Non-ferromagnetic retinal tacks are a tolerable risk in magnetic resonance imaging.

Should patients with cobalt alloy (ASTM F563) retinal tacks (Grieshaber cat. #611.95) in their eyes be subjected to the magnetic fields used in magnetic resonance imaging? Although the tacks are not ferromagnetic, they will experience a retarding torque when they are moved at the high angular velocities of human eye motion. Because retinal tacks are small (2.85 mm x 0.9 mm), the torque is difficult to measure. Rather, we measured the torque on a model 25.4 times larger and used a scaling law derived from Maxwell's equations to calculate the force on the tack. The scaling law states that the torque varies with the cube of the object's length. To mimic the motion, models of retinal tacks were attached to Plexiglas rods and the assemblies were swung as pendulums. The pendulums were oriented in the magnetic field of a 1.5 T imager to experience the greatest retardation. Retarding torques were estimated from the rate of decrease of the pendulum amplitude, both inside and outside the magnet. Even if the retinal tacks were as conductive as 6061T6 aluminum alloy (25 MS/m) and the velocity of the surface of the eye were 24 cm/s (angular vel. of 1130 deg/s), the retarding torque would be only 1.6 times the weight of the tack acting with a lever arm as long as the distance from its tip to its center of gravity. The maximum retarding torque on an implanted retinal tack in a 1.5 T magnet is similar to the torque produced by gravity alone acting on the tack and is a tolerable risk.(ABSTRACT TRUNCATED AT 250 WORDS)

MR imaging of stump neuroma.

Painful stump neuromas in the postamputation patient are difficult to diagnose and treat. We report a case in which magnetic resonance allowed precise preoperative diagnosis and localization of such a lesion. Magnetic resonance, with its ability for long axis imaging in the extremity, is ideally suited for the workup of stump neuromas.

CNR enhancement in the presence of multiple interfering processes using linear filters.

Given several images of the same slice, a linear filter can produce an image in which the contrast-to-noise ratio (CNR) between pathological and normal tissues is greater than in any of the initial images. To distinguish the pathology from more than one tissue, the filter should optimize the set of CNRs between the pathology and each of the interfering tissues. We define the optimal filter as the one which provides the largest value for the minimum CNR in the set and show how it is selected from a field of only four possibilities. The filter is demonstrated with both experimental phantom studies and clinical cases. Filter performance is compared with that of other techniques for distinguishing a desired feature from more than one interfering process.

A method for evaluating the bronchial circulation. Preliminary observations.

Due to the small size of the bronchial arteries, the bronchial circulation is difficult to study and remains poorly understood. We have utilized single photon emission computed tomography (SPECT) as a noninvasive means of studying the bronchial circulation. Imaging studies were performed in sheep before and after the introduction of a single pulmonary embolus. Digital bronchial arteriograms demonstrated the dilatation of the bronchial artery which is present on studies one week after embolization. SPECT bronchial perfusion scans reveal a wedge of increased activity that corresponds to a wedge of decreased activity on SPECT pulmonary perfusion images. Axial images available with SPECT provide information about regional alterations of bronchial perfusion not readily apparent on bronchial arteriograms.

Practical aspects of vascular imaging using MRI.

MRI is an accurate means of noninvasively assessing vascular abnormalities. However, multiple potential pitfalls exist. In order to minimize error, a knowledge of the flow phenomena seen on various pulse sequences, along with an understanding of pulse sequence modifications that alter these flow phenomena, is necessary. With such an understanding, the current utility of MRI for solving commonly encountered clinical problems of the vascular tree is addressed. Emphasis is placed on a "how-I-do-it" approach, realizing that the reader will make modifications where necessary due to differences in hardware, software, and experience. As appropriate, references to the literature are made in an attempt to justify the use of MRI for the particular problems being discussed.

MR-tutor: a program for teaching the interdependence of factors which influence signal intensity in magnetic resonance imaging.

