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D B Plewes

Publications and source records attributed to D B Plewes.

At least 37 records · Page 2Linked to original sources
PubMed1995

Phase constrained encoding (PACE): a technique for MRI in large static field inhomogeneities.

In spin echo imaging, magnetization is assigned to a location defined by its frequency of rotation. In the presence of a static magnetic field inhomogeneity, however, this location does not correspond to the true location of the magnetization. This paper describes a magnetic resonance imaging technique called phase constrained encoding (PACE) that assigns magnetization to its true location through the use of a spin echo train and alternating readout gradients. Small artifactual side-bands occur in the point spread function but can be minimized or eliminated using higher gradient strengths, more echoes, and/or additional acquisitions. Implementation of a simple version of this technique confirms simulations.

Artifacts↗
PubMed1995

Role of equalisation mammography of dense breasts.

Parenchymal patterns characteristic of dense breasts are known to degrade the mammographic detection of small breast cancers and microcalcifications. This arises from large variations in exposure of the film, resulting in reduced image contrast over areas of suboptimal exposure. Based on sensitometric measurements of mammograms from a typical patient population, it is shown that over 60% of a typical mammogram in Wolfe's DY classification was found to be exposed suboptimally, suggesting a significant margin for improving mammography for these patients. In order to address this problem, a prototype mammographic version of scanning equalisation radiography (MSER) has been developed, which delivers a patient-specific spatially non-uniform distribution of breast exposure, adjusted to maintain optimal film exposure and contrast over the entire mammogram. Anthropomorphic phantom MSER images show a marked improvement in subjective image quality relative to conventional mammograms, while exhibiting a similar radiation risk. The detection of small microcalcifications and fibrils over clinically significant breast densities is found to be improved by factors eight and four, respectively. Such a system may be clinically practical through the use of multiple-beam equalisation methods with available X-ray tube technology.

Breast Neoplasms↗
PubMed1995

An MR compatible flow simulator for intravascular pressure simulation.

An MR compatible flow simulator is described which generates physiologically realistic pressure and flow waveforms. The simulator is based on a servomotor-driven gear pump which produces pulsatile flow by modulation of the servomotor rotation rate. Operation of the simulator is under the control of a personal computer, which executes an iterative feedback loop to minimize errors between measured and desired pressure waveforms. The simulator is totally automatic, requiring only a few minutes of iteration to generate the desired pressure waveform. Accurate sinusoidal waveforms with frequencies up to 10 Hz have been generated using the simulator, with high-frequency contamination of the measured waveform at least 80 dB below the fundamental frequency. Aortic waveforms have been produced with realistic flow rates and pressure variations. The pump assembly is mechanically straightforward and can operate at an 8-m distance from the flow phantom to allow the device to be isolated from the MR magnet room.

Biophysical Phenomena↗
PubMed1995

A method for practical equalization mammography of the radiographically dense breast.

It has been shown that equalization radiography can overcome the well-known problem of limited film latitude encountered in mammography of the radiographically dense breast. Current equalization geometries based on single scanning beam (SER) or multiple-beam techniques approach the heat-loading limits of mammographic x-ray sources and require excessively long scan times. The authors have proposed an alternative geometry for equalization mammography, rotary scanning equalization radiography (RSER), which uses a slot beam in a translate-rotate geometry. RSER provides the simplicity of a single-beam geometry while offering improved tube efficiency over multiple-beam geometries. Numerical simulations and a prototype imaging system are used to show that equalized mammograms exhibiting high contrast throughout the breast can be obtained with a large scanning beam translated over the image at only four scanning angles. These results indicate that RSER is an efficient, simple, and practical means of imaging the dense breast.

Female↗
PubMed1994

Rotary scanning equalization radiography: an efficient geometry for equalization mammography.

The detection of cancer in the radiographically dense breast is problematic, since the breast will produce a range of exposure that exceeds the useful dynamic range of high contrast film-screen combinations. It has been shown previously that mammographic scanning equalization radiography (MSER) can be used to overcome the latitude limitations of film-screen mammography. However, the tube loading of MSER is orders of magnitude greater than conventional mammography. A new rotary geometry for equalization radiography is proposed, in which the image receptor is exposed by repeated scans of a modulated slot beam, oriented at a variety of scanning angles with respect to the object. The superposition of the exposure from appropriately modulated, rotated slot beams produces an entrance exposure that will effectively equalize the film exposure. The principle advantages of this geometry is its simplicity and reduced tube loading. To determine the effectiveness and feasibility of RSER the effect of conventional, MSER, and RSER have been numerically simulated on the appearance of clinical mammograms, the relative heat loading, and the fraction of the breast imaged with high contrast are calculated. It is found that RSER produces images that are free of artefacts, and exhibit a similar degree of equalization, as found in MSER images. RSER accomplishes this with only four scanning angles, and a beam that is approximately 4 cm wide. The resulting tube loading is only three times greater than that found in conventional imaging. Numerical simulations indicate that RSER is a simple, feasible means of overcoming the latitude limitations of film-screen mammography.

