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Biomedical subjects

D S Brettle

Publications and source records attributed to D S Brettle.

11 recordsLinked to original sources
PubMed1998

Experiences with functional magnetic resonance imaging at 1 tesla.

Functional magnetic resonance imaging (fMRI) has been performed on a standard 1 T system using a pulse sequence developed to utilize blood oxygen level dependent (BOLD) contrast and an off-line analysis routine using correlation techniques. The sequence and the data analysis routine have been validated by reproducing the conventional hand movement paradigm studies reported by numerous other workers. Our work has then been extended to investigate cerebral foci for a tonic pain stimulus and the cortical representation of oesophageal stimulation. Both these studies relate to paradigms where the expected BOLD signal is significantly less than that encountered for motor or visual cortex paradigms. The results show good agreement with other modalities (positron emission tomography, magnetoencephalography and cortical evoked potentials). Performing fMRI at 1 T is slightly controversial. However, our successful study of demanding paradigms, using a standard clinical 1 T imaging system, has important implications for many other users operating at this field strength.

Brain↗
PubMed1997

Physical performance measures of radiographic imaging systems.

New systems and technologies for dental radiographic imaging are being introduced at an increasing rate. Established methods of physical measurement allow the imaging performance of these systems to be quantified. These quantitative measures are important when comparing the imaging performance of competing systems. This review presents an overview of the main concepts in the assessment of the physical performance of imaging systems including signal transfer, as described by both the characteristic curve and modulation transfer function (MTF), and image noise, as described by the Noise Power Spectra. These measures can be combined to produce signal-to-noise ratio descriptors such as Noise Equivalent Quanta and Detective Quantum Efficiency which give an overall description of imaging system performance. Subjective means of measuring image quality, which naturally include the performance of the image display system and the observer, are also considered as a gauge of system performance.

Artifacts↗
PubMed1997

Visibility of microcalcifications in computed and screen-film mammography.

Due to the clinically and technically demanding nature of breast x-ray imaging, mammography still remains one of the few essentially film-based radiological imaging techniques in modern medical imaging. There are a range of possible benefits available if a practical and economical direct digital imaging technique can be introduced to routine clinical practice. There has been much debate regarding the minimum specification required for direct digital acquisition. One such direct digital system available is computed radiography (CR), which has a modest specification when compared with modern screen-film mammography (SFM) systems. This paper details two psychophysical studies in which the detection of simulated microcalcifications with CR has been directly compared to that with SFM. The first study found that under scatter-free conditions the minimum detectable size of microcalcification was approximately 130 microns for both SFM and CR. The second study found that SFM had a 4.6% higher probability of observers being able to correctly identify the shape of 350 microns diameter test details; there was no significant difference for-either larger or smaller test details. From the results of these studies it has been demonstrated that the modest specification of CR, in terms of limiting resolution, does not translate into a dramatic difference in the perception of details at the limit of detectability. When judging the imaging performance of a system it is more important to compare the signal-to-noise ratio transfer spectrum characteristics, rather than simply the modulation transfer function.

Breast Diseases↗
PubMed1997

Estimation of fetal and effective dose for CT examinations.

Doses from CT examinations are difficult to estimate. However, they are requested more frequently due to the increase in CT examinations. In particular, fetal dose estimations are frequently required for patients who have discovered, subsequent to the examination, that they were pregnant when the examination was conducted. A computer model has been developed to facilitate such dose calculations. This model combines empirical beam data with anatomical information. The model has been verified using thermoluminescent dosemeter (TLD) readings of internal and surface dose from both phantoms and patients, including intrauterine doses for patients undergoing afterloading gynaecological intracavitary treatment. Although only limited experimental data were available, the results indicate that the model accurately predicts uterine doses within acceptable errors. This approach has been validated for fetal dose estimation. The model was also used in a comparison with the nationally available CT dose data from the National Radiological Protection Board (NRPB). The two models were found to be in agreement for fetal dose estimations.

Computer Simulation↗
PubMed1996

The potential medico-legal implications of computed radiography.

Computed radiographic (CR) systems are available which allow dentists to obtain direct digital intra-oral images. These CR images, however, can be readily altered using image processing software. This paper demonstrates the potential medico-legal problems of image manipulation and calls for manufacturers of CR equipment to be pressed to explore methods of preventing such abuse.

Computer Security↗
PubMed1996

The imaging performance of a storage phosphor system for dental radiography.