Magnetic resonance (MR) imaging is a valuable diagnostic radiologic procedure. The appearance of tissues on the MR scan is dependent upon a complex relationship among a number of variables. We have written a computer program which teaches students of magnetic resonance imaging the interdependence of these variables and how they affect the appearance of tissues on MR images. The program is written in BASIC for the IBM* and compatible computers. A listing of the program appears in the Appendix of this article.

Carcinoid tumors: iodine-131 MIBG scintigraphy.

Eighty-two patients with pathologically proved carcinoid tumors were examined with iodine-131 metaio-dobenzylguanidine (MIBG) scintigraphy. Localization scores of I-131 MIBG accumulation in the primary tumor or metastatic site ranged from 0 to 3+ on the basis of comparison with normal liver. I-131 MIBG uptake varied greatly in different patients with carcinoid tumors. The localization scores in known tumor sites were related to the location of the primary tumor in the stomach (1-3+ in two of five patients), pancreas (1-3+ in four of five patients), cecum (3+ in two of two patients), appendix (0 in one of one patient), jejunum (0 in one of one patient), Meckel diverticulum (3+ in one of one patient), terminal ileum (2-3+ in 19 of 28 patients), bronchus (3+ in one of nine patients), thymus (1+ in one of two patients), and unknown (2-3+ in 18 of 28 patients). Tumors of midgut origin concentrated I-131 MIBG more frequently than those of foregut origin. Uptake of I-131 MIBG was more likely if neurohumor levels, particularly serum serotonin, were elevated. There was no relationship of I-131 MIBG uptake to carcinoid syndrome. I-131 MIBG is useful in the determination of the location and extent of some carcinoid tumors, particularly those of midgut origin.

Effects of turbulence on signal intensity in gradient echo images.

Although the appearance of laminar vascular flow in magnetic resonance (MR) images has been characterized, there is no general agreement about the effect of turbulent flow on MR signal intensity. This study uses a fast scan gradient echo pulse sequence to evaluate nonpulsatile turbulent flow in two different models. The first model simulated flow in normal vascular structure. It generated nonpulsatile, laminar and turbulent flow in straight, smooth-walled Plexiglas tubes. The second model simulated flow through a vascular stenosis. It generated nonpulsatile, laminar, and turbulent flow through an orifice. Velocities and flow rates ranged from low physiologic to well above the physiologic range (velocity = .3 to 280 cm/second, flow rate from .15 to 40 L/minute). Transition from laminar to turbulent flow was observed with dye streams. Turbulent flow in straight, smooth-walled vessels was not associated with a decrease in MR signal intensity even at the highest velocities and flow rates studied. The transition from laminar to turbulent flow through an orifice is not associated with a decrease in gradient echo signal intensity. As the intensity of the turbulent flow increases, however, there is a threshold above which signal intensity decreases linearly as turbulence increases (r = .97). This study suggests that flow in normal vascular structures should not be associated with decreased signal intensity in gradient echo images. Turbulent flow through areas such as valves, valvular lesions or vascular stenoses, may be associated with a decrease in gradient echo signal intensity.

Deep venous thrombosis evaluation with limited-flip-angle, gradient-refocused MR imaging: preliminary experience.

Sixteen patients (17 lower extremities) were prospectively examined with venography and limited-flip-angle, gradient-refocused magnetic resonance (MR) imaging for the presence or absence of deep venous thrombosis. Thrombosed vessels showed decreased-to-absent signal intensity, while patent vessels had high signal intensity. In 16 of 17 extremities, MR images allowed accurate detection and localization of the thrombi found with venography. In the remaining extremity, MR imaging allowed correct identification of thrombus in the iliac and femoral veins but incorrectly demonstrated clot in the calf and popliteal veins. MR imaging with limited-flip-angle, gradient-refocused pulse sequences appears to be a sensitive, noninvasive means of detecting deep venous thrombosis.