Computer Simulation↗
PubMed1994

The AAPM/RSNA physics tutorial for residents. Contrast mechanisms in spin-echo MR imaging.

The majority of sequences used in routine clinical magnetic resonance imaging rely on the concepts involving the spin echo. Spin-echo sequences require long acquisition times (1-10 minutes), but compared with faster gradient-recalled echo methods, spin-echo methods are relatively immune to signal loss and distortions from field inhomogeneity and tissue-induced susceptibility variations. Through modifications of intersequence repetition time (TR), echo formation interval (echo time [TE]), and various gradient moments, image contrast can be altered to emphasize tissue relaxation times T1, T2, or proton density. The TR and TE values control the amount of T1 weighting and T2 weighting, respectively. At long TR intervals (approximately 10 x tissue T1 values) and minimum TE values, the difference in signal intensity arising from relaxation vanishes, and contrast arises solely from the differences in proton density between the two tissues. Images formed with short TR intervals and long TE values exhibit very low signal-to-noise ratio and negligible contrast and should be avoided. Recently, fast spin-echo sequences have partially overcome the limitation of long acquisition times, with up to 16-fold reduction, by acquiring multiple lines in k space with multiecho sequences.

Artifacts↗
PubMed1994

Detection of pituitary microadenomas: comparison of dynamic keyhole fast spin-echo, unenhanced, and conventional contrast-enhanced MR imaging.

OBJECTIVE: Pituitary microadenomas may not be detected on conventional MR images. We supplemented conventional unenhanced and contrast-enhanced MR imaging with a dynamic keyhole fast spin-echo (kFSE) method in order to compare how frequently a microadenoma could be detected with the three different methods. SUBJECTS AND METHODS: Eighteen consecutive patients with clinical and laboratory evidence of pituitary microadenomas had unenhanced, dynamic kFSE, and conventional contrast-enhanced MR imaging of the pituitary gland. A control group of 13 subjects without pituitary disease also had dynamic kFSE MR imaging. Hard copies of all the studies were obtained in an identical fashion, and then the MR images of the patients and control subjects were randomly mixed. The studies were reviewed by a neuroradiologist who had no knowledge of the clinical status of the subjects. The presence or absence of any pituitary focal hypointensity consistent with the appearance of a microadenoma was noted. RESULTS: A pituitary lesion consistent in appearance with a microadenoma was detected on dynamic kFSE images in 13 of the 18 patients, on unenhanced images in nine patients, and on conventional contrast-enhanced images in 10 patients. In four patients, a microadenoma was detected on dynamic kFSE images only. Dynamic kFSE images showed a lesion in four of the 13 control subjects. CONCLUSION: Dynamic kFSE MR imaging is a useful supplemental sequence in patients undergoing MR imaging because of pituitary endocrinopathy. It may show lesions that would otherwise escape detection.

Adenoma↗
PubMed1991

Improved imaging of bone with scan equalization radiography.

Use of scan equalization radiography (SER) for imaging bone in the head, face, neck, and shoulder was evaluated in a clinical comparison study with conventional radiographs of randomly selected patients. Two hundred nine pairs of normal and abnormal SER images and conventional radiographs were compared by four radiologists in a side-by-side viewing situation. The radiologists compared the visibility of specific anatomic features and rated the SER images as better than, equal to, or worse than the conventional radiographs. In the evaluation of the images of the cervical spine, the radiologists much preferred the SER images 63% of the time because of slight improvement in bone detail and marked improvement in detail of the soft tissues. In the evaluation of the images of the shoulder, the radiologists preferred the SER image 58% of the time and preferred the conventional radiograph only 5% of the time (p less than .05). In the evaluation of the images of the skull, face, and mandible, the radiologists preferred the SER images 62% of the time and the conventional images 4% of the time (p less than .05). The superior image quality with the SER technique was recognized by all radiologists in the study and was the overwhelmingly preferred way of imaging the shoulder, neck, head, and face.

