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Biomedical subjects

T W Redpath

Publications and source records attributed to T W Redpath.

At least 19 recordsLinked to original sources
PubMed2001

The effect of partial removal of the nucleus pulposus from the intervertebral disc on the response of the human annulus fibrosus to compression.

OBJECTIVE: To determine how partial removal of the nucleus changes the response of the annulus to compression. DESIGN: The deformation of the annulus in the mid-sagittal plane, during compression, was determined from digital video images. BACKGROUND: Several studies have shown that removal of the nucleus changes the external behaviour of the intervertebral disc, but few studies have investigated changes to internal behaviour. METHODS: Six frozen human lumbar discs were bisected in the sagittal plane to produce 12 specimens. The cut surfaces were marked with seven dots of Alcian blue stain. The specimens were sealed, enabling their internal structure to be viewed directly by a digital video recording system, and thawed. The video system recorded the response of each specimen as it was compressed by up to 1.8 mm at a rate of 0.2 mm s(-1). The displacements of the Alcian blue marks were measured using an image analysis program. Magnetic resonance imaging was used to investigate the validity of this technique. RESULTS: Partial removal of the nucleus changed the way that the disc deformed under compression. A highly significant change in direction of movement was seen in the inner posterior region of the annulus. CONCLUSIONS: Partial removal of the nucleus changes the response of the annulus to compression. RELEVANCE: Partial denucleation of the human intervertebral disc is shown to change the direction of bulging of the inner annulus when the disc is compressed. Increases in shear stress, arising from these changes, may lead to further disc degeneration in the form of circumferential tears.

Biomechanical Phenomena↗
PubMed2000

Magnetic resonance imaging screening in women at genetic risk of breast cancer: imaging and analysis protocol for the UK multicentre study. UK MRI Breast Screening Study Advisory Group.

The imaging and analysis protocol of the UK multicentre study of magnetic resonance imaging (MRI) as a method of screening for breast cancer in women at genetic risk is described. The study will compare the sensitivity and specificity of contrast-enhanced MRI with two-view x-ray mammography. Approximately 500 women below the age of 50 at high genetic risk of breast cancer will be recruited per year for three years, with annual MRI and x-ray mammography continuing for up to 5 years. A symptomatic cohort will be measured in the first year to ensure consistent reporting between centres. The MRI examination comprises a high-sensitivity three-dimensional contrast-enhanced assessment, followed by a high-specificity contrast-enhanced study in equivocal cases. Multiparametric analysis will encompass morphological assessment, the kinetics of contrast agent uptake and determination of quantitative pharmacokinetic parameters. Retrospective analysis will identify the most specific indicators of malignancy. Sensitivity and specificity, together with diagnostic performance, diagnostic impact and therapeutic impact will be assessed with reference to pathology, follow-up and changes in diagnostic certainty and therapeutic decisions. Mammography, lesion localisation, pathology and cytology will be performed in accordance with the UK NHS Breast Screening Programme quality assurance standards. Similar standards of quality assurance will be applied for MR measurements and evaluation.

Adult↗
PubMed2000

Assessment of maxillary sinus volume for the sinus lift operation by three-dimensional magnetic resonance imaging.

OBJECTIVES: To calculate sinus and bone graft volumes and vertical bone heights from sequential magnetic resonance imaging (MRI) examinations in patients undergoing a sinus lift operation. METHODS: MRI scans were obtained pre-operatively and at 10 days and 10 weeks post-operatively, using a 0.95 tesla MRI scanner and a three-dimensional (3D) magnetisation prepared, rapid acquisition gradient-echo (MP-RAGE) sequence. RESULTS: Estimates of the bone graft volumes required for a desired vertical bone height were made from the pre-operative MRI scan. Measurements of the graft volumes and bone heights actually achieved were made from the post-operative scans. The MRI appearance of the graft changed between the 10 day and 10 week scans. CONCLUSIONS: We have proposed a technique which has the potential to give the surgeon an estimate of the optimum volume of graft for the sinus lift operation from the pre-operative MRI scan alone and demonstrated its application in a single patient. Changes in the sequential MRI appearance of the graft are consistent with replacement of fluid by a matrix of trabecular bone.

Adolescent↗
PubMed2000

Inflammatory abdominal aortic aneurysms: diagnosis with gadolinium enhanced T1-weighted imaging.

