-year-old diabetic woman presented with progressive binocular vertical diplopia and right eye pain. Examination revealed a pupil-involving partial right third cranial nerve palsy, with development of anisocoria over the course of several hours. MRI of the brain showed no mass lesion. MRA, even with retrospective review of the images, failed to clearly identify a 1 cm right posterior communicating artery aneurysm detected by subsequent conventional cerebral angiography. While MRA has been reported to be highly sensitive in cerebral aneurysm detection at some centers, other investigators have indicated less favorable data. Standardized protocols for data acquisition and meticulous attention to proper post-processing and image interpretation are essential if MRA is to supplant invasive arteriography. Currently, conventional (x-ray) angiography remains the gold standard for aneurysm detection, while MRA possesses excellent potential in this regard.

Dynamic magnetic resonance imaging of the rat brain during forepaw stimulation

A magnetic resonance (MR) imaging brain mapping method was used to localize an activated volume of brain tissue in chloralose-anesthetized rats during electrical stimulation of the forepaw. Physiologically-induced changes are characterized by alterations of the magnetic properties of blood as determined by the oxygenation state of hemoglobin. Stimulation of the left forepaw led to an increase in MR signal intensity of the contralateral frontal and parietal cortices, which corresponded to forelimb motor and somatosensory areas. The activation was contiguous in coronal planes between +5 and +2 mm anterior to the bregma, and its volume was calculated to be 20-30 mm3. Each activated region was revealed using a paired t-test statistical analysis method and the activated volume was calculated from regions exposed by thresholding at p < 0.005. Physiologically-induced fractional signal changes, delta S/S, in the motor and somatosensory areas were 0.06 +/- 0.04 and 0.17 +/- 0.06, respectively.

Asymptomatic arteriovenous malformation of the pancreas. Demonstration by Doppler ultrasonography and magnetic resonance imaging

We reported a case of asymptomatic pancreatic AVM, incidentally found on routine ultrasonography and diagnosed noninvasively by means of Doppler ultrasonography and MRI. The diagnosis was confirmed by angiography. This is the first report to demonstrate the characteristics of pancreatic AVM by these two imaging techniques. They proved to be useful in identifying the vascular nature of AVM without the use of a contrast material, which is a definite advantage over DSA and CT. The quality of the image obtained was equivalent or superior to that obtained by DSA and contrast-enhanced CT. Doppler ultrasonography and MRI may serve as the primary imaging techniques of choice in suspected cases of AVM.

A novel flow-suppression technique using tailored RF pulses

The pulsatile nature of blood flow makes zipper-like artifacts along the coding direction in the two-dimensional Fourier transform NMR image. So far, spatial presaturation, one of the correction methods, is known to be effective in eliminating flow artifacts when the Fourier spin echo acquisition is employed. However, this method requires an additional RF pulse and a spoiling gradient for presaturation. Described in this paper is a new flow suppression technique, based on spin dephasing, using a set of tailored RF pulses. The proposed method does not require additional saturation RF pulses or spoiling gradient pulses, making it advantageous over other methods. In addition, the method is relatively robust to flow velocity. The proposed technique is equivalent to the existing flow saturation technique except that the elimination of the flow component is achieved by a pair of tailored 90-180 degrees RF pulses in the spin echo sequence. The principle of the proposed method is the creation of a linear phase gradient within the slice along the slice selection direction for the moving material by use of two opposing quadratic phase RF pulses, i.e., 90 degrees and 180 degrees RF pulses with opposing quadratic phase distributions. That is to say, all the spins of the moving materials along the slice selection direction become dephased. Therefore, no observable signal is generated. Computer simulations and experimental results obtained using a 2.0-T whole-body imaging system on both a phantom and a human volunteer are also presented.

Emotional stress and coping in multiple sclerosis (MS) exacerbations

Ninety-five pairs of MS patients in exacerbation and remission were compared on emotional stress in the previous three months. Patients in exacerbation scored higher on emotional disturbance and intensity of stressful events than patients in remission, but lower on frequency of compensating uplifts. There was also a tendency for more patients in exacerbation than remission to favour emotion-focused coping techniques over problem-solving or social support. Whether patients building to an exacerbation over-react to various events or unresolved emotional stress precipitates exacerbations, MS patients might benefit from counselling in stress reduction techniques.

Time-of-flight effects in MR imaging of flow

This article provides a brief overview of one of the two fundamental physical principles which lead to a modulation of the magnetic resonance signal of moving spins: the time-of-flight effect. It then discusses some of its characteristic manifestations in spin-echo imaging and reviews strategies for exploiting the effect to quantitate vascular flow.