Adult↗
PubMed1989

Hybrid methods of chemical-shift imaging.

We propose a family of hybrid chemical-shift sequences which combines two physical principles for water/lipid separation to minimize artifacts introduced by B0 and B1 inhomogeneities. Hybrid sequences provide improved species discrimination over earlier methods without resorting to postprocessing while maintaining a multislice/multiecho capability.

Abdomen↗
PubMed1989

Comparison of scanning equalization and conventional chest radiography.

A clinical comparison study of scanning equalization radiography (SER) and conventional chest radiography was performed with the latest prototype SER system. Conventional chest radiography was performed at 120 kVp with Lanex regular screens (Eastman Kodak, Rochester, NY) and Kodak Ortho-G or Ortho-C film (Eastman Kodak). The 253 volunteer patients were examined with both techniques. The chest radiographs were interpreted by four radiologists. The study group was composed of 58 normal and 195 abnormal posteroanterior and lateral chest radiographs. In 31 cases there were two major radiologic diagnoses. The number of correct interpretations increased when the SER images were examined, compared with the conventional Ortho-G (chi 2 = 4.17, P less than .05) and conventional Ortho-C (chi 2 = 16.9, P less than .001) radiographs. The overall accuracy of disease detection improved for all radiologists with the SER system. There was no disease category in which the accuracy of interpretation decreased when the SER system was used. The SER system is a clinically reliable method of improving image quality and increasing diagnostic accuracy.

Humans↗
PubMed1989

Lung weight in vivo measured with computed tomography and rebreathing of soluble gases.

In nine anesthetized dogs, accuracy of noninvasive measurements of lung weight (W) and gas volume in vivo was determined from volume and density determined by computed tomography (CT) and by rebreathing helium and the soluble gases dimethyl ether (WDME) and acetylene (WC2H2). Reference standards were obtained from the postmortem scale weight of the frozen lungs (Wscale) and compared with the CT lung weights measured in the living dog (WCT-38) and the frozen carcass (WCT-cold). WCT-cold did not significantly differ from Wscale [-2 +/- 9% (SD), P = 0.7]. WCT-cold was 10% greater than WCT-38 (0.10 greater than P greater than 0.05), suggesting an increase in lung weight despite immediately commencing freezing after death. WDME measured 64 +/- 6% and WC2H2 56 +/- 12% of WCT-38. Serial multiple measurements in three dogs over 14 wk showed a coefficient of variation (CV) of 10 +/- 2% for WDME, 18 +/- 2% for WC2H2, 4.1 +/- 0.9% for WCT, 2.6 +/- 0.8% for CT density, and 3.5 +/- 1.6% for functional residual capacity (FRC) by CT. FRC calculated from CT consistently underestimated FRC measured by rebreathing helium by 18 +/- 8% (P less than 0.005). This error, despite good agreement between WCT and Wscale, was explained by underestimation of CT total lung volume and overestimation of lung density by factors known to affect CT readings, such as partial volume effects, beam hardening, and limited number of input signals. These data show that CT scanning can provide serial measurement of the mass, density, and volume of the lungs with a CV in the order of 5%, but the rebreathing of soluble gases gives more than double this variability. Measurements of WDME performed on the same day had a CV of 3 +/- 1%, so that WDME provides a precise noninvasive means to measure lung weight in acute studies.

Acetylene↗
PubMed1988

Fat suppression in the time domain in fast MR imaging.

Two gradient-recalled lipid suppression sequences are proposed. A two-excitation sequence cycles the TE interval between excitations to alter the lipid phase which is followed by complex subtraction. A four-excitation variant which improves the extent of lipid suppression by partially compensating for errors resulting from spin-spin relaxation and B0 inhomogeneities is outlined.

Humans↗
PubMed1988

Improved pulmonary nodule detection with scanning equalization radiography.

The potential for improved pulmonary nodule detection with scanning equalization radiography (SER) was evaluated by means of observer performance testing during the interpretation of posteroanterior conventional radiographs and SER images of an anthropomorphic chest phantom with simulated nodules. A test set of 200 conventional and 200 SER radiographs of phantoms containing either one nodule or none was interpreted by four radiologists attempting to detect a nodule and indicate a confidence value. Their ability to detect nodules positioned over the lung was slightly improved with SER compared with conventional radiography (sensitivity, .56 vs .70); for nodules over the mediastinum or diaphragmatic areas, it was much improved (sensitivity, .29 vs .64). The results were also analyzed with receiver-operating characteristic methods, which revealed a significant improvement in lesion detect-ability over the thicker body parts with SER images. The capability of equalized chest radiographs to provide improved lesion detectability suggests that SER may set a new standard for film-based chest radiography and have a large clinical application.

