Biomedical subjects

R S Hinks

Publications and source records attributed to R S Hinks.

At least 19 recordsLinked to original sources

PubMed2000

Comparison of thermal damage calculated using magnetic resonance thermometry, with magnetic resonance imaging post-treatment and histology, after interstitial microwave thermal therapy of rabbit brain.

Clinical application of high-temperature thermal therapy as a treatment for solid tumours requires an accurate and close to real-time method for assessing tissue damage. Imaging methods that detect structural changes during heating may underestimate the extent of thermal damage. This is due to the occurrence of delayed damage manifested at tissue locations exposed to temperatures lower than those required to cause immediate structural changes. An alternative approach is to measure temperature and then calculate the expected damage based on the temperature history at each tissue location. Magnetic resonance (MR) imaging methods now allow temperature maps of the target and surrounding tissues to be generated in almost real-time. The aim of this work was to evaluate whether thermal damage zones calculated on the basis of MR thermometry maps measured during heating correspond to actual tissue damage as measured after treatment by histological methods and MR imaging. Four male rabbits were treated with high-temperature thermal therapy delivered in the brain by a single microwave antenna operating at 915 MHz. MR scanning was performed before, during and after treatment in a 1.5 T whole-body scanner. Temperature maps were produced using the proton resonance frequency (PRF) shift method of MR thermometry. In addition, conventional T1-weighted and T2-weighted spin-echo images were acquired after treatment. Thermal damage zones corresponding to cell death, microvascular blood flow stasis and protein coagulation were calculated using an Arrhenius analysis of the MR temperature/time course data. The calculated zones were compared with the lesions seen on histopathological examination of the brains which were removed within 6-8 h of treatment. The results showed that calculated damage zones based on MR thermometry agreed well with areas of damage as assessed using histology after heating was completed. The data suggest that real-time calculations of final expected thermal damage based on an Arrhenius analysis of MR temperature data may provide a useful method of real-time monitoring of thermal therapy when combined with conventional T2-weighted images taken after treatment.

Animals↗

PubMed2000

Prostate cancer: MR imaging and thermometry during microwave thermal ablation-initial experience.

Percutaneous interstitial microwave thermoablation of locally recurrent prostate carcinoma was continually guided with magnetic resonance (MR) imaging. Phase images and data were obtained with a rapid gradient-echo technique and were used to derive tissue temperature change on the basis of proton-resonance shift. Thermally devitalized regions correlated well with the phase image findings. MR imaging-derived temperatures were linearly related to the fluoroptic tissue temperatures. MR imaging can be used to guide thermoablation.

Aged↗

PubMed1999

Heat-source orientation and geometry dependence in proton-resonance frequency shift magnetic resonance thermometry.

The proton-resonance frequency (PRF) shift method of thermometry has become a promising tool for magnetic resonance image-guided thermal therapies. Although the PRF thermal coefficient has recently been shown to be independent of tissue type when measured ex vivo, significant discrepancy remains on its value for tissues measured in vivo under a variety of experimental conditions. The authors identify a potential source of variation in the PRF thermal coefficient that arises from temperature-induced changes in the volume magnetic susceptibility of tissue and is dependent on the orientation and geometry of the heat-delivery device and its associated heat pattern. This study demonstrates that spatial variations in the apparent PRF thermal coefficient could lead to errors of up to +/-30% in the magnetic resonance estimated temperature change if this effect is ignored.

Algorithms↗

PubMed1998

Virtual frequency encoding: achieving symmetry in k-space.

Chemical shift artifacts and other off-resonance spatial shifts in 2DFT MRI arise from the linear time dependence in the k-space data in the readout direction. Introduction of a view-dependent time shift of the readout window adds a time dependence to the phase-encoding direction and results in a virtual frequency-encoding direction that is a linear combination of the phase-encode and readout axes. By this method, the readout and phase-encode directions can be made identical in their sensitivity to off-resonance effects and can be arbitrarily swapped with no change in chemical shift or inhomogeneity effects, improving previously reported methods that swap these axes for signal averaging or reduction of motion artifacts.

Artifacts↗

PubMed1998

Ex vivo tissue-type independence in proton-resonance frequency shift MR thermometry.

The temperature sensitivity of the proton-resonance frequency (PRF) has proven valuable for the monitoring of MR image-guided thermal coagulation therapy. However, there is significant inconsistency in reported values of the PRF-thermal coefficient, as measured from experiments encompassing a range of in vivo and ex vivo tissue types and experimental conditions. A method of calibrating the temperature dependence of the PRF is described and results are presented that indicate a tissue-type independence. To this end, other possible mechanisms for variations in the PRF-thermal coefficient are suggested, including physiological perturbations and volume magnetic susceptibility effects from geometry and orientation.