AIM: Inflammatory abdominal aortic aneurysms (IAAA) are a variant of abdominal aortic aneurysms, which are associated with an increased morbidity. The diagnosis of IAAA has traditionally been established with a combination of clinical and laboratory findings together with contrast enhanced CT. There is a high incidence of renal impairment in this group, and therefore contrast enhanced CT may be harmful. PATIENTS AND METHODS: Five patients with IAAA underwent T1-weighted spin echo and gradient echo gadolinium enhanced abdominal MRI. A total of eight examinations were performed, including three patients who underwent repeat MRI following steroid therapy. RESULTS: The inflammatory cuff was clearly identified following gadolinium administration in all eight examinations. The cuff enhanced homogeneously in all patients with no alteration in appearance following steroid therapy. The intravenous administration of gadolinium DTPA enabled clearer definition of locally involved structures. CONCLUSION: Gadolinium enhanced MRI readily demonstrates features of IAAA. In view of potential renal impairment in this patient group, we recommend this technique in preference to contrast enhanced CT in the initial investigation of inflammatory abdominal aortic aneurysms.

Aged↗
PubMed2000

Estimating achievable signal-to-noise ratios of MRI transmit-receive coils from radiofrequency power measurements: applications in quality control.

The inverse relationship between the radiofrequency (RF) power needed to transmit a 90 degree RF pulse, and the signal-to-noise ratio (SNR) available from a transmit-receive RF coil is well known. The theory is restated and a formula given for the signal-to-noise ratio from water, achievable from a single-shot MRI experiment, in terms of the net forward RF power needed for a rectangular 90 degree RF pulse of known shape and duration. The result is normalized to a signal bandwidth of 1 Hz and a sample mass of 1 g. The RF power information needed is available on most commercial scanners, as it is used to calculate specific absorption rates for RF tissue heating. The achievable SNR figure will normally be larger that that actually observed, mainly because of receiver noise, but also because of inaccuracies in setting RF pulse angles, and relaxation effects. Phantom experiments were performed on the transmit-receive RF head coil of a commercial MRI system at 0.95 T using a projection method. The measured SNR agreed with that expected from the formula for achievable SNR once a correction was made for the noise figure of the receiving chain. Comparisons of measured SNR figures with those calculated from RF power measurements are expected to be of value in acceptance testing and quality control.

Magnetic Resonance Imaging↗
PubMed2000

Dobutamine magnetic resonance imaging as a predictor of myocardial functional recovery after revascularisation.

OBJECTIVE: To assess the use of dobutamine magnetic resonance imaging (MRI) as a preoperative predictor of myocardial functional recovery after revascularisation, comparing wall motion and radial wall thickening analyses by observer and semi-automated edge detection. PATIENTS: 25 men with multivessel coronary disease and resting wall motion abnormalities were studied with preoperative rest and stress MRI. MAIN OUTCOME MEASURES: Observer analysis for radial wall thickening was compared with a normal range, while wall motion analysis used a standard four point scale. Semi-automated analysis was performed using an edge detection algorithm. Segments displaying either improved or worsened thickening or motion with dobutamine were considered viable. Postoperative rest images were performed 3-6 months after coronary artery bypass grafting (CABG) for comparison. RESULTS: For observer analysis the values for sensitivity and specificity were 50% and 72% for wall motion, with respective values of 50% and 68% for thickening. With semi-automated edge detection the figures for motion were 60% and 73%, with corresponding values of 79% and 58% for thickening. Combining thickening and motion for the semi-automated method to describe any change in segmental function yielded a sensitivity of 71% and specificity of 70%. CONCLUSIONS: Dobutamine MRI is a reasonably good predictor of myocardial functional recovery after CABG. The use of semi-automated edge detection analysis improved results.

Aged↗
PubMed1999

Accuracy of T1 measurement in dynamic contrast-enhanced breast MRI using two- and three-dimensional variable flip angle fast low-angle shot.

In vivo T1 measurements, used to monitor the uptake of contrast agent by tissues, are typically performed as a first step in implementing compartmental analysis of contrast-enhanced breast magnetic resonance imaging (MRI) data. We have extended previously described methodology for in vivo T1 measurement (using a variable flip-angle gradient-recalled echo technique) to two-dimensional (2D), fast low-angle shot (FLASH). This approach requires computational modeling of slice-selective radiofrequency (RF) excitation to correct for nonrectangular slice profiles. The accuracy with which breast tissue T1 values can be measured by this approach is examined: T1 measurements from phantom and in vivo image data acquired with 2D and 3D FLASH imaging sequences are presented. Significant sources of error due to imaging pulse sequence quality and RF transmit field nonuniformity in the breast coil device that will have detrimental consequences for compartmental analysis are identified. Rigorous quality assurance programs with calibrated phantoms are thus recommended, to verify the accuracy with which T1 measurements are obtained.

