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Little information exists on the performance of x-ray backscatter machines now being deployed through UK, US and other airports. We implement a Monte Carlo simulation using as input what is known about the x-ray spectra used for imaging, device specifications and available images to estimate penetration and exposure to the body from the x-ray beam, and sensitivity to dangerous contraband materials. We show that the body is exposed throughout to the incident x-rays, and that although images can be made at the exposure levels claimed (under 100 nanoGrey per view), detection of contraband can be foiled in these systems. Because front and back views are obtained, low Z materials can only be reliable detected if they are packed outside the sides of the body or with hard edges, while high Z materials are well seen when placed in front or back of the body, but not to the sides. Even if exposure were to be increased significantly, normal anatomy would make a dangerous amount of plastic explosive with tapered edges difficult if not impossible to detect.

Objective: To characterize the ability of an automatic lung nodule segmentation algorithm to measure small nodule dimensions and growth rates. Methods: A phantom of 20 sets of 6 balls each (11 different nylon balls and 9 acrylic balls) of 1 to 9.5 mm in diameter, in foam, was imaged using x-ray computed tomography with slice thicknesses of 5, 2.5, and 1.25 mm, pitches of 3 and 6, and standard and lung resolution. Measurements consisted of volume and maximum in-plane cross-sectional areas and their derived maximum and effective diameters. Growth rates were simulated using pairs of groups of balls. Results: Volume measurements overestimate volume, more so for thicker slices. For the largest balls, the error is 60% for 5-mm slices and 20% for 1.25-mm slices. Effective diameter calculated from volume better approximates actual diameter. For area measurements, errors are 0% to 5% for the largest balls, and the effective and actual diameters are closely matched. Conclusions: Below 5 mm in diameter, changes in volume should reach 100% for reliable indication of growth. Above 6 mm, the threshold for detecting change is on the order of 25% growth. Even under ideal conditions, results indicate the need for caution when making a diagnosis of malignancy on the basis of volume change.

MRI yields cross-sectional image acquisition in multiple planes with very high contrast resolution. Additionally, in MRI of the breast, evaluation of the entire implant is possible. Not only is MRI very sensitive to loss of implant integrity and rupture, but also MRI can evaluate and characterize adjacent fluid collections or soft-tissue masses. Anatomic relationships of abnormalities and normal structures are well seen. In some situations, MRI may offer clinically relevant information not adequately addressed by other imaging modalities.

Rationale and objectives Low-field magnetic resonance imaging (MRI) evaluation of breast implants is described, with emphasis on the capabilities provided by different imaging sequences. Methods Open architecture MR images using a .064-T permanent magnet and three-dimensional Fourier transform and inversion recovery sequences were obtained. A breast coil was designed and built for this project. Symptomatic and asymptomatic patients with silicone, saline-silicone and saline implants, and phantoms were part of this study. Phase images were used to differentiate protons in silicone, water, and fat. Results and conclusions Low-field MRI permitted differentiation of silicone, water, and fat. Implant anatomy and surrounding pathology could be imaged and identified.

In disorders which involve the bone marrow, the fat/water ratio of the marrow is often an indicator of the progress of disease. A noninvasive method of monitoring this ratio in bone marrow could be clinically useful. We have investigated a method of bone marrow assessment, using magnetic resonance imaging at low field (0.064 Tesla). The method has been tested using calibrated phantoms; we also present results from normal human subjects.

A multisection, two-dimensional, Fourier transform, double-gradient-echo magnetic resonance imaging sequence with partial flip-angle excitation and section doubling by radiofrequency encoding approximately doubles the signal-to-noise ratio obtainable from a conventional spin-echo sequence at low field strength, while maintaining essentially equivalent contrast.

Background Recent studies indicate that medication resistant depressed patients can be successfully treated by a series of sessions of High Frequency repetitive Transcranial Magnetic Stimulation (HF-rTMS), delivered on the left dorsolateral prefrontal cortex (DLPFC). However, changes in subjectively experienced mood give only limited insight into the underlying physiological responses. Previous studies in depressed patients, as well as in healthy volunteers, have reported a possible impact of HF-rTMS on the hypothalamic-pituitary-adrenal (HPA) axis. Objective We wanted to evaluate the emotional and neurobiological impact of one session of HF-rTMS applied on the left DLPFC in a sample of unipolar treatment resistant depressed patients of the melancholic subtype. Methods 20 right-handed antidepressant-free depressed patients were studied using a sham-controlled, ‘single’ blind, crossover design. We examined subjective mood changes with Visual Analogue Scales (VAS). To examine HF-rTMS effects on the HPA-axis, we analyzed salivary cortisol levels. Mood assessment and salivary cortisol levels were assessed before and immediately after stimulation. To detect any delayed effects, all measurements were also re-assessed 30 min post HF-rTMS. The left DLPFC was determined under MRI guidance. Results One session of HF-rTMS did not result in any subjectively experienced mood changes. However, salivary cortisol concentrations decreased significantly immediately and 30 min after active HF-rTMS. Conclusions Although one session of HF-rTMS on the left DLPFC did not influence mood subjectively in melancholic unipolar depressed patients, we found support for the hypothesis that a single session has a significant impact on the HPA-axis, as measured by salivary cortisol. Our results may provide more insight into the underlying working mechanisms of HF-rTMS in unipolar melancholic depression, and could add further information about endocrinological functioning in affective disorders.

