Search PubMed⌕ Search

Biomedical subjects

G D Fullerton

Publications and source records attributed to G D Fullerton.

At least 55 records · Page 3Linked to original sources
PubMed1984

Characterization of proton NMR relaxation times in normal and pathological tissues by correlation with other tissue parameters.

To help understand which tissue parameters best account for the water proton NMR relaxation times, the longitudinal relaxation time (T1), the transverse relaxation time (T2), and the water content of 16 tissues from normal adult rats were measured at 10.7 MHz and 29 degrees C. Regression analyses between the above and other tissue parameters were performed. These other tissue parameters included: the amounts of various organic and inorganic components, protein synthetic rate, oxygen consumption rate, and morphological composition. In addition, the differences in T1, T2, and water content values between normal liver and malignant tumor (Morris #7777 a transplantable hepatoma) were studied to help understand how a disease state can be detected and characterized by NMR spectroscopy. The results of this study and information from the literature allow the following generalizations to be made about tissue T1 and T2 values: (1) Each normal tissue has rather consistent and characteristic T1 and T2 relaxation times which are always shorter than the T1 and T2 of bulk water; (2) tissues with higher water content tend to have longer T1 relaxation times; (3) tissue T2 values are not, however, as well correlated with water content as T1 values; (4) tissues with shorter T1 values have higher calculated hydration fractions, greater amounts of rough endoplasmic reticulum, and a greater rate of protein synthetic activity; (5) tissues with higher lipid content, associated with intracellular non-membrane bounded lipid droplets, tend to have longer T2 values; (6) tissues with greater overall surface area, whether in the form of cellular membranes or intracellular or extracellular fibrillar macromolecules, tend to have shorter T2 values; (7) the differences between T1 and T2 values between tumor and normal tissues correlated with differences in the volume fraction (amounts) of extracellular fluid volumes and in the amounts of membrane and fibrillar surface area in the cells. The above generalizations should be useful in predicting T1 and T2 changes associated with specific tissue pathologies.

Amino Acids↗
PubMed1984

Frequency dependence of magnetic resonance spin-lattice relaxation of protons in biological materials.

The frequency dependence of spin-lattice (T1) relaxation times of protons in tissues and other macromolecular solutions was investigated. This dependence can be principally assigned to reduced mobility of water molecules "hydrated" on the surface of macromolecular structures. The T1 relaxation time of "hydration water" was found to increase linearly with frequency in the range from 5 to 100 MHz (T1 = 1.83 f + 25.0). It is assumed that the remainder of water in the tissue has a relaxation rate that is independent of frequency, as is characteristic of bulk tap water. Variations occur in the fraction of water hydrated or bound from one organ to the next. As the observed relaxation rate is a weighted average of the two rates as described by the fast exchange model, the above empirical relationship can be used to take a tissue T1 relaxation time measured at one frequency and then calculate the tissue T1 relaxation time that would be expected at another frequency. Good agreements were obtained between such calculated tissue T1 values and the T1 values actually measured at the second frequency. Variations between the calculated and measured T1 values in some classes of tissues indicate that there are also less important secondary factors such as lipid content.

Achilles Tendon↗
PubMed1984

Post mortem changes in skeletal muscle proton spin-lattice relaxation times.

A previously published report indicated that there are early post mortem changes in the pulsed NMR proton spin-lattice relaxation time (T1) of skeletal muscle which must be taken into account in in vitro tissue analysis. We re-examined this subject. When T1 measurements were done by allowing the signal intensity to decay over two orders of magnitude from the original intensity, the T1 decay curves showed more than a single exponential relaxation time component as reported earlier. However, when T1 measurements were done by allowing signal intensity to decay to about one order of magnitude from the original intensity only a single exponential relaxation time component was found. We postulate that the latter method gives the average T1 value of the various macroscopic tissue components present and that this method gives a representative T1 value for the entire tissue. Using data obtained in this manner we could not find significant post mortem changes in the muscle T1 relaxation times during the first four hours but found change at a later time.