Animals↗

PubMed1997

Magnetic resonance imaging of temperature changes during interstitial microwave heating: a phantom study.

Changes in magnetic resonance (MR) signals during interstitial microwave heating are reported, and correlated with simultaneously acquired temperature readings from three fiber-optic probes implanted in a polyacrylamide gel phantom. The heating by a MR-compatible microwave antenna did not interfere with simultaneous MR image data acquisition. MR phase-difference images were obtained using a fast two-dimensional-gradient echo sequence. From these images the temperature-sensitive resonant frequency of the 1H nuclei was found to decrease approximately by 0.008 ppm/ degree C. The method and results presented here demonstrate that noninvasive MR-temperature imaging can be performed simultaneously with interstitial microwave thermal treatment.

Biophysical Phenomena↗

PubMed1995

Nontraumatic pediatric musculoskeletal MR imaging: comparison of conventional and fast-spin-echo short inversion time inversion-recovery technique.

PURPOSE: To compare conventional short inversion time inversion-recovery (STIR) with fast spin-echo (FSE) STIR techniques to evaluate suspected nontraumatic musculoskeletal abnormalities. MATERIALS AND METHODS: Thirty STIR and FSE-STIR examinations in 26 pediatric patients with suspected nontraumatic musculoskeletal abnormalities were prospectively evaluated. Qualitative (subjective) and quantitative (five-point rank score) analyses of the images were performed. RESULTS: FSE-STIR was faster than STIR (mean, 2 minutes 25 seconds and 6 minutes 35 seconds, respectively). Fat suppression was slightly better with STIR. Image degradation due to motion was judged similar. Lesion contrast to muscle was slightly better with STIR than FSE-STIR, and lesion contrast to fat was equivalent. Qualitatively, lesion conspicuity was similar: All lesions were seen with both techniques. CONCLUSION: FSE-STIR can replace STIR when an inversion-recovery fat-suppression sequence is desired. Considerable imaging time is saved.

Adolescent↗

PubMed1995

MR evaluation of vertebral metastases: T1-weighted, short-inversion-time inversion recovery, fast spin-echo, and inversion-recovery fast spin-echo sequences.

PURPOSE: To compare the detectability of vertebral metastatic disease on T1-weighted, short-inversion-time inversion recovery (STIR), fast spin-echo (FSE), fat-saturated FSE, and inversion recovery FSE (IRFSE) MR sequences using percent contrast and contrast-to-noise ratios. METHODS: Patients with proved metastatic disease underwent imaging on a 1.5-T MR system with sagittal T1-weighted (800/20/2 [repetition time/echo time/excitations]) (91 patients), STIR (1400/43/2; inversion time, 140) (91 patients), FSE (4000/180/2) (46 patients), fat-saturated FSE (4000/180/2) (16 patients), and IRFSE (29 patients) sequences. Percent contrast and contrast-to-noise ratio were calculated for the lesions. The number of metastatic lesions detected with each of the pulse sequences was also calculated. RESULTS: Mean percent contrast was, for T1-weighted sequence, -42.2 +/- 1%; STIR, 262 +/- 34%; FSE, 121 +/- 21%; fat-saturated FSE, 182 +/- 6%; and IRFSE, 272 +/- 47%. The mean contrast-to-noise ratio for T1-weighted was -4.63 +/- 1.7; STIR, 10.8 +/- .98; FSE, 4.16 +/- .76; fat-saturated FSE, 4.87 +/- .19; and IRFSE, 5.2 +/- .87. STIR and IRFSE showed the highest number of lesions, followed by T1-weighted, fat-saturated FSE, and FSE sequences. T1-weighted sequences showed 94%, FSE 55%, and fat-saturated FSE 78% of the lesions detected. Epidural metastatic lesions were better depicted on T1-weighted, FSE, and fat-saturated FSE sequences. CONCLUSION: STIR was superior to both T1-weighted and FSE (with and without fat saturation) for detection of metastatic lesions, in terms of both percent contrast and contrast-to-noise ratio and visibility. IRFSE was equal to STIR for the detection of metastasis by both subjective and objective criteria. T1-weighted, FSE, and fat-saturated FSE sequences were superior to STIR and IRFSE in the detection of epidural metastatic disease. IRFSE provided faster scanning time, which could be translated into greater resolution.

Adolescent↗

PubMed1994

Gradient moment nulling in fast spin echo.