Direct digital imaging systems are becoming increasingly common in dental radiography. The majority of these systems are based on charge coupled device (CCD) technology. A relatively new system, based on photo-stimulable phosphor luminescence (PSPL), is now commercially available. This system appears to overcome some of the restrictions of CCD systems including those associated with the bulky detector, connecting wire, limited image size and limited exposure latitude. A physical evaluation of the PSPL system was conducted including measurements of sensitometric response, modulation transfer function (MTF), noise power spectrum (NPS), noise equivalent quanta (NEQ) and detective quantum efficiency (DQE). These measurements were compared with results obtained previously for Kodak Ektaspeed (E-speed) film. The results of the evaluation indicate that the PSPL system may be capable of operating at exposures up to 80% lower than for film with comparable image quality.

Humans↗
PubMed1995

Technical note: dual compression mammography using computed radiography.

On mammography most malignant growths may be seen as a tethered mass or architectural distortion. Benign lesions such as cysts and fibroadenomas have no focal tethering and are usually well circumscribed. Computed radiography (CR) is a direct digital system which may be used for mammography. One of the main advantages of CR is its wide exposure latitude allowing images to be acquired at lower exposures than required for film-screen systems. A technique was developed, utilizing the exposure latitude of CR, to acquire two mammograms of the same breast at two slightly different compressions, but using approximately the same exposure as a single film-screen mammogram. These images were then "animated" on a computer allowing the fibrous septa movement to be visualized. The movement of the fibrous septa in the presence of an abnormality was used to help confirm whether a lesion was benign or malignant. This technique may have clinical application in classifying architectural distortions and masses which are ultrasonically benign but later found to be malignant. Microcalcifications cannot be used as a diagnostic indicator with this technique because of the increased noise associated with the lower exposures. The increased noise did not affect the visualization of masses.

Breast Neoplasms↗
PubMed1994

A clinical comparison between conventional and digital mammography utilizing computed radiography.

Previous work utilizing a specially designed mammographic test object has indicated that the image quality of computed radiography (CR) approaches that of a conventional film-screen technique of equivalent radiographic speed. In an attempt to correlate these results with the clinical situation a subjective rating study of the primary physical parameters of noise, sharpness, and contrast has been conducted. Using a randomized viewing sequence two radiologists experienced in mammography were asked to score these parameters utilizing a 5 point scale. A total of 138 images was used, 62% containing no abnormalities and 38% containing either microcalcifications, masses or both. Additionally, the observers scored their confidence of correct classification, and these results were then used to construct receiver operating characteristic (ROC) curves. The subjective rating of the physical parameters results produced similar results to those obtained using the test object, with CR images providing greater contrast than film. The resolution of CR, however, was lower than for film. Overall the noise levels were similar for both modalities, but the digital images occasionally contained high levels of dust artefacts. In one case these artefacts were mistaken for a cluster of microcalcifications. The test object results and the parameter rating results indicate that CR approaches the performance of film-screen images. Statistically the ROC curves produced indicate CR images to be comparable to the film-screen combination. This may be because the increased contrast compensates for resolution limitations and these findings correlate with our clinical experience.

Breast Diseases↗
PubMed1994

Physical evaluation of computed radiography as a mammographic X-ray imaging system.

The physical imaging properties of a computed radiography (CR) system operating under mammographic exposure conditions have been measured. These measurements include modulation transfer function (MTF), sensitometric response and noise power spectrum (NPS). These figures were used to derive signal-to-noise ratio (SNR) descriptors of the system performance. The same measures were derived for a mammographic film-screen system. The SNR properties of the CR system indicate superior low frequency performance over a wider dynamic range than the film-screen system. The spatial frequency dependent SNR properties of the CR system are, however, limited. The results of a psychophysical test support the results obtained from physical measurements. These results indicate that despite its limited sharpness, details of dimensions relevant to diagnosis in mammography (circular details 0.25 mm diameter to represent microcalcifications and 3 mm diameter to represent small masses) are reproduced using CR with SNRs which are at least comparable with those of a mammographic film-screen system.

Breast Diseases↗
PubMed1992

A preliminary investigation of the imaging performance of photostimulable phosphor computed radiography using a new design of mammographic quality control test object.

Leeds Test Object TOR[MAM] has been designed to supplement the current FAXIL mammography test object TOR[MAX]. It contains a range of details that have a more natural radiographic appearance and has been designed as a test that more closely approximates the image quality achieved in clinical mammography. Physical aspects of the design and implementation of TOR[MAM] are presented. The TOR[MAM] has been used in a preliminary physical evaluation of the comparative image qualities produced by conventional (screen-film) and photostimulable phosphor computed mammography and the results are discussed. TOR[MAX] results are also presented. The influence of digital image processing (enhancement) on the image quality of computed mammograms is also considered. The results presented indicate the sensitivity of TOR[MAM].

Female↗