Magnetic forces and torques were measured in a 640-G permanent magnet. The forces on external objects were directed in such a way and of such a strength that the likelihood of injury from unsupported ferromagnetic objects was very small. The forces and torques on ferromagnetic objects within the subject were comparable to those produced by gravitational and inertial efforts in daily living.

Excessive obesity can pose a limitation to both clinical and radiographic evaluation. Although CT and MR have revolutionized head and body imaging, patients with weights above 300 lb present a restriction of these imaging modalities. Magnetic resonance imaging (MRI) is well suited for imaging excessively obese patients, because the RF used does not have difficulty in penetrating large amounts of adipose tissue as ionizing radiation or sound waves does. The limitations of conventional MR imaging in these obese patients are the gantry size and the table weight limit. The recent development of a new low field MR imager with a larger gantry size and greater weight capacity, has the potential for imaging obese patients that cannot be evaluated by standard CT or MR. In this paper, we report our experience in imaging nine excessively obese patients with weights between 350 and 490 lb using a permanent magnet operating at 0.064 T.

Magnetic resonance (MR) imaging has proved beneficial in the evaluation of internal derangements of the knee. A limitation to general acceptance of MR imaging of the knee has been availability and cost. The recent introduction of low-field-strength MR imaging has shown promise in decreasing the cost and increasing the availability of this modality. High-resolution (pixel size, 0.7 mm2), three-dimensional Fourier transform (3DFT), thin-section (3.5 mm) imaging performed on a 0.064-T permanent magnet was used to evaluate 117 knees in 114 consecutive patients. The appearance of normal anatomy and internal derangements of the knee at low-field-strength imaging is described. Arthroscopic correlation was available for 28 knees. Findings from low-field-strength MR imaging and arthroscopy agreed in 79% of cases in the determination of meniscal tears. Partial-flip-angle techniques with 3DFT produced thin-section images of the knee of diagnostic quality. The authors conclude that in patients with internal derangements of the knee, low-field-strength (0.064-T) MR imaging may provide useful information.

The open architecture of a permanent magnet MR system, a thin solenoidal coil and the addition of a simple positioner permit knee imaging under extreme degrees of flexion.

Previous studies in healthy volunteers reported a possible impact of high frequency repetitive transcranial magnetic stimulation (HF-rTMS) on stress hormones, like cortisol. In this sham-controlled, "single blind", crossover study, we examined whether HF-rTMS had an effect on the hypothalamic-pituitary-adrenal (HPA) axis, by analysing salivary cortisol levels. Two studies were conducted. First, HF-rTMS on the left dorsolateral prefrontal cortex (DLPFC) was performed in 28 young healthy female volunteers. Second, in a comparable, but different group of 26 healthy females, HF-rTMS was performed on the right DLPFC. Salivary cortisol levels were assessed before, immediately after and 30 min after real and sham HF-rTMS. We found no support for the hypothesis that one single session of HF-rTMS on the left or the right DLPFC has an immediate or delayed impact on the HPA-axis, as measured by salivary cortisol. Although we controlled for several methodological problems in HF-rTMS research, the hypothesis that one single session of HF-rTMS on the left or on the right DLPFC can influence the HPA-axis in healthy volunteers was not supported.

The sections in this article are 1 Introduction 2 Physics of Low Field Strength Operation 3 Magnets for Low-Field MRI 4 Sequencing for Low-Field Imaging 5 Sequences for Mid-Field Imaging 6 Signal-to-Noise Ratio and Field Strength 7 Discussion 8 Biographical Sketches Related Articles

Three-dimensional Fourier transform (3-DFT) MR imaging Offers advantages in terms of SIN per unit of time for the case where a large number of slices is desired. This advan- sidered briefly in this paper. The first, contrast, is prob- ably the single most important one. Nevertheless, the at- tage is enhanced when T, is short. Because time limitations in tention Of the community is focused On the 3-DFT imaging force the use of short TR times, lesion contrast second One, and in MR in particular, the perception is often undesirable at mid- and high-field strength even when the SIN is good. At low fields, where T, values are short, high S/Nand contrast can both be achieved with 3-DFT MR images. The conceptual and analytical aspects of low-field 3-DFT MRI are presented and demonstrated at 640 G. Of its relation to field strength has important COIlSequenCeS in terms of cost.