Humans↗
PubMed1987

Separation of lipid and water MR imaging signals by chopper averaging in the time domain.

A convenient alternative to Dixon's water/lipid imaging technique is proposed. The sequence exploits the automatic paired averaging capability that is used in commercial magnetic resonance imaging systems for data averaging to achieve lipid suppression. This can be easily implemented via a minor modification to an existing partial saturation sequence. This approach provides linear suppression of lipid signals that is independent of the relative abundance of water and lipid contributions, unlike the characteristics of the original Dixon sequence. The proposed sequence can be tuned before the imaging procedure to ensure optimum timing and frequency adjustment.

Body Water↗
PubMed1987

Physiologic changes during high field strength MR imaging.

High field strength MR imaging systems may require several kilowatts of RF power to obtain images. A fraction of this power is absorbed by the patient, and changes in body temperature have been measured in experimental animals. The purpose of this study was to quantify changes in body surface temperature and other physiologic parameters in humans during MR scanning at 1.5 T. Blood pressure, heart rate, respiration, and axillary temperature measurements were obtained on 27 normal volunteers. Measurements were made at RF power levels of 0, 0.2, and 0.8 W/kg, with the power sequence randomized. In 14 volunteers receiving lumbar scans, statistically significant increases in temperature were observed at RF power levels of 0.2 (+0.2 +/- 0.1 degree C) and 0.8 (+0.5 +/- 0.1 degree C) W/kg. No significant changes related to RF power were observed in blood pressure or respiratory rate. At the 0.8 W/kg level there was a slight increase in heart rate (3 +/- 1.3 beats per minute). In the 13 patients receiving head scans, physiologic changes were substantially smaller. The temperature increases and other physiologic changes observed during MR scanning with the 1.5 T imager at RF powers of 0.2 and 0.8 W/kg were small and of no clinical concern. Additional studies should be performed in patients with cardiac failure, vascular occulusion, and metallic implants or prostheses.

Adult↗
PubMed1986

Measurement of lung gas volume and regional density by computed tomography in dogs.

To determine if computed tomography (CT) can accurately measure lung volume, we compared lung gas volume measured by helium dilution with the equivalent volume calculated from CT total lung volume and density in 13 supine dogs. CT lung gas volume underestimated helium volume by 34% (range: -63 to 0%). Studies of wooden lung phantoms varying in density from 0.082g/cc to 0.776g/cc showed that only 15% of this error could be mimicked by the phantoms. The rest of the discrepancy is attributed to the lung's irregular borders, and the sharp density gradients surrounding and within the lung that result in x-ray beam hardening, sampling limitations, and partial volume measurement errors. Serial biweekly measurements in three dogs for 14 weeks showed CT gas volume to be highly reproducible with less scatter than seen in the helium measurements. Density in the lungs of all dogs showed a uniform gradual decrease from approximately 0.60g/cc at the dependent surface to 0.20g/cc at the superior surface with relatively constant density at any horizontal level. These studies show that whereas CT underestimates gas volume in the lungs, serial measurements are highly reproducible in experimental studies and are a promising technique to monitor diseases or response to therapy. Density gradients in the lungs were sufficiently uniform so that disruption of the normal gradient may be an indicator of early lung disease.

Animals↗
PubMed1986

Normal and abnormal temporomandibular joint: MR imaging with surface coil.

The normal temporomandibular joint (TMJ) was evaluated using magnetic resonance (MR) imaging with a surface coil in five subjects and compared with the abnormal joint in 37 patients (aged 14-59 years; total joints studied, 76). Multisection 3-mm-thick sagittal, coronal, and axial images were obtained with a 1.5-T MR system and 6.5-cm-diameter surface coil using both partial saturation and spin-echo sequences (TR = 1,000 msec, TE = 20 or 25 msec). A comparison with arthrography (n = 13 joints), computed tomography (CT) (n = 11), and surgical (n = 5) findings demonstrated that MR imaging with a surface coil provided an accurate depiction of both normal and abnormal TMJs. MR provided information about meniscal position, morphology, and histology that was not available with either arthrography or CT alone. The imaging potential of MR and its noninvasive characteristics warrant priority for further examination of MR as a useful modality in the diagnosis of TMJ pain and dysfunction.

Adolescent↗