Breast↗
PubMed1999

Sources of artifact and systematic error in quantitative snapshot of FLASH imaging and methods for their elimination.

Snapshot FLASH preceded by a preparation sequence is used to measure NMR parameters. Some preparation information is lost during snapshot FLASH acquisition as the magnetization evolves towards the steady state. This problem is often addressed by assuming that image intensity is proportional to the central k-space line, which may be sampled immediately after preparation using a centric-reordered scheme. Although some limitations of this assumption have been discussed, they are often ignored in parameter measurement. In this study, magnetization evolution effects in snapshot FLASH were investigated by computer simulation and phantom experiments in the context of T2 measurement. It was found that the standard analysis may lead to significant error in parameter measurement. Some improvement may be obtained using a more sophisticated analysis to take account of magnetization evolution. However, at the expense of a 50% reduction in signal intensity, accurate values may be obtained by a novel method that uses crusher gradients to eliminate evolved magnetization.

Artifacts↗
PubMed1999

A novel fast split-echo multi-shot diffusion-weighted MRI method using navigator echoes.

Difficulties in obtaining diffusion-weighted images of acceptable quality using conventional hardware and in a reasonable time have hindered the clinical application of diffusion-weighted magnetic resonance imaging (DWI). Diffusion-weighted fast spin-echo (FSE) sequences offer the possibility of fast DWI on standard hardware without the susceptibility problems associated with echoplanar imaging. However, motion in the presence of diffusion-sensitizing gradients can prevent fulfilment of the Meiboom Gill phase condition, leading to destructive interference between echo components and consequent signal losses. A recently proposed single-shot FSE sequence employed split-echo acquisition to address this problem. However, in a segmented FSE sequence, phase errors differ between successive echo trains, causing "ghosting" in the diffusion-weighted images that are not eliminated by split-echo acquistion alone. A DWI technique is presented that combines split-echo acquisition with navigator echo phase correction in a segmented FSE sequence. It is shown to be suitable for diffusion measurements in vivo using standard hardware.

Brain↗
PubMed1999

Determination of normal regional left ventricular function from cine-MR images using a semi-automated edge detection method.

A semi-automated edge detection method for the delineation of the endo- and epicardial borders of the left ventricle from cine MR images has been developed. The feasibility of this was demonstrated by processing end diastolic and end systolic ECG-gated images of four short axis images in 10 healthy subjects. The first derivative method combined with a 2D weighted polynomial fitting procedure was used to determine the endo- and epicardial borders, which then allowed determination of the wall motion, wall thickening, and ejection fraction, of the left ventricle. The results show that the end-systolic radial wall motion varies from (32+/-8)% to (76+/-12)%, and wall thickening from (0.60+/-0.46) cm to (1.26+/-0.50) cm. An average ejection fraction of (69+/-6)% was found which agrees well with literature values. The method described, for the delineation of the borders, reduces considerably the long and tedious operator time inherent in manual measurement and greatly increases the reproducibility of the measurements.

Adult↗
PubMed1999

Assessment of the sinus lift operation by magnetic resonance imaging.

Sinus lift operations are done to increase bone thickness before placement of osseointegrated implants in areas of the maxilla where there is insufficient bone between the lower border of the maxillary sinus mucosa and the oral mucoperiosteum. Full sectional information should be available to the surgeon before the sinus lift operation so that the outcome can be predicted, and to allow for assessment of the volume of bone graft required. It is also useful to assess the success of the procedure before placement of the implant, and to choose the implant length and orientation required for maximum stability. We describe the use of a 0.2 tesla magnetic resonance imaging (MRI) scanner to image a patient before and after sinus lift. MRI gave full sectional information of the bone on both occasions without using ionizing radiation. The quality of the images was good, and should be considered as an alternative to the accepted gold standard of computed tomography (CT), which exposes the patient to a high dose of radiation.

Bone Transplantation↗
PubMed1998

Magnetic resonance imaging: a useful tool for evaluation of bone prior to implant surgery.

OBJECTIVE: To illustrate the potential for use of magnetic resonance imaging (MRI) to gain full sectional information before placement of osseo-integrated dental implants, with no patient exposure to ionising radiation. MATERIALS AND METHODS: Four typical cases are illustrated, the patients being imaged in a one tesla MRI scanner. The setting up of the sequences is explained, along with the use of an imaging/surgical template with gadolinium markers. RESULTS: MRI clearly shows full sectional detail of available bone for safe implant placement, and allows the delineation of cortical and cancellous bone to attain maximum implant length and stability. Vital structures and the floor of the maxillary sinus are clearly shown. CONCLUSIONS: MRI allows the surgeon to assess cases for suitability to place dental implants with confidence. The ability to scan directly at any desired plane, with no reformatting, and to relate this information to a surgical template gives predictable surgery. MRI is a sectional imaging modality giving information about the 3-dimensional relationship of the vital structures, without using ionising radiation. It therefore deserves consideration as an alternative to computed tomography. However, further work is indicated to investigate the relative technical merits of the two imaging modalities.