MR imaging with surface coils in primary hyperparathyroidism.

The use of magnetic resonance (MR) to preoperatively evaluate patients with primary hyperparathyroidism was assessed using a 1.5 T system and surface coil reception. Twenty-five patients with primary hyperparathyroidism were studied before surgical exploration. Axial images, 5 mm thick, were obtained from the thyroid cartilage to the sternal notch. Both T1-weighted [short repetition time (TR), short echo time (TE)] and T2-weighted (long TR, long TE) spin echo sequences were performed in most cases. Parathyroid adenomas typically demonstrated greater signal than surrounding tissues on T2-weighted sequences, yet demonstrated signal intensity that was less than or equal to normal thyroid tissue on T1-weighted sequences. Using these criteria, MR correctly identified 17 of 20 surgically proven parathyroid adenomas in the neck. Magnetic resonance appeared less sensitive in two patients with parathyroid hyperplasia, identifying only one of six hyperplastic glands. We conclude that MR with surface coils provides high contrast, anatomic delineation of the neck and is useful for preoperative localization of parathyroid tumors.

Evaluation of steady and pulsatile flow with dynamic MRI using limited flip angles and gradient refocused echoes.

Magnetic resonance imaging sequences utilizing limited flip angles and gradient echoes yield rapid (less than 2 min) dynamic images of the cardiovascular system. These images contain both accurate anatomical and functional information. Using a gradient refocused acquisition in the steady state (GRASS) in the CINE mode, we studied the relationship between gradient echo signal intensity and velocity of steady and pulsatile flow in a phantom simulating medium to large vessels. Images were acquired on a 1.5 Tesla system (repetition time = 21 ms, echo time = 12 ms, flip angle = 30 degrees). Data from each pulse interval were sorted in 16 images. Signal intensities from flow tube lumina and surrounding stationary water jacket were used to calculate contrast ratios which were compared to velocity measurements made with electromagnetic (EM) flow probes outside the magnet room. During steady flow, signal intensity contrast ratios increased with increasing flow and in a 10 mm thick slice, reached a peak at 48 cm/s, and declined for velocities up to 90 cm/s. Changes in instantaneous velocity during pulsatile flow correlated well (r greater than .88) with signal intensity changes up to a maximum mean velocity of 17 cm/s. Total signal intensity from the lumen for an "R to R" interval correlated extremely well (r greater than .97) with mean pulsatile flow velocities up to 30 cm/s. The excellent correlation between gradient echo signal intensity and actual flow velocities suggests that this imaging sequence might be useful for evaluating normal and pathologic flow phenomena.

In vitro function of indium-111 oxine-labeled human monocytes.

Mononuclear phagocytes (monocytes and macrophages) are widely distributed throughout the body. Indium-111 (111In) oxine has been a useful radioactive label in studies of in vivo cellular kinetics and distribution. In preparation for in vivo monocyte investigations in humans, this study was designed to determine the effects of labeling human blood monocytes with 111In on their function in vitro. The monocytes labeled with an efficiency of 57.9 +/- 14.2% (range of 47-95%). The 111In eluted from the cells at a rate of 0.25%/h over 48 h in vitro. The labeled monocytes had a slight, but statistically significant, reduction in their ability to phagocytize or mediate antibody-dependent cellular cytotoxicity of antibody-coated sheep erythrocytes. The labeled cells were not different than control monocytes with respect to viability (trypan blue exclusion and lactate dehydrogenase release), adherence to plastic, hydrogen peroxide production, chemotaxis to N-formyl methionylleucylphenylalanine, and antibody-independent lysis of the tumor cells U937 and HeLa. 111In-labeled human monocytes should be useful for in vivo studies of mononuclear phagocyte kinetics and distribution in normal and diseased subjects.

Iodine-131 metaiodobenzylguanidine scintigraphy of carcinoid tumors.

