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P S Tofts

Publications and source records attributed to P S Tofts.

At least 19 recordsLinked to original sources
PubMed1992

In vivo T1 values from guinea pig brain depend on body temperature.

T1 and T2 values were calculated from guinea pig brain in vivo at 0.5 T. T1 values showed significant dependence on body temperature, but the effect varied significantly from animal to animal and from one tissue type to another. Mean dependencies were 8.2 ms/degrees C for corpus callosum, 14.7 ms/degrees C for gray matter, and 21.5 ms/degrees C for hemispheric white matter (1.7, 2.6, and 4.5%/degrees C, respectively), all with respect to core temperature. These findings suggest that body temperature monitoring and control may be needed when sensitive measurements of T1 are being made. There was evidence for regulation of brain temperature within the hyperthermic range of body temperature.

Animals↗
PubMed1992

Measurement of blood-brain barrier permeability using dynamic Gd-DTPA scanning--a comparison of methods.

Two recently published methods of blood-brain barrier permeability measurement using Gd-DTPA scanning are compared by authors representing each group. The physiological models are reconciled. Results from both groups agree. These show that the transfer constant (the permeability surface area product per unit volume of tissue) of the defective blood-brain barrier in multiple sclerosis is in the range 1-12.10(-4) s-1.

Blood-Brain Barrier↗
PubMed1992

Volume measurement of multiple sclerosis lesions with magnetic resonance images. A preliminary study.

The ability to visualise multiple sclerosis lesions in vivo with magnetic resonance imaging suggests an important role in monitoring the course of the disease. In order to help the long-term assessment of prospective treatments, a semi-automated technique for measuring lesion volume has been developed to provide a quantitative index of disease progression. Results are presented from a preliminary study with a single patient and compared to measurements taken from lesion outlines traced by a neuroradiologist, two neurologists and a technician. The semi-automated technique achieved a precision of 6% compared to a range of 12-33% for the manual tracing method. It also reduced the human interaction time from at least 60 min to 15 min.

Brain↗
PubMed1992

Semiautomated quality assurance for quantitative magnetic resonance imaging.

It is now well established that MRI can be used for quantitative (as opposed to simply qualitative) measurements, and good accuracy and precision have been obtained in phantom experiments. To make routine quantitative measurements as part of a clinical scanning protocol, however, quality assurance (QA) methods particularly suited to quantification must be developed. We describe a set of QA tests using clinical protocols on test phantoms, with which we have assessed quantitative performance of our Picker 0.5-T scanner (Picker International, Cleveland, OH) over 2 years. We also describe the automated data processing methods we have developed to deal with the large amounts of data generated by these tests.

Humans↗
PubMed1992

Accurate and precise measurement of blood-retinal barrier breakdown using dynamic Gd-DTPA MRI.

Dynamic T1-weighted magnetic resonance imaging (MRI) after the injection of Gd-DTPA is a promising method for investigating breakdown of the blood-retinal barrier (BRB). Previously, the authors demonstrated that in a T1-weighted image, the initial rate of change in the vitreous water MRI signal as gadolinium diethylenetriaminepentaacetic acid (Gd-DTPA) enters the vitreous space strongly correlated with the extent of BRB breakdown. Here, a practical approach to measuring a more relevant physiologic parameter is presented: the permeability surface area product (PS). The theory is a development of earlier work used in investigating the breakdown of the blood-brain barrier. The accuracy and precision of this approach was investigated in rabbits pretreated with sodium iodate (30 mg/kg intravenously). The MRI-derived PS normalized to the area of leaky retina (5.65 +/- 0.25 x 10(-4) cm/min, mean +/- standard error of the mean; n = 6) was compared to a similarly normalized PS calculated using a classical physiologic method (4.12 +/- 0.73 x 10(-4) cm/min; n = 6). Good agreement between the two methods was found (P = 0.09). This result demonstrates that the MRI-derived PS is an accurate and precise measure of BRB breakdown under these conditions. The mathematical model of Gd-DTPA distribution in vivo also is validated. Based on these results, several potential sources of error are discussed, including the effect of back-flow of Gd-DTPA from the vitreous space to the plasma, the underlying vascular patency, and MRI slice selection.