Adult↗
PubMed1998

Comparison of four magnetization preparation schemes to improve blood-wall contrast in cine short-axis cardiac imaging.

Improvements in short-axis blood-myocardium contrast in the heart with the use of four magnetization preparation schemes applied before the imaging sequence are demonstrated. Gradient-echo cine cardiac images are acquired and compared at 0.95 T incorporating T2, T1rho, magnetization transfer, and double inversion (black blood) preparations in a series of volunteer studies over the first 550 ms of the cardiac cycle. T2 and T1rho preparations exhibit improvements of 100% and above in image contrast. Magnetization transfer preparation exhibits improvements of 50% in image contrast, whereas an initial improvement (50%) followed by a large loss in contrast is observed using the black blood preparation. Improvements in contrast are dependent on tissue relaxation parameters and therefore are suitable for studies involving patients exhibiting poor in-flow enhancement of blood caused by poor heart function.

Computer Simulation↗
PubMed1998

Split-and-merge segmentation of magnetic resonance medical images: performance evaluation and extension to three dimensions.

Intensity- or edge-based methods of segmentation are often insufficiently robust to be applied to images containing complex anatomical objects, such as those seen in high-resolution magnetic resonance imaging systems. Split-and-merge techniques attempt to overcome these difficulties by using homogeneity measures. Simple modifications to the basic 2D split-and-merge method, based on the principles of simulated annealing and controlled boundary elimination, are developed and discussed. Simulated annealing reduced the number of regions by 22% with a further reduction of 21% achieved through boundary elimination. Smoother regional boundaries are also produced. These methods are extended to true 3D and quantitatively compared with their 2D counterparts. The main advantage of 3D methods is that they produce segmented volumes by directly preserving the connectivity between slices, whereas in 2D, segments have to be grouped together in a post-split-and-merge process. Finally, the properties of the 3D approach are demonstrated by the automatic quantitation of brain ventricle volume, producing estimates to within 7% of validated manual methods.

Algorithms↗
PubMed1998

Estimating spatial resolution of in vivo MR images using spatial modulation of magnetization.

Spatial Modulation of Magnetization is shown to provide a means of estimating perceived spatial resolution directly in vivo. On the first magnetic resonance system tested, resolution in conventional spin echo images was found to be stability limited in the phase encoding direction and voxel limited (via the Nyquist sampling theorem) in the frequency encoding direction both in vitro and in vivo. As the voxel size approaches half the stripe separation, fringes of resolved and unresolved stripes are formed across the image. This phenomenon is explained and described mathematically. On a second magnetic resonance scanner, resolution in the phase encoding direction of fast spin echo images with centrically ordered phase encoding is shown to be voxel limited in substances with long T2, with poorer resolution in substances with short T2. Resolution in fast spin echo images with linearly ordered phase encoding was shown to be voxel limited in the phase encoding direction.

Artifacts↗
PubMed1998

Low-field magnetic resonance imaging for implant dentistry.

OBJECTIVE: To evaluate the use of a low-field magnetic resonance scanner for assessment of available bone for placement of osseo-integrated dental implants. METHODS: Eleven Patients were examined to assess suitability for implant placement using a 0.2 tesla 'open' scanner. Imaging/surgical templates were constructed, with gadolinium markers to allow accurate location of the implant sites. RESULTS: In all cases, localisation of potential implant sites was easily made and full information in all three planes readily available. Artefacts were few and localised, (noted on one site in one case only). Vital structures (nerves and vessels), and the variable geometry of the floor of the maxillary sinus were clearly seen, and cortical bone delineated from cancellous. The appearance of soft tissues in the scan allowed the surgeon to assess the final profile of the patient. CONCLUSION: Low-field magnetic resonance imaging has definite potential for pre-implant assessment. Full sectional information is readily available at any desired plane with no need for reformatting. The information for accurate and safe implant placement is clear. The technique uses no ionising radiation. Further work is needed to evaluate spatial distortion caused by magnetic susceptibility effects at air/tissue interfaces, but our calculations indicate that at low field, using an appropriate protocol, the effect will not be substantial.

Alveolar Process↗