The fast spin echo sequence combines data from many echo signals in a Carr-Purcell-Meiboom-Gill echo train to form a single image. Much of the signal in the second and later echoes results from the coherent addition of stimulated echo signal components back to the spin echo signal. Because stimulated echoes experience no dephasing effects during the time that they are stored as Mz magnetization, they experience a different gradient first moment than does the spin echo. This leads to flow-related phase differences between different echo components and results in flow voids and ghosting, even when the first moment is nulled for the spin echo signal. A method of gradient moment nulling that correctly compensates both spin echo and stimulated echo components has been developed. The simplest solution involves nulling the first gradient moment at least at the RF pulses and preferably at both the RF pulses and the echoes. Phantom and volunteer studies demonstrate good suppression of flow-related artifacts.

Artifacts↗

PubMed1994

Spin-echo and gradient-echo EPI of human brain activation using BOLD contrast: a comparative study at 1.5 T.

In this study, Blood Oxygenation Level Dependent (BOLD) contrast in the detection of human brain activation was compared between spin-echo and gradient-echo echo-planar sequences at 1.5 T. Time course series of spin-echo and gradient-echo images containing the primary motor cortex were collected during rest (no finger movement) and activation (finger movement). Each time course series was collected using a different TE. Resting and active state signal intensities at each TE were measured in identical regions in the motor cortex. From these data, resting and active state R2 (1/T2) and R2* (1/T2*) values were obtained. Across four subjects, brain activation produced an average R2 change of -0.16 +/- 0.02/s (+/- SE), and an average R2* change of -0.55 +/- 0.08/s. The average delta R2*/delta R2 ratio was 3.52 +/- 0.56. The average gradient-echo/spin-echo ratio of activation-induced signal changes at the TE for maximal BOLD contrast for each sequence (TE approximately T2* and T2) was calculated to be 1.87 +/- 0.40.

Adult↗

PubMed1993

Stabilization of echo amplitudes in FSE sequences.

The classical CPMG sequence and its extension as an imaging sequence, fast spin echo (FSE, based on RARE), suffer from signal magnitude variations in the early echoes when the refocusing pulses are not set exactly to 180 degrees. It has been suggested that by varying the value of the nutation angle of each refocusing pulse the signal magnitude could be made constant. This article describes an algorithm permitting the generation of sequences of nutation angles yielding series of echoes with constant signal magnitudes. This result is then used to design selective pulses for the FSE imaging technique.

Algorithms↗

PubMed1992

Time course EPI of human brain function during task activation.

Using gradient-echo echo-planar MRI, a local signal increase of 4.3 +/- 0.3% is observed in the human brain during task activation, suggesting a local decrease in blood deoxyhemoglobin concentration and an increase in blood oxygenation. Images highlighting areas of signal enhancement temporally correlated to the task are created.

Brain↗

PubMed1990

Cine MR in the evaluation of normal and abnormal CSF flow: intracranial and intraspinal studies.

Evaluation of intracranial and intraspinal CSF flow was accomplished by the use of cardiac gated gradient echo magnetic resonance (MR) technique. Normal patterns of pulsatile flow within the ventricles, cisterns and cervical subarachnoid space were established by this technique and these observations were compared to prior description of CSF flow. With systole there is downward (caudal) flow of CSF in the aqueduct of Sylvius, the foramen of Magendie, the basal cisterns and the dorsal and ventral subarachnoid spaces while during diastole, upward (cranial) flow of CSF in these same structures is seen. The relationships between the cardiac cycle and the CSF pulsations are demonstrated on both magnitude reconstruction and phase reconstruction MR images. Calculations of actual fluid velocity within CSF containing spaces can be obtained from the phase reconstruction images and holds promise for a more accurate analysis of CSF flow. In conditions which result in alterations of flow, cine MR dramatically shows either obstruction or excessively turbulent flow within the CSF pathways. The site of obstructed flow whether in the third ventricle, aqueduct, fourth ventricle, or subarachnoid space can be appreciated by changes in or absence of the normal hypointense signal. Cystic cord lesions such as congenital syringohydromyelia and posttraumatic spinal cord cysts may show pulsatile flow of CSF, a fact which can relate to progressive enlargement of these cysts. The distinction between myelomalacia and cyst formation in the cord is facilitated by the technique. Although the use of cine MR for the analysis of CSF flow is in its infancy, our experience indicates that this technique is useful in a wide range of pathological conditions including, but not limited to, conditions resulting in hydrocephalus or cystic cord lesions.

Brain↗

PubMed1989

MR technique for localization and verification procedures in episcleral brachytherapy.

