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Biomedical subjects

M D Harpen

Publications and source records attributed to M D Harpen.

At least 37 records · Page 2Linked to original sources
PubMed1990

Magnetic resonance imaging of bone marrow in sickle cell disease: clinical, hematologic, and pathologic correlations.

A longitudinal, prospective, controlled evaluation of magnetic resonance images (MRI) of long bones in sickle cell patients was undertaken simultaneously with assessment of clinical status and hematologic parameters, including dense erythrocytes. MRI of bone marrow in sickle cell patients during steady states appeared patchy and were markedly different from those in matched controls (P approximately 0). Patients with severe patchiness were older than those with mild or moderate patchiness (P less than .03). Sixty-nine MRI were performed during 28 painful episodes occurring in 14 subjects with sickle cell disease (SCD). Increased signals on intermediate and T2-weighted images were detected in 35.7% of painful episodes. These abnormalities were distinct and not observed to occur spontaneously during the steady-state examinations (P approximately 0). Bone marrow infarcts were confirmed by biopsy in two instances and autopsy in one instance. Dense red cells decreased by 40.81% of baseline during pain crises (P = .00005), more remarkably in those who had pain in the lower extremities (P = .0145). Patients with change in MRI during pain crises had a greater percentage change in the dense cells than those without the change in MRI (69.7% v 31.3%, P = .0120).

Anemia, Sickle Cell↗
PubMed1989

Eddy current distributions in cylindrical samples: effect on equivalent sample resistance.

We present a general technique for the determination of eddy current distributions within an irregularly shaped conducting sample in the uniform field of an NMR RF coil. Also presented is a general expression for the sample-induced coil resistance. The technique is applied specifically to a conducting cylindrical solid. Unlike previous descriptions of cylindrical samples in solenoidal coils where the induction is parallel to the axis of the cylinder and eddy current streamlines are circular, we treat the case where the induction is perpendicular to the cylindrical axis and where consequently eddy current streamlines take on an irregular shape.

Electric Conductivity↗
PubMed1988

Influence of skin depth on NMR coil impedance.

A derivation of the change in electrical impedance of an NMR surface coil produced by inductive coupling with a conducting medium is presented. The effect of skin depth is treated explicitly. Theoretical predictions of coil sample resistance and frequency offset are compared with those observed experimentally in a variety of circular surface coils and sample conductivities.

Electric Conductivity↗
PubMed1988

Influence of skin depth on NMR coil impedance. Part II.

We present a theoretical description of the interaction between an NMR coil system and a lossy conducting sphere. The influence of the finite skin depth or RF penetration and the relatively large dielectric constant of biological samples on the sample-induced electrical impedance of the coil is treated explicitly. The results are shown to be in good agreement with experiment and reduce to the more traditional expression for the coil impedance of Hoult and Lauterbur (1979) in the low conductivity or small sample size limit.

Electric Conductivity↗
PubMed1987

Magnetic resonance imaging in neuronal ceroid lipofuscinosis.

Neuronal ceroid lipofuscinosis is a group of neurodegenerative disorders characterized by accumulation of lipofuscin and/or ceroid within the tissues of the body. These entities are manifest by visual, intellectual, and motor deterioration as well as recurrent seizures. Computed tomography has been shown to demonstrate changes of cerebral atrophy in more severely affected patients. Seven patients with neuronal ceroid lipofuscinosis were examined with both computed tomography and magnetic resonance imaging, and the results were correlated with the clinical severity of the disorder. Two less severely affected patients had normal results on computed tomography and magnetic resonance imaging studies. Varying degrees of cerebral atrophy were seen in the remaining five patients with both computed tomography and magnetic resonance imaging. Severity of atrophy correlated with the severity of disability in these patients. Abnormal white matter was seen in the two most severely affected patients only with magnetic resonance imaging. Although the findings in patients with neuronal ceroid lipofuscinosis were nonspecific, the increased sensitivity of magnetic resonance imaging for subtle white matter abnormalities over computed tomography may prove helpful in monitoring the progression of this rare disorder.

Adolescent↗
PubMed1987

Quantitative analysis of NMR spectroscopic imaging.

A quantitative analysis of recovery curves derived from a series of standard and spectroscopic images is presented. Inversion recovery sequences were modified to produce difference (W-F) images of a water and fat phantom. Adding and subtracting these data from those obtained from standard inversion recovery images resulted in separate recovery curves for the fat and water fractions of emulsions making up the phantom. The procedure yielded consistent reproducible values of longitudinal relaxation times for the constituents of the emulsions.