Multiple sclerosis and brain tumor: a diagnostic challenge

Multiple sclerosis may present as a lesion indistinguishable clinically and radiologically from brain tumor. A case of multiple sclerosis is reported with clinical and radiologic features of a brain tumor. A brain biopsy was later undertaken that provided pathological confirmation of the diagnosis of this demyelinating disease. This case report in conjunction with those in the literature indicate that conservative treatment with steroid therapy and serial computed tomography should establish the diagnosis of multiple sclerosis without the need for surgical intervention.

Computed tomography versus magnetic resonance imaging of the brain. A collaborative interinstitutional study

A retrospective study (1983-1984) of magnetic resonance imaging (MRI) and computed tomography (CT) examinations in 471 patients with known pathology in the brain and craniocervical junction was conducted in order to determine the relative efficacy of MRI versus CT. All MRI examinations involved slice thickness greater than 10 mm, and only single-slice single-echo or multislice single-echo imaging techniques were available. These studies were evaluated independently by two neuroradiologists from a panel of six for anatomic abnormalities, lesion contrast, and radiologist's impression. Results, which excluded microadenomas of the pituitary and approximately 9% of studies in which consensus was not achieved by the readers, were as follows: (1) 14% of the studies were positive on MRI but normal on CT; (2) in 55% of the studies, MRI was better than CT; (3) MRI was equal or better than CT in 95% of the studies; and (4) CT was better than MRI in 5% (21/421) of the examinations. There were no patients in this series where CT was positive but MRI missed the abnormality.

Flow-induced phase effects and compensation technique for slice-selective pulses

Flowing spins experience a time-varying frequency during the application of slice-selective radiofrequency (RF) pulses. As a result the flowing spins accumulate phase relative to stationary spins as the spins are rotated toward the transverse plane. Using finite difference techniques to solve the Bloch equations for flowing spins, the phase of the transverse magnetization after a slice-selective pulse was evaluated for varying flow velocities and for tip angles which ranged between 0 degrees and 180 degrees. The following results were obtained for constant slice thickness: (1) For a fixed tip angle, the phase varies nonlinearly with velocity. (2) For a fixed velocity, the phase varies nonlinearly with tip angle. For small tip angles the nonlinearity in the variation as a function of velocity is very small but increases for tip angles greater than 90 degrees and becomes especially severe near 180 degrees. A method is proposed to desensitize the phase of flowing spins during the application of slice-selective pulses. The compensation scheme depends upon tip angle but is virtually independent of flow velocity. The technique was tested and verified with clinical images.

Patterns of gliosis in Alzheimer's disease and aging cerebrum

The distribution of astrocytic gliosis in Alzheimer's disease (AD) and aging cerebrum, as marked by immunoperoxidase staining for glial fibrillary acidic protein (GFAP), was examined in whole-hemisphere coronal sections. Cortical gliosis in AD had an obvious laminar pattern. There were two heavy bands of staining, one in layers II-III and another in layer V. Normal aging cases sometimes displayed considerable cortical gliosis, but no specific patterns were apparent. Most AD cases, and some normal aging cases, displayed hypertrophy of immunoreactive astrocytes at grey matter-white matter interfaces, especially the cortico-medullary junction. Subcortical grey matter gliosis was common in both normal aging and AD, but there was no consistent pattern in either group. The deep cerebral white matter, which is stained evenly and heavily in young, healthy individuals, showed uneven staining in both normal elderly and AD brains. In both AD and aging, perivascular gliosis was prominent throughout the cerebrum and especially in the putamen. In conclusion, both AD and aging cerebri show extensive gliosis: AD cortical gliosis has a specific laminar pattern, but there does not appear to be an AD-specific pattern of subcortical gliosis.

Approaches to MR angiography

One of the most revolutionary recent imaging advances is the use of magnetic resonance to study and produce morphologic representations of flowing blood vessels known as MR angiography. The ability to produce an image of even moderate spatial resolution of the three dimensional course of blood vessels with MR could have significant advantages over conventional invasive angiography which requires ionizing radiation and contrast material injection. By definition, MR angiography does not require the addition of any intravascular contrast agents and the images are produced entirely by the effect of the radio frequency pulses and magnetic field gradients on the spinning protons. Several researchers are already producing relatively high resolution MR angiograms using a variety of techniques. Essentially all techniques of MR angiography use variations of three steps to produce the image: (1) a projection image, (2) suppression of background static material, and (3) production of a flow sensitive image. This report will survey some of the more commonly used approaches to MR angiography that are currently under investigation.