Animals↗
PubMed1982

An electron beam pseudoarc technique for irradiation of large areas of chest wall and other curved surfaces.

An electron beam technique has been developed for the treatment of large areas of the chest wall and other curved surfaces employing the stationary field electron beam mode ordinarily available on a linear accelerator. The technique simulates an arc through the use of multiple fixed fields. The electron collimator is not used. Field shaping is achieved through the combined effects produced by the photon jaw settings, the arc limits, and secondary lead shielding on the patient's skin. Thus, there is no limitation on the circumferential extent of the field size that can be used. Electron beam energies of 6 MeV, 9 MeV, 12 MeV, 15 MeV and 18 MeV have been studied. Our findings indicate that, for these energies, this technique produces isodose curves parallel to the cylindrical surface at all depths beyond maximum build-up. This paper discusses the physical characteristics of the single beams, the degree of dose homogeneity achieved with the multiple fields, and the dosimetry technique developed to implement the therapy.

Electrons↗
PubMed1982

Advantages and limitations of computed tomography scans for treatment planning of lung cancer.

Forty-five Chest computed tomography (CT) scans performed on patients with lung carcinoma (LC) were evaluated in an attempt to understand the pattern of intrathoracic tumor spread and the advantages and limitations this technique offers for treatment planning when compared to planning done by conventional X rays. The following findings can help treatment planning. (1) When regular X rays do not show tumor location (i.e., hemithorax opacification), CT scan will show it in 68% of patients. If regular X rays show a well localized mass, unsuspected tumor extensions were disclosed in 78% of these patients. Hence, CT scans should be done in all LC patients prior to treatment planning; (2) Mediastinal masses frequently spread anteriorly toward the sternum and posteriorly around the vertebral bodies toward the cord and costal pleura. This should be considered for radiotherapy boost techniques; (3) Lung masses spread in one third of cases toward the lateral costal pleura. Thus, the usual 1-2cm of safety margin around the LC are not sufficient in some cases; (4) Tumor size can appear much smaller in regular X rays than in CT scans. Hence, CT scans are necessary for accurate staging and evaluation of tumor response. Some CT scan limitations are: (1) Atelectasis blends with tumor in approximately half of the patients, thus obscuring tumor boundaries; (2) CT numbers and contrast enhancement did not help to differentiate between these two structures; and (3) Limited definition of CT scan prevents investigation of suspected microscopic spread around tumor masses.

Humans↗
PubMed1982

Evaluation of computed tomography numbers for treatment planning of lung cancer.

Computerized tomography numbers (CTN) were evaluated in 32 computerized tomography scans performed on patients with carcinoma of the lung, with the aim of evaluating CTN in normal (lung, blood, muscle, etc) and pathologic tissues (tumor, atelectasis, effusion, post-radiation fibrosis). Our main findings are: 1. Large individual CTN variations are encountered in both normal and pathologic tissues, above and below mean values. Hence, absolute numbers are meaningless. Measurements of any abnormal intrathoracic structure should be compared in relation to normal tissue CTN values in the same scan. 2. Tumor and complete atelectasis have CTN basically similar to soft tissue. Hence, these numbers are not useful for differential diagnosis. 3. Effusions usually have lower CTN and can be distinguished from previous situations. 4. Dosimetry based on uniform lung density assumptions (i.e., 300 mg/cm3) might produce substantial dose errors as lung CTN exhibit very large variations indicating densities well above and below this value. 5. Preliminary information indicates that partial atelectasis and incipient post-radiation fibrosis can have very low CTN. Hence, they can be differentiated from solid tumors in certain cases, and help in differential diagnosis of post radiation recurrence within the radiotherapy field versus fibrosis.

Humans↗
PubMed1982

NMR relaxation of protons in tissues and other macromolecular water solutions.