Emulsions↗
PubMed1986

Radiation dose reduction in pediatric CT.

The relationship between image noise and radiation dose was investigated in computed tomography (CT) images of a pediatric abdomen phantom. A protocol which provided a minimum absorbed dose consistent with acceptable image noise criteria was determined for a fourth generation CT scanner. It was found that pediatric abdominal CT scans could maintain diagnostic quality with at least a 50% reduction in dose from the manufacturers' suggested protocol.

Child↗
PubMed1986

Longitudinal relaxation time measurements with non-uniform tilt angles.

The effect of non-uniform tilt angles across the slice thickness on the determination of T1 in NMR imaging is investigated in saturation-recovery, spin-echo and inversion-recovery pulse sequences. It is shown that for each of these imaging protocols, non-uniform tilt angles imply a multi-exponential dependence of NMR signal on the pulse repetition interval TR.

Magnetic Resonance Spectroscopy↗
PubMed1986

Nuclear magnetic resonance of perfused tissue.

The effect of perfusion on the NMR signal observed in NMR imaging is studied in a phantom and in two isolated perfused canine lungs. It is observed that perfusion in tissue has little effect on longitudinal relaxation times. Transverse relaxation rates are observed to correlate linearly with rates of perfusion, in accordance with a model presented.

Animals↗
PubMed1985

Radionuclide determination of right- and left-ventricular mixing.

Previous models for the dynamics of the transit of the bolus of activity of labeled red cells through the chambers of the heart have assumed the complete and instantaneous mixing of tracer with the cardiac blood pool. This assumption can be conveniently tested with a radionuclide procedure. We present a model of the bolus transit through the right and left ventricle that allows the quantitation of the mixing of labeled cells with the residual blood of the cardiac chambers. This model was used to analyze the time-activity curves obtained from a variety of patients undergoing first-pass dynamic scans of the heart. It is shown that technetium-labeled red blood cells and technetium pertechnetate have indistinguishable mixing properties, and the blood in the left ventricle is more thoroughly mixed than that in the right ventricle.

Cardiac Output↗
PubMed1985

Indicator dilution approach to NMR signal-flow curves.

An indicator dilution model for the dependence of NMR signal on flow rate is presented. The model explains the enhancement in NMR signal observed at low flow rates and the reduction in NMR signal at high flow rates. It also produces an excellent fit to original NMR flow data, which are also presented.

Humans↗
PubMed1984

Radionuclide determination of cardiac outputs and indices.

A radionuclide procedure for the determination of cardiac output is presented. Cardiac outputs estimated by the proposed method are compared to those obtained by the temperature dilution method during right heart catheterization. A new index of cardiac performance is also presented: cardiac output as a percentage of total blood volume. This index is determined by a totally external count-based procedure.

Adult↗
PubMed1984

Quantitative differences between the thyroid uptake of 123I and 99mTc.

Unlike 123I, sodium pertechnetate Tc99m is known to be trapped by the thyroid gland without being further processed. Whether this property alone explains the total difference between the early thyroid uptake of these two isotopes was studied in a group of goitrous patients. The absolute activities of 99mTc were measured by a conjugate-view counting method and those of 123I by a coincidence counting method. The uptake was continuously registered to give uptake curves which were analyzed according to a three-compartment model of the thyroid. Both isotopes are postulated to bind to some carriers and then to be transported into the thyroid gland before they are further metabolized. Although the differences observed between the two uptakes were somewhat model dependent, the application of compartmental analysis provides a more detailed description of the differences between the two uptakes which occur even before the organification step.

Adolescent↗
PubMed1983

Sonographic determination of renal volumes in normal neonates.

Renal diseases that affect renal size without altering renal architecture require a quantitative means of detection. A prospective study was undertaken to establish normal values for renal volumes in healthy neonates using sonography. Volumes were determined by two methods; (1) the serial area-volume method using parallel transverse images; and (2) the prolate ellipsoid model of the kidney using orthogonal diameters taken from ultrasound images. Renal volumes for both the right and left kidneys in both sexes were found to be approximately 10 ml. There was no significant difference between the results obtained by either method, nor were there significant differences between the volumes of the right and left kidneys within either sex. No difference in renal volume was noted between sexes. The mean greatest renal length was also computed for right and left kidneys in both sexes. Knowledge of normal renal volumes may aid in the diagnosis of urinary system disorders in neonates.

Female↗