Animals↗
PubMed1991

Measurement of the blood-brain barrier permeability and leakage space using dynamic MR imaging. 1. Fundamental concepts.

Leakage of Gd-DTPA through a defective blood-brain barrier is measured quantitatively using dynamic MRI scanning, in which repeated scans are made after a bolus injection. Image registration artifacts are minimized; a dose of 0.1 mM/kg and an IR sequence enable enhancement to be measured quantitatively. The triexponential enhancement curve is fitted to a theoretical model based on compartmental analysis. The transfer constant, or permeability surface area product per unit volume of tissue (k), and leakage space per unit volume of tissue (v1) are measured. Estimates for a quickly enhancing multiple sclerosis lesion are k = 0.050 min-1, v1 = 21%; for a slow one k = 0.013 min-1, v1 = 49%. This implies permeability in the range 4-17 x 10(-6) cm s-1, in broad agreement with other physiological methods. The method is noninvasive and can be used to make serial measurements in patients and in experimental animal models. The time course of pathological aspects of diseases with blood-brain barrier breakdown, such as multiple sclerosis, tumors, and infections (e.g., HIV) can be studied, along with their response to therapy. The measurements are of physiological variables and are therefore independent of imaging equipment and field.

Adult↗
PubMed1991

The measurement of electric field, and the influence of surface charge, in magnetic stimulation.

A circular magnetic stimulator coil placed perpendicularly to the surface of a large uniform conductor induces surface charge. The resulting electrostatic field reduces the total electric field within the conductor to 58% of the value in the absence of surface charge. The properties of 3 kinds of probe for measuring the effect of a magnetic stimulator are considered. A short dipole electric field probe is the only one which correctly measures the total electric field, including the contribution from any surface charge. A search coil generally gives incorrect results, since it is insensitive to the electrostatic field.

Electric Conductivity↗
PubMed1991

Sources of T1 variance in normal human white matter.

The major factors contributing to T1 variance at 0.5 T in white matter were studied in healthy people. Anatomical location of the white matter sampled and differences between individuals contributed 74% of the total variance in serial measurements of the same subjects. There was also significant change over time within an individual subject that could not be attributed to machine drift. This information permitted estimates to be made concerning adequate sample size in future studies that examine for pathological white matter T1 change.

Adolescent↗
PubMed1990

Towards quantitative measurements of relaxation times and other parameters in the brain.

The nature and physical significance of the relaxation times T1 and T2 and of proton density are described. Methods of measuring T1 and T2 are discussed with emphasis on the establishment of precision and the maintenance of accuracy. Reported standards of success are briefly reviewed. We expect sensitivities of the order of 1% to be achievable in serial studies. Although early hopes of disease diagnosis by tissue characterisation were not realised, strict scientific method and careful calibration have made it practicable to apply relaxation time measurement to research into disease process. Serial measurements in patients and correlation with similar studies in animal models, biopsy results and autopsy material taken together have provided new knowledge about cerebral oedema, water compartmentation, alcoholism and the natural history of multiple sclerosis. There are prospects of using measurement to monitor treatment in other diseases with diffuse brain abnormalities invisible on the usual images. Secondarily derived parameters and notably the quantification of blood-brain barrier defect after injection of Gadolinium-DTPA also offer prospects of valuable data.

Brain↗
PubMed1990

The distribution of induced currents in magnetic stimulation of the nervous system.