Nuclear magnetic resonance (NMR) longitudinal (T1) and transverse (T2) relaxation parameters have been evaluated for protein solutions, cellular suspensions and tissues using both data from our laboratory and the extensive literature. It is found that this data can be generalized and explained in terms of three water phases: free water, hydration water, and crystalline water. The proposed model which we refer to as the FPD model differs from similar models in that it assumes that free and hydration water are two phases with distinct relaxation times but that T1 = T2 in each phase. In addition there is a single correlation time for each rather than a distribution as assumed in most other models. Longitudinal decay is predicted to be single exponent in character resulting from a fast exchange between the free and hydration compartments. Transverse decay is predicted to be multiphasic with crystalline (T2 approximately 10 microseconds), hydration (T2 approximately 10 msec) and free (T2 approximately 100 msec) water normally visible. The observed or effective transverse relaxation times for both the hydration and free water phases are greatly affected by the crystalline phase and are much shorter than the inherent relaxation times.

Animals↗
PubMed1982

Prospective tissue-mimicking materials for use in NMR imaging phantoms.

Water-based proteinaceous gels, which--with appropriate additives--are stable with time and possess a high melting point, have been used as base materials in ultrasonically tissue-mimicking materials. In the present work, versions of these gels having various concentrations of glycerol and graphite particles were studied regarding their NMR T1 and T2 dependencies at a proton Larmor frequency of 10.7 MHz. It has been found that T1 depends primarily on the concentration of glycerol and T2 depends primarily on the graphite particle concentration. Also, the ranges of T1 and T2 likely span those which exist for soft tissue parenchymae. Thus, these materials are good candidates for use as NMR tissue-mimicking materials. T1 and T2 also vary with gelatin concentration. The latter fact, together with the strong dependence of T2 on graphite concentration, mean that effective contrast-resolution phantoms and anthropomorphic phantoms with stable T1 and T2 distributions can be produced.

Gelatin↗
PubMed1982

Basic concepts for nuclear magnetic resonance imaging.

The basic concepts necessary to understand the physical basis of NMR imaging are presented in this didactic article. It is intended as a starting point for the radiologist or medical physicist who is addressing the topic of NMR for the first time. The basic of the NMR phenomena is described with introduction of the concepts of magnetic moment, magnetic fields, magnetic resonance, net magnetic moment of a sample, NMR excitation and NMR emission. The equipment necessary to observe these NMR properties of matter is summarized as well as the procedures for basic pulsed NMR experiments. The physical concepts for spatial localization of NMR emissions are introduced with physical analogies to stringed musical instruments. Several alternative imaging modalities are compared with greatest emphasis on the inversion recovery technique which yields images weighted by tissue T values. The six subsystems of an NMR imaging device (primary magnet, computer, radio equipment, magnetic gradient, data storage and display subsystems) are described in an overview fashion. The paper is followed by a series of study questions to test the reader's comprehension of basic NMR imaging concepts.

Humans↗
PubMed1982

Methods for routine calibration of brachytherapy sources.

Two types of routine geometry calibrators, the nuclear medicine dose calibrator and a Lucite jig holding a 30-ml external beam ion chamber with the source in a rigid geometry, were compared with open-air measurements for 137Cs, 192Ir, and 226Ra brachytherapy sources. The proximity of scattering surfaces in the second apparatus resulted in significant distortion of the buildup effect and deviation from the inverse square law (5% to 15%). For the dose calibrator, the response/Roentgen was found to be dependent not only on source energy, but on source capsule thickness as well. Approximately one half of the observed variation (27%) in calibration factors was accounted for by differences in filtration among sources. A mathematical logarithm that corrects for these geometric and filtration effects is presented. In addition, the activity assay procedure provided by the manufacturer of the dose calibrator is shown to be unsuitable for brachytherapy sources. Steps to overcome these problems are discussed.

Brachytherapy↗
PubMed1982

Urography with metrizamide in children.