OBJECTIVE. We introduce stratification data for three methods (Agatston, volume, mass) obtained from one single patient population.MATERIALS AND METHODS. Measurements in 11,490 individuals scanned from 1999 to 2002 with electron-beam CT were used for this study.RESULTS. Our Agatston score ranges agree reasonably well with the Kondos values except for measurements in patients at the extreme ages, at which we sampled a wider age range and consequently had different biases of averages. Neither method is preferable because except for a small percentage of individuals near the dividing lines, stratification is the same for the three methods. When we matched them against a known “lesion” phantom, the Agatston and volume scores behave nonlinearly, and the latter grossly overestimates volume. The mass method is linear except for lesions near the edge of detectability and matches known volumes to within a small percentage.CONCLUSION. We provide validated risk stratification data for use with mass scoring methods.

The purpose of this study was to evaluate the impact of spatial resolution on coronary calcium scoring by x-ray CT, to assess the scoring performance of different CT scanners as they are operated in the field and to correct for the effects of CT scanner spatial resolution on coronary calcium scoring. A phantom consisting of five aluminium wires of known diameter in water was used to measure spatial resolution and to assess scoring performance. Fourteen CT scanners (three helical, two dual, two electron-beam and seven multi-detector) from four manufacturers were evaluated, some under different operating conditions. One scanner was monitored over a 3 month period and again 6 months later. Both spatial resolution and image pixel size significantly affect calcium scoring results. Spatial resolution can be measured with a precision of about 2%. Scanner spatial resolution ranged from 1 to 1.7 mm full-width-half-maximum (FWHM), and pixel size from 0.25 to 0.86 mm. Spatial resolution differences introduce systematic scoring differences that range from 38% to 1100% depending on wire size. Significant temporal variations in spatial resolution were observed in the monitored scanner. By correcting all the scanners to the same target spatial resolution, the standard deviation of individual scanners with respect to a mean value (the spread) can be reduced by 25-70% for different wires. In conclusion, scanner spatial resolution significantly affects calcium scoring and should be controlled for. Scanner performance can change over time. Under ideal conditions, CT scanners should be operated with a standard spatial resolution for calcium scoring. When this is not possible, post-processing correction is a viable alternative.

With the introduction of direct digital radiography into clinical practice, there are opportunities to provide faster and more reliable ways of performing traditional radiological tasks. A technique was developed for evaluating and measuring scoliosis, which involves the computerized stitching of a radiograph of the thoracic spine with a radiograph of the lumbar spine. A technique is provided for real-time, graphic feedback during the stitching process, as well as during calculation and display of the Cobb angle to ensure accuracy. Initial data indicates an accuracy within 1 to 2 degrees. The time required to stitch the two images and measure the Cobb angle is on the average of 5 minutes per case. Using computerized techniques of image stitching and angle calculation, with real-time graphical feedback, the task of scoliosis evaluation is more accurate, less time consuming, and prone to fewer human errors. The requirement for special film cassettes is also eliminated.

One-hundred and nineteen implants were imaged in an open-sided low-field magnetic resonance imaging system. Thirty-four of these implants were removed, and postoperative reports were made available. We found a variety of signs associated with the implants, including internal structures, loss of saline in double-lumen implants, intracapsular and extracapsular fluid accumulations, signal dropout regions most likely associated with calcifications, and debris and fluid in the silicone, as well as degradation of the silicone, disruptions of the capsule and bulging, and extracapsular silicone. Conclusions of intact versus ruptured were made on the basis of these findings and found to be confirmed in 32 of 34 cases, with 1 false-positive and 1 false-negative result. Based on MRI criteria for a broad sample of the population, 27 percent of the implants were considered ruptured.

In principle, tomographic imaging of breast implants provides the information needed to compute implant volume. We have investigated the reliability of this measurement as a means of diagnosing the loss of gel from the implant. We find that measurement errors, postimplantation changes of the implant, uncertainties in the patients' records, and lack of knowledge of the implant temperature in the body can lead to significant discrepancies, making this measurement unreliable as a diagnostic tool.

Switched magnetic fields (in addition to the usual pulsed magnetic gradient fields) aiding and/or opposing the usual constant static magnetic field Bo are utilized so as to increase the signal-to-noise ratio for given available imaging sequence times and/or to provide special imaging effects.