Magnetic stimulation of the nervous system is being used as an alternative to electrical stimulation, principally because it is painless. The spatial distribution of induced currents from the stimulating coil is calculated from a computer model with graphical output. Two configurations of a plane circular coil are considered: parallel to the tissue surface and perpendicular to the surface. The surface is assumed planar and infinite in extent. The tissue is modelled as a uniform, isotropic volume conductor. A quasi-static approximation is made in calculating the electric field. Maps of current density, J, as a function of position, including depth, are shown. In both configurations, J is always parallel to the surface, and is maximum at the surface. There is no perpendicular (vertical) current. For a one-turn 10 cm diameter coil, spaced 1 cm from conducting tissue and parallel to it, with rate of change of current 10(8) A s(-1), Jmax = 6.8 A m(-2) (assuming conductivity 0.2 omega -1 m(-1)). In the perpendicular configuration Jmax = 4.1 A m(-2). These results suggest that nerve fibres running parallel to the skin surface are more likely to be stimulated than those running obliquely; and that it is extremely difficult to stimulate nerve fibres running perpendicularly. This model can be used to characterise the performance of other shapes of stimulating coils and the dependence on fibre orientation.

Computer Simulation↗
PubMed1990

Duration and selectivity of blood-brain barrier breakdown in chronic relapsing experimental allergic encephalomyelitis studied by gadolinium-DTPA and protein markers.

Gadolinium-DTPA (Gd-DTPA) enhancement seen with magnetic resonance imaging in chronic relapsing experimental allergic encephalomyelitis (CREAE) corresponded with sites of blood-brain barrier breakdown judged by traditional markers in areas of inflammatory demyelination. Duration of Gd-DTPA leakage for individual lesions in CREAE varied from 5 days to more than 5 wks. By contrast, in acute EAE leakage was of shorter duration (always less than 5 days). Selective enhancement was observed in CREAE lesions using Gd-protein markers. Gd-albumin enhancement was not always seen in areas of leakage of the smaller molecular weight compound Gd-DTPA. The addition of immunoglobulin to the gadolinium complex led to enhancement of lesions not seen with Gd-albumin alone. From the similarities between the histology and the patterns of Gd-enhancement in CREAE and multiple sclerosis, it is probable that Gd-enhancement reflects active inflammation (with or without demyelination) in the human disease.

Animals↗
PubMed1990

Heterogeneity of blood-brain barrier changes in multiple sclerosis: an MRI study with gadolinium-DTPA enhancement.

We performed 15 dynamic gadolinium-DTPA (Gd-DTPA)-enhanced MRI studies in 8 patients with relapsing and remitting multiple sclerosis; 7 were follow-up studies. We measured the time course of enhancement in 102 enhancing lesions for up to 384 minutes, with rest breaks. Immediate postcontrast MRIs demonstrated many different patterns of enhancement. We observed both uniformly enhancing and ring enhancing lesions. The enhancing regions were often less extensive than the corresponding high signal on T2-weighted images. Three lesions were seen with Gd-DTPA but not on unenhanced scans; 1 was seen on unenhanced scans 10 days later, suggesting that blood-brain barrier disturbance may precede other MRI signs of MS lesions. Three months later, some high-signal areas on T2-weighted scans had decreased in size to resemble the areas previously outlined by Gd-DTPA. This technique provides useful information about the pathogenesis and behavior of MS lesions.

Adolescent↗
PubMed1989

Precise relaxation time measurements of normal-appearing white matter in inflammatory central nervous system disease.

Precise relaxation time (RT) measurements have been made, with a standard deviation of 3% for T1 and T2 in white matter in normal volunteers. This sets an upper limit to the instrumental random errors (imprecision). Achieving this precision requires careful adjustment and use of the imager. The wide variation in RTs seen by other workers may be in part due to larger instrumental errors. We have measured RTs (both T1 and T2) in normal-appearing white matter in 16 normal controls and patients with multiple sclerosis (MS, 18), systemic lupus erythematosis (SLE, 16) and cerebral sarcoidosis (8). Both RTs were significantly higher in MS than in other patient groups and controls (P less than .05), possibly caused by microscopic lesions. T2 was elevated in SLE patients relative to controls and sarcoidosis patients (P less than .05), possibly because of microhemorrhages. Lesion RTs were abnormal but more variable and no significant differences between diseases were found.

Adolescent↗