A mathematical model for signal from spins flowing during the application of spin echo pulse sequences

Models are presented for both laminar and plug flow that predict the signal from spins flowing during the application of slice-selective spin echo pulse sequences. The models permit calculation of the total signal from a cylindrical vessel lying perpendicular to the slice and incorporate the effect of the physical displacement of the spins between successive excitations. This time-of-flight effect gives a signal which is composed of contributions from a finite number of spin populations, with each population signal weighted by the fractional volume of that spin population within the cylindrical vessel segment. The signal and fractional volume from each spin population are derived analytically for ten different spin echo pulse sequences. The models for plug and laminar flow have important application for predicting and interpreting flow effects observed in clinical images. They are shown to be useful for selecting pairs of pulse sequences that can be used to obtain digitally subtracted MR images which provide optimum contrast for flowing blood with essentially complete suppression of stationary anatomy. These models provide a means for quantitatively comparing the expected signal from flowing spins for the many techniques presently being investigated for MR angiography.

Projective Fourier angiography

The Fourier imaging of flow is discussed, and the limitations of imaging time, spatial and velocity resolution, imposed on this technique when applied in conjunction with ECG synchronization are examined. It is argued that this method is best used in projective format, and in vivo examples are presented. The relative velocity spectrum obtained is largely system independent, and quantification of velocity is implicit in the Fourier technique.

Time-resolved magnetic resonance angiography

A time-resolved phase contrast magnetic resonance angiography technique is described. This technique provides a series of angiograms obtained at different phases of the cardiac cycle. Such a series of angiograms can be used to evaluate blood flow dynamics. For example, turbulent flow in the regions of vessel bifurcations is easily demonstrated and followed during systole and diastole. Retrograde flow can also be observed. Dynamic angiography can be particularly useful in distinguishing transient image features, such as signal voids due to turbulent flow, from static features arising from vessel morphology.

The MRI appearance of cavernous malformations (angiomas)

The angiographic, computerized tomography (CT), and magnetic resonance imaging (MRI) findings were compared in 10 patients with a total of 16 pathologically verified cavernous angiomas. Only three lesions had abnormal vasculature in the form of venous pooling or a capillary blush. The CT scans were positive in seven patients and detected 14 lesions, while high-field strength (1.5 Tesla) MRI was positive in each case and demonstrated 27 distinct lesions. On T2-weighted MRI, the combination of a reticulated core of mixed signal intensity (SI) with a surrounding rim of decreased SI strongly suggests the diagnosis of a cavernous malformation. Smaller lesions appear as areas of decreased SI (black dots). The sensitivity of MRI is based on magnetic susceptibility and possibly diffusion effects related to field heterogeneity that is more conspicuous on high-field imaging and caused by the presence of excessive iron (hemosiderin).

Rapid scan magnetic resonance angiography

The change in phase of transverse spin magnetization induced by macroscopic spin motion in the direction of an applied magnetic field gradient is used to generate projection angiograms. The method can provide a quantitative measure of laminar and pulsatile flow. Cardiac synchronization is not required provided that data are acquired at many points in the cardiac cycle. The use of short TR and a large number of excitations provides better suppression of stationary tissue and patient motion artifacts than is possible with cardiac gated studies. In addition to improvements in image quality, a substantial shortening of scan time is obtained.

Balance of T1-weighted images before and after application of a paramagnetic substance (Gd-DTPA)

Ten patients with encephalitis disseminata and 10 patients with cerebral ischemia underwent computed tomography and magnetic resonance imaging examinations. The results before and after the application of contrast media were compared. It is shown that T1-weighted MRI images, even after application of gadolinium-DTPA, are of little value for differential diagnosis of these diseases. In contrast, T2-weighted images did support the diagnosis of an acute stage of encephalitis disseminata versus acute ischemia. This was also true for lesions that had not been adequately depicted by computed tomography.

The utility of principal component analysis for the image display of brain lesions. A preliminary, comparative study

Principal component analysis (PCA), a common tool from multivariate statistical analysis, has been implemented into the computer display system of a MR imaging device. PCA allows the calculation of images in which the information in a defined region of interest inherent in the basic acquired images is condensed. PCA image calculation has been applied to acquired MR studies of 13 patients with brain lesions. The appearance of the brain lesions on the resultant PCA images was scored in comparison to the acquired images before and after administration of Gd-DTPA as well as to other calculated images including T1, T2, hydrogen density, and contrast-optimized images. The conspicuity of a lesion and the number of distinguishable components within a lesion were slightly superior on PCA than on the acquired images. PCA is an analytical tool for MR imaging that should be helpful in revealing information that is inherent in, but not readily visible on, standard acquired MR images.