Noionic contrast material could have particular value in pediatric urography because of the absence of osmotic diuresis and associated fluid loss. Excretory urograms were obtained in 23 children using metrizamide at 300 mg l/ml and administered at a dose of 2.2 ml/kg. The radiographs gave excellent urinary tract opacification without changes in serum osmolality. The calyces appeared sharp but undistended, there was increased occurrence of ureteral ridging, and the urinary bladders often remained undistended at the completion of the studied. The dense opacification attainable with isoosmolar metrizamide also gave the opportunity for performing urography with higher than normal kilovoltage and thus reducing radiation exposure to the child. Also in vitro studies demonstrated the significant radiation dose reduction possible with the higher kilovoltage technique.

Blood↗
PubMed1982

Correlation between initial salivary flow rate and radiation dose in the production of xerostomia.

Dose and initial flow rate are two of several factors that influence the outcome of xerostomia when irradiating head and neck tumors. These two parameters were analysed in 11 irradiated patients where most salivary glands were in the radiation field. Unstimulated whole saliva samples were collected before, during and after the irradiation. The results indicate a statistically significant linear correlation between initial saliva flow rate and the accumulated dose causing irreversible xerostomia. This phenomenon is explained. A diagram is made that could be used (in head and neck tumors where one side could be spared) as a indicator of the maximum dose the salivary glands in the contralateral non-involved side can receive before reaching irreversible dryness.

Head and Neck Neoplasms↗
PubMed1979

Anthropomorphic test objects for CT scanners.

Numerous types of artifacts can result in clinical misinterpretations in computed tomography. The authors have evaluated phantom materials and techniques of construction which would produce objects similar enough to the human body to test the accuracy of CT in vivo. The object used for this purpose is a thoracic section with five soft-tissue materials, representing adipose tissue, muscle, heart, esophagus, and lung, and three bone materials representing cortical bone, average rib, and "inner" (trabecular) bone. The accuracy of CT numbers for studies of different organs was assessed and sources of error in five commercial scanners were identified.

Adipose Tissue↗
PubMed1978

CT determination of parameters for inhomogeneity corrections in radiation therapy of the esophagus.

Accurate dose prediction for megavoltage photon therapy of carcinoma of the esophagus requires information on tumor depth, lung thickness, and lung density. The authors found that CT localization of internal and external contours is accurate within +/- 1 mm. Lung density can be measured with an error of less than 0.02 g/cm3 in the range 0.25-1.00 g/cm3. Variance between predicted and measured dosage was less than 3% in all patients and in most RANDO phantom measurements. Accurate radiation therapy planning is possible with CT information from a commercial scanner.

Computers↗
PubMed1978

Mailed thermoluminescent dosimeter determination of entrance skin exposure and half-value layer in mammography.

A method of providing in vivo measurement of entrance skin exposure for mammography using mailed thermoluminescent dosimeters has been developed. The dosimeters are calibrated to 137Cs (662 keV), and a correction is applied to account for energy response. A mailed dosimeter packet which can be used to measure the half-value layer at mammography energies is also described. The results for both systems agree well (to within 10%) with careful ion chamber determinations. These dosimeter measurements provide a simple. Inexpensive means to reliably determine exposure and to monitor machine parameters on a periodic basis.

Cesium Radioisotopes↗
PubMed1977

Physical aspects of electron-beam arc therapy.

The effect of different parameters on dose distribution in electron-beam arc therapy was studied in order to develop a technique for routine clinical use. A special diaphragm was designed to facilitate telecentric rotation. Dosimetry was performed with an ion chamber, film, and LiF powder in cylindrical polystyrene phantoms and an Alderson Rando phantom. Dose distributions were evaluated with regard to dose homogeneity, and a method of sharpening the dose fall-off near the ends of the arc was proposed. Criteria for selection of isocenter depth and field size were developed. Methods of dose calculation, calibration, and treatment planning are discussed.

Electrons↗