Spatial definition of an intraocular tumor and subsequent determination of the actual position of an implanted eye plaque are essential for adequate ocular brachytherapy treatment planning. However, a method for verification of the plaque placement which would provide required 3-dimensional information is not available at present. In addition, tumor localization procedures, including ultrasonography and CT techniques, cannot always offer the precision needed for 3-dimensional definition of an intraocular target. This communication describes a magnetic resonance imaging technique specifically developed for both localization and verification procedures. A 1.5 Tesla magnetic resonance scanner, spin-echo pulse sequence (echo time 30 msec, repetition time 700 msec), and commercially available surface coil were used to obtain a series of transverse, coronal, and sagittal images of a slice thickness of 3 mm. Usually, eight scans in each of the three planes were needed for adequate coverage of the orbit. The required patient set-up and data acquisition time did not exceed 40 minutes. With a data matrix size of 256 X 256 pixels and 13 cm field of view, localization and verification were accomplished with a precision of 0.5 mm. Our results suggest that the magnetic resonance imaging technique permits precise integration of diagnostic and therapeutic procedures, and in addition provides adequate data for accurate treatment planning. We conclude that magnetic resonance imaging is the preferred diagnostic technique for episcleral brachytherapy.

Brachytherapy↗

PubMed1989

MR imaging of intracranial carotid occlusion.

The MR scans of seven patients with intracranial carotid occlusion (five proved, two presumed) were reviewed to evaluate the MR signal characteristics seen in this disorder. Five patients had clinical signs of cerebral infarction. Of the remaining two patients, one was asymptomatic and the other had a long-standing occlusion and headaches. We correlated the MR findings with cerebral angiography in five patients and with CT scans in six patients. All occluded vessels demonstrated MR signal predominantly isointense to brain on proton-density- T1- and T2-weighted images. Since there is an absence of flow, the MR signal is based on the intrinsic properties of the arterial thrombus and possibly on the chronicity of the occlusion. The pathogenesis and histopathology of intravascular thrombus differ significantly from extravascular hematoma, which accounts for the differences in their MR signal characteristics. The demonstration of occluded intracranial vessels may solidify the diagnosis of stroke in cases in which clinical and/or CT findings are equivocal. In patients presenting with infarction, an occluded carotid artery by MR may obviate the need for angiography; however, the demonstration of a patent carotid in conjunction with infarction suggests the possibility of an embolus, which may require angiography. We believe that MR is a valuable adjunct to CT in evaluating patients with cerebrovascular infarction.

Adult↗

PubMed1988

Magnetic resonance imaging for monitoring relapse of acute myeloid leukemia.

Magnetic resonance provides a non-invasive tool for monitoring normal and leukemic bone marrow. Measurements of the T1 relaxation times are elevated in acute myelogenous leukemia. However, interpatient variability diminishes the usefulness of MR measurements for diagnosing leukemia. In following the time course of individual patients, T1 relaxation time measurements appear to give an early and sensitive indication of leukemic relapse after remission and this may serve a clinical role in the management of leukemia by reducing the regularity of necessary marrow aspirates and biopsies.

Bone Marrow↗

PubMed1988

Improved MR imaging of the brain by using compensating gradients to suppress motion-induced artifacts.

Sixty patients were examined with and without extra gradient pulses, which compensate for motion-induced phase errors, in order to determine the effect those gradients had in suppressing the motion artifacts frequently present in the brainstem, temporal lobes, and basal ganglionic regions on routine T2-weighted brain MR imaging. Two comparative studies were performed: (1) in 50 patients the motion-artifact suppression technique (MAST) was compared with a single-echo MR examination, and (2) in 10 patients the MAST technique was compared with the second echo of a symmetric dual-echo sequence. In the first study 39 patients were examined at 1.5 T and 11 patients were examined at 0.5 T with the same pulse sequences. We found that MAST resulted in a significant improvement of image quality in 24 of 39 patients on the high-field-strength system and in two of 11 patients on the mid-field-strength system. In the second study, we found that in four of the 10 patients, MAST resulted in a suppression of artifacts greater than that achieved by even-echo rephasing alone. With MAST, artifacts were eliminated that not only obscured normal structures but that could have left doubt about the presence of a true signal abnormality. There was, however, marked suppression of the CSF flow-void phenomenon and increased signal from flowing blood, particularly in the cortical veins and dural sinuses. Because of this, the use of additional pulse sequences in which these motion-compensating gradients were not used was necessary under certain clinical circumstances. We conclude that, with these motion-compensating gradients, artifacts are reduced or eliminated, and a marked improvement in image quality can be obtained without the need for cardiac gating.

Adolescent↗

PubMed1988

Motion artifacts in brain and spine MR.

As a result of the effects of cerebrospinal fluid and blood motion, motion artifacts are common in brain and spine MR imaging. These artifacts, which are most common on T2-weighted spin-echo sequences, show up as streaking and ghosting in the phase-encoding (PE) direction (PE artifacts) and as loss of signal from flowing material (flow void). A review of the physical and physiologic causes of these artifacts and a review and explanation of several techniques that are useful in reducing or eliminating them are presented in this article.

Blood Circulation↗