Iodine-131 metaiodobenzylguanidine ([131I]MIBG) is concentrated in pheochromocytomas and can be detected by external imaging. We administered [131I]MIBG to 23 patients with carcinoid tumors to determine if it would be useful in scanning patients with these tumors. The carcinoid tumors of 14 of the 23 patients (61%) were visualized. Iodine-131 MIBG was concentrated most avidly by tumors of midgut origin (ileum, cecum); it was concentrated less avidly by some tumors of foregut origin (pancreas, stomach); it was not significantly concentrated by other tumors of foregut origin (bronchus). We conclude that 131I is a useful imaging agent for carcinoid tumors.

MRI with surface coils for parathyroid tumors: preliminary investigation.

The ability of MRI to detect parathyroid gland enlargement was assessed using 1.5-T systems and surface coil reception. Nine patients with primary hyperparathyroidism were studied before surgical neck exploration. Five-millimeter-thick axial images were obtained from the thyroid cartilage to the sternal notch. Both T1 and T2 contrast-weighted spin-echo sequences were performed in most cases. MRI correctly identified six out of seven parathyroid adenomas. In the false-negative case, T2 contrast-weighted sequences were not performed for technical reasons. In the one case of surgically proven parathyroid hyperplasia, a lesion identified by MRI as a parathyroid adenoma coincided at surgery with the largest of three hyperplastic parathyroid glands. A single case demonstrated a potential pitfall in the search for ectopic parathyroid glands: A large colloid cyst in the posterior portion of the thyroid gland showed signal characteristics indistinguishable from a parathyroid adenoma. The enlarged parathyroid glands were best visualized on T2 contrast-weighted sequences, with the lesions demonstrating greater signal than surrounding tissues.

Diagnostic imaging of abdominal fluid collections and abscesses.

With today's advanced technology, the clinician and radiologist have a number of techniques with which to evaluate the abdomen for fluid collections and/or abscesses. Three points are clear from the plethora of literature concerning this subject: the need to individualize the clinical and imaging approach to each patient suspected of an abnormal fluid collection or abscess, the need to individualize the imaging procedure based upon what is done best at the institution, and the need for percutaneous aspiration of all fluid collections for diagnosis. No single imaging test is totally sensitive or specific for the detection of an infected fluid collection. Overall, CT probably does provide the most accurate means to detect an intra-abdominal abscess. Because of this, and because it is less operator dependent than ultrasound, CT is used most often as the procedure of choice at our institution.

Single photon emission computed tomography (SPECT). Part II: Clinical applications.

The clinical applications of SPECT are just beginning to be defined since complete systems have only recently become available. SPECT studies are more difficult to perform than planar imaging studies, and close attention to quality control is important to obtain optimal studies. SPECT has higher lesion contrast and is able to detect smaller lesions than planar imaging in Tc-99m sulfur colloid liver studies. Preliminary results of SPECT T1-201 studies are encouraging, but further work comparing SPECT and planar imaging of T1-201 is needed. SPECT does give more information than planar imaging in certain bone imaging cases such as suspected avascular necrosis of the hip. Although interesting results have been published using SPECT in brain and lung perfusion studies, the clinical utility of this work has not been determined. The development of certain radiopharmaceuticals would enhance the future of SPECT. Technetium-99m labeled brain and myocardial perfusion agents would be ideally suited for SPECT studies. The ability to quantitate lesion volume (eg, in liver metastases) has not been studied and could be useful in following patents on chemotherapy. SPECT could give additional information if monoclonal antibodies labeled with I-123 or Tc-99m can be demonstrated to have appropriate sensitivity. The advantages of SPECT over planar imaging will be greater as new agents are developed.

Evaluation of pulmonary embolism.

Pulmonary embolism is a common disease entity that is difficult to diagnose accurately by noninvasive methods. Ventilation-perfusion is a pivotal study in the evaluation of patients with suspected pulmonary embolism. The role of standard and newer modalities in evaluating pulmonary embolism is discussed in this article.