Magnetic resonance imaging of acute intracerebral hematomas: in vivo and in vitro studies

The mechanisms causing unclear differentiation of ICH in the acute phase by MRI are discussed. The increase of the hemoglobin concentration in the hematoma, caused by loss of the plasma component directly after onset, is one of the reasons for the shortening of relaxation times. The clotting system is also responsible for this shortening. Thus, the hematoma reaches the same intensity as the brain in the acute stage. Although knowledge from our animal experiments, in vivo as well as in vitro, cannot be directly transferred to the human, such experiments are useful for the understanding of the pathomechanism influencing MR imaging.

Magnetic resonance imaging in vascular malformations: studies with and without gadolinium-DTPA

MRI and gadolinium-DTPA enhanced dynamic MRI increase the information available for and from diagnosis of vascular malformations since it appears possible to obtain nearly specific information about these lesions. Further investigations are needed with fast imaging, flow imaging, and MR substraction angiography to obtain information similar to that obtained by angiography and CT. The value of X-ray CT for the diagnosis of vascular malformations seems to be inferior to MRI since MRI can delineate vessels and flow better and also allows direct multiplanar imaging.

Sampling time effects on signal-to-noise and contrast-to-noise ratios in spin-echo MRI

This work discusses the effect of sampling time on noise, signal-to-noise and contrast-to-noise ratios in magnetic resonance imaging. A simple imaging experiment is performed to demonstrate the effect of sampling time on noise, confirming theoretical expectations that doubling the sampling time while decreasing the read gradient strength by a factor of two reduces statistical noise to 1/square root 2 of its original level. This result suggests that sampling time should be maximized within the constraints of the pulse sequence: namely, that sampling time should be increased and read gradient strength decreased as TE is increased. Revised expressions for signal-to-noise and contrast-to-noise ratios are presented based on the assumption that sampling time increases linearly with echo delay time above a certain minimum TE value. The revised expressions are then used to derive new predictions of the interpulse delay times that maximize signal-to-noise and contrast-to-noise ratios in spin-echo imaging. It is demonstrated that sampling times are critical in determining whether T1-weighted or T2-weighted sequences produce superior tissue contrast in spin-echo imaging.

Arteriovenous fistula associated with renal cell carcinoma: demonstration by magnetic resonance imaging

Two cases of arteriovenous fistula associated with renal cell carcinoma were studied using multiple techniques, including magnetic resonance imaging (MR). These techniques clearly delineated the vascular abnormalities without the use of a contrast agent; they also demonstrated the extent of the tumors. Data derived from MR were confirmed upon surgical exploration and were useful in planning the approach to the kidney.

Natural history of the vascular dementias: a prospective study of seven cases

The clinical course of vascular dementia has been described as a stepwise deterioration over time. We studied the chronologic course of cognitive performance over 1-4 years in seven patients with known ischemic cerebrovascular disease whose dementia subtype was assigned according to the clinical history and the pattern of white matter lesions seen on magnetic resonance imaging (MRI). One patient had the lacunar state, three had subcortical arteriosclerotic encephalopathy and three had multiple cortical and subcortical strokes. All were being treated with an antiplatelet agent and six received antihypertensive therapy. Four of the seven vascular dementia patients improved cognitively over the first year. A fluctuating course was eventually seen in all patients. None showed stepwise deterioration of cognitive function over time. The MRI was useful in subclassifying vascular dementia patients, but the clinical course did not appear to vary as a function of the dementia subtype.

Reproducibility of relaxation and spin-density parameters in phantoms and the human brain measured by MR imaging at 1.5 T

The reproducibility of T1, T2, and proton density, measured in phantoms and the human brain was evaluated by proton imaging techniques. The sequence used to derive T1 and density values was a multiple-saturation recovery which consists of four pairs of 90 degrees pulses, followed by a 180 degrees phase reversal pulse, generating four T1-weighted images. T2 was derived from a multiple-echo sequence, generating four T2-weighted images. The data were analyzed by fitting the pixel intensities to the respective equations by means of nonlinear multiparameter least-squares analysis. Short-term reproducibility between four consecutive scans was evaluated to be 1-4% depending on location with a phantom covering the entire span of physiologic T1 and T2 values. A second phantom containing a series of identical samples served to study the dependence of the apparent T1 and T2 on position, both radially and axially, with respect to magnet isocenter. Reproducibility across the field of view was found to be better than 7% (T1 and T2). This phantom was further used to evaluate effects of long-term reproducibility, which at each location varied from 5-14% (T1) and 2-10% (T2). Finally, interinstrument reproducibility, tested by means of the same protocol on three different instruments, all operating at the same magnetic field and using largely identical hardware for each location, was found to be 1-14% (T1) and 2-10% (T2). The positional dependence of the apparent relaxation times appears to be systematic and may be due to variations in the effective field, caused by magnet and rf inhomogeneity. Finally, brain tissue relaxation and spin-density data were determined using the same protocol in 37 scans performed on 27 normal volunteers. The tissues analyzed were putamen, thalamus, caudate nucleus, centrum semiovale, internal capsule, and corpus callosum. Excellent accordance was further obtained between left and right hemispheres.