We describe here an implementation of a pulse sequence in which one of the normally fixed scan parameters is varied through the scan. The image contrast that is normally obtained with long sequence repetition time TR, and consequently long scanning times, can be achieved with relatively shorter scan times by systematically varying TR during the data acquisition. By employing the longest TR only while collecting the data for the lowest spatial frequencies and using the shortest available TR elsewhere a flexible trade-off between image features (contrast and resolution) and scan time is possible. Examples of 2-D and 3-D FT MR images at low and middle field strength are presented with sequence details and some comparison scans of similar scan times.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Surface descriptions are difficult to specify. Though image processing techniques are well established to generate nearly any planar or three-dimensionally curved surface, methods to describe such complex shapes are often disorienting. Even the best intentioned interface for surface description can confuse a seasoned user. This paper introduces a surface definition technique that is simple, accurate and intuitive for the needs of routine medical image analysis. We describe a procedure to define a curved surface based on surface intersection points in a series of parallel images. In this medical context, points selected describe a surface that contains pathology of diagnostic interest. Using this technique diagnostic views are generated that conform to natural anatomic shape, physicians are no longer restricted to orthogonal or even single curve surfaces. This user interface provides analytic descriptions to produce surface views that use a Fourier-shift technique for reconstruction. Surfaces through a volume are produced with resolution equal to that of the original data set. Example images are illustrated.

A total of 58 multiple sclerosis lesions from 16 patients were used to characterize the performance of a low field system for the detection of edematous (e.g., water-elevating) brain lesions. Contrast, signal-to-noise, and their product (signal difference to noise) were measured for two-dimensional and three-dimensional Fourier transform techniques at low field strength (640 G) and compared to two-dimensional sequences at mid field strength (3,500 G). The results showed numerically that low-field strength magnetic resonance imaging can reliably detect multiple sclerosis lesions, and, by extension, other water-elevating lesions, although with lower confidence levels.

Due to the nature of three-dimensional Fourier transform (3-DFT) data acquisition in magnetic resonance imaging (MRI), the spatial relation between a resolved volume element (a voxel) and the object can be manipulated easily. Those manipulations have practical consequences in terms of registering slice positions with respect to features of interest in producing oblique reformatted images where volume resolution is preserved, and in generating reformatted images that project the viewing plane onto a surface through the object that has an arbitrary shape.

Magnetic resonance imaging (MRI) offers a unique and extraordinary window into the body. Since 1983 the growth and acceptance of this modality as a diagnostic tool has been prodigious, and the enthusiasm with which it is received still grows. The original uses were centered on head imaging, and have since expanded to spine and extremities. Some acceptance is now being seen for body imaging, more for central portions of the chest and in the pelvis than in the abdomen. The routine use of MRI for cardiovascular work is generally limited to large centers. Because of the patient demand for MRI, and because of the desire to obtain revenue from the units, MRI has not been fully exploited in areas where the primary payoff is a contribution towards understanding disease processes. For this latter purpose, one of the most important characteristics of MRI is that it is non-invasive, and therefore ideally suited for both cross-sectional and longitudinal studies. There is an obvious match between MRI and the needs of those who aim to understand the evolution of the human atherosclerotic plaque.

In magnetic resonance imaging (MRI), the signal intensity depends indirectly on particular physical and chemical characteristics of the tissues being imaged. These tissue properties influence the behavior of the nuclei undergoing resonance and their behavior is what directly affects the MRI signal. The parameters of interest that describe this behavior are the relaxation times Tl and T2, the spin density [for hydrogen, N(H)], and the microscopic (diffusion) and macroscopic (flow, motion) motional states of the nuclei. As has often been repeated, different imaging techniques result in different responses to these magnetic resonance (MR) parameters. Early speculation suggested that the MR parameters, in particular the relaxation times, would provide very specific diagnostic information on the state of tissue within the body. In fact the results in this area have been disappointing. While MRI has been extremely sensitive to disease within the body’s soft tissue, the specificity is much poorer. It is often difficult to make specific comments on the character of the lesion based on the MR data alone. Yet when properly used, the MR parameters can provide valuable information.

An electromagnet shim coil is utilized for temporarily altering the shape of a volume in which there is provided a substantially homogeneous NMR polarizing field. By temporarily energizing the electromagnet shim coil and thus altering the shape of the volume, magnetic resonance imaging can take place in other than a substantially spherical volume (e.g., in an elongated ellipsoidal-like volume extending axially along a patient so as to encompass a longer section of the spinal column). In the exemplary embodiment, the electromagnet shim coil takes the form of a pancake-like coil with windings positioned so as to create fourth power spherical harmonic in a transverse magnet-type of MRI system.