Magnetic resonance imaging of the extremities

MRI offers the best soft-tissue contrast of any radiographic modality. It provides better delineation of soft-tissue tumors and bone tumors than CT, but several different pulse sequences may be needed to make optimum use of this potential. MRI is also well suited to evaluation of abnormalities of the bone marrow, as it is not subject to the dense bone artifact that may occur with CT. Changes in signal intensity may be used to gain some appreciation of blood flow in the major vessels. Examples of each of these applications are discussed.

Magnetic resonance imaging of the hila and mediastinum

In the diagnosis of chest diseases, MR imaging has been shown to be most helpful in the evaluation of hilar and mediastinal lesions. The detection of intrathoracic abnormalities is technique-dependent, and imaging with both short and long repetition and echo times is necessary. ECG gating and sagittal or coronal images supplement transaxial scans in some patients. MR allows the diagnosis of mediastinal vascular lesions and the differentiation of mediastinal mass and vessel without contrast agents. It provides diagnostic information quite similar to that of CT in patients with mediastinal mass, but spatial resolution is somewhat poorer. In patients with hilar mass, MR is superior to CT in identifying the mass and distinguishing it from normal structures.

Magnetic resonance imaging of aneurysms and thrombi

This is a report of the first systematic investigation of the qualitative and quantitative diagnosis of aneurysms in the regions of the left ventricle and thoracic and abdominal aorta plus proof of intracavitary thrombi in the heart and the aorta, as well as aneurysms in the superior and inferior vena cava. For diagnosis of the heart, ECG gating is an absolute necessity, but for the analysis of abdominal aortic aneurysms it only leads to a considerable improvement of the spatial resolution. For differential diagnosis of the blood flow and intracavitary clots in the heart and the aorta, use of a second or even multiple echoes is needed. Also, digital subtraction between the first and second echoes (magnetic resonance digital subtraction) can assist in assessing flow. When dissecting aortic aneurysm is suspected and in cases when risk of perforation of ventricular and aortic aneurysms is present. MR offers particular advantages, since it is noninvasive and few scans can provide all the information that is required.

Potential approaches to evaluating the cardiovascular system using NMR

The current status and some of the future possibilities for nuclear magnetic resonance imaging of the cardiovascular system have been described. With many of these possibilities there is overlap with existing techniques. For example, functional analysis of the left ventricle can be obtained using either echocardiography or radionuclide techniques. With current instrumentation and current costs, these conventional techniques could frequently provide a more cost-effective approach for morphologic and functional assessment of the cardiovascular system. Nevertheless, because of the excellent resolution, the inherent contrast, the sensitivity to blood motion, the three-dimensional nature, and the lack of ionizing radiation, the cardiovascular morphologic imaging capabilities of NMR may provide justification for such applications. However, for NMR to achieve its most important status as a cardiovascular imaging technique, some of its unique possibilities will need to be developed. These include the ability to reproducibly depict the proximal coronary arteries, to define regional myocardial blood flow distribution, to evaluate regional high energy phosphate or other metabolic activity; and to characterize myocardial disease using proton T1 and T2 alterations.

Magnetic resonance imaging (MRI) in the diagnosis of cerebral arteriovenous angiomas

Eight patients with angiographically confirmed arteriovenous malformations (AVMs) were studied by CT and MRI. MRI scans were performed with a 0.35 Tesla wholebody scanner using three spin-echo sequences (SE 400/35, SE 1600/35, SE 1600/70). In CT and MRI, pathological findings were obtained in all cases. In MRI AVMs were displayed as lesions of low signal intensity in the applied sequences. Full extent of the lesions as well as the relationship to the surrounding structures were clearly demonstrated in MRI in all patients. Based on the characteristic sequence dependent signal intensity property of the lesions, the differential diagnosis in the sense of an AVM could be obtained by MRI in all cases. Concerning topographical imaging and/or differential diagnosis, MRI was superior to CT in 4 out of 8 cases. MRI offers advantages in the demonstration of AVMs of the cerebral midline, especially in brain stem angiomas.