Moses A. Greenfield

Current status of physical measurements of the skeleton.

An overview is presented of some of the major methods of measuring the skeleton in the past 30 years. These include single photon absorptiometry (SPA), dual photon absorptiometry (DPA), quantitative computed tomography (QCT), and recently dual energy radiographic absorptiometry (DRA), also called DEXA (dual energy x-ray absorptiometry). In addition to these methods, all attempting to measure bone mineral density, regional and total body calcium have been determined by in vivo neutron activation analysis (IVNAA). An attempt to determine bone quality as contrasted with bone quantity has been made using ultrasound, with measurements of speed of sound and of attenuation as useful parameters to characterize bone tissue. While the various methods for measuring bone density have been most useful, no one method includes all the features required to be entirely satisfactory: excellent precision and accuracy and the ability to measure volumetric density in gm/cm3. Least successful has been the ability to predict fracture risk, an essential goal in helping the patient.

Measurement of the Velocity of Ultrasound in Human Cortical Bone and Its Potential Clinical Importance: An In Vivo Preliminary Study

The velocity of ultrasound in human cortical bone has been measured in vivo by a pulse echo technique. The velocity is reduced in a small group of older females, compared with a small group of young males. It is suggested that this change results from a proportionately greater decrease in the elastic constant of the bone, compared with its density. This method measures a new combination of physical properties of bone, which may have important clinical significance in the assessment of the diagnosis and course of osteoporosis.

Effect of Coronary Blood Flow on Radioisotope Dilution Curves Measured by Precordial Scintillation Detection

Two series of animal experiments are reported. In the first of these, the disappearance rates of intravenously injected radioactive boluses were monitored over the heart and from a peripheral artery. The disappearance rate over the heart was shown to be slower than in the artery. In a second series of experiments in which injection was made into the left ventricle or into a coronary artery while sampling from the right side of the heart, it was shown that the coronary transit time was several times longer than the left ventricular transit time. The prolonged presence of the radioactivity in the coronary vascular bed accounts for the difference in the heart and arterial disappearance slopes. A ratio of these two slopes may provide an index of coronary blood flow.

The measurement of trabecular bone mineral density using coherent and Compton scattered photons in vitro.

A photon scattering method for measuring the trabecular bone mineral density (TBMD) in vitro is described. This method involves the measurement of the ratio of coherent to Compton 90 degrees scattered photons from Am-241 by using a narrow beam geometry with an intrinsic germanium detector. The feasibility of using smaller scattering angles for better counting efficiency and the associated problems in their application for in-vivo measurements were investigated. Calibration of the system with fresh trabecular bone samples showed a linear relationship between the coherent to Compton ratio R of the detected counts and the TBMD (r = 0.94). The effect of the overlying soft tissue on the R ratio was significant while the effect of self-attenuation by the trabecular bone itself and the cortical layer was negligible. It was found that the marrow fat content could alter the value of the R ratio. Our results show that for a 10% increase in the fat content in the interstices of the trabecular bone there is a 2.5% decrease in the R ratio. This technique together with soft tissue corrections will enable us to measure the TBMD of the calcaneum in vivo, assuming a small variation in the trabecular fat content. The estimated absorbed dose to the bone marrow is about 139 mrad.

The modulus of elasticity of human cortical bone: an in vivo measurement and its clinical implications.

The modulus of elasticity was derived by combining the velocity of ultrasound measurements and photon absorption (Norland-Cameron method) in human cortical bone (proximal radius) in vivo. The results compare favorably with published values of the elasticity modulus obtained in vitro. Values obtained for a heterogeneous group of patients with bone and joint complaints differed from those of normal volunteers.

A study of the homogeneity of the trabecular bone mineral density in the calcaneus

In the past our laboratory has reported a method of measuring trabecular bone mineral density (TBMD) in the calcaneus in vivo by using the coherent-to-Compton scattering ratio. In the present work the distribution of TBMD in the calcaneus has been studied, and the reproducibility of this technique in vivo has been determined. It is found that although the TBMD may vary within the calcaneus, a region exists over which the variation in density is not large. This region coincides with the midportion of the heel and is the site chosen for the measurement of TBMD by the coherent-to-Compton scattering ratio technique. The reproducibility of this technique in vivo has been determined to be 3.4%.

Characterization of tissue via coherent‐to‐Compton scattering ratio: Sensitivity considerations

It is known that the ratio (R) of the detected coherent and Compton scattered photons from bone can be used in order to determine its mineral density. This technique utilizes the dependence of the coherent scattering on the effective atomic number (Z) of the scattering medium. It is generally accepted that a small scatter angle is preferred in order to ensure adequate counting statistics by favoring the detection of more coherent photons. Moreover, it has been assumed that a change in the scatter angle does not affect the sensitivity of the measurement. Our theoretical calculations for 60-keV photons and for the range of Z that corresponds to trabecular bone, indicate that increasing the scatter angle results in a stronger power dependence of the measured ratio on Z. This implies that by increasing the scatter angle, smaller changes in the mineral density can be detected, thus improving the sensitivity of the measurement. This effect was investigated experimentally by using a collimated beam of 59.54-keV photons from Am-241 (44.4 GBq) and a collimated intrinsic germanium detector. Solutions of K2HPO4 with different concentrations were used in order to simulate trabecular bone. The scatter spectra were recorded for all solutions at six scatter angles between 37 degrees and 98 degrees and the value of R was computed for each spectrum. The sensitivity of the measurement, evaluated from these experiments increased, with the increase of the scatter angle.

The effect of the momentum transfer on the sensitivity of a photon scattering method for the characterization of tissues.

The ratio of coherent to Compton photon scattered by a tissue-like material depends on its effective atomic number. This ratio can, therefore, be used for the in vivo characterization of tissues. The intrinsic sensitivity of this measurement is defined as the change in the coherent-to-Compton ratio for a given change in the atomic number. The effect of the scatter angle on the sensitivity has already been described by us in a paper recently submitted to this journal. In this study, the dependence of the sensitivity on the energy of the incident photons is investigated in two ways. The first approach is quasitheoretical and is based on computations of the cross sections of the coherent and Compton scattering for various energies. The second approach is experimental and it involves the measurement of the scatter ratio from a series of K2HPO4 solutions for three primary photon energies: 60, 81, and 140 keV. The combined effect of both the photon energy and the scatter angle on the sensitivity can be described by a single parameter which is the momentum transfer. It is concluded that for the limited range of the atomic numbers which apply to trabecular bone (8 less than or equal to Z less than or equal to 11) the momentum transfer reflects completely the effect of the scatter angle and photon energy on the sensitivity.

Optimization of the radiotherapy treatment plan

A nonlinear minimization search method is proposed for optimizing the radiotherapy treatment plan. The basic implicit assumptions in clinical radiotherapeutic practice are reviewed in light of their implications for treatment-plan optimization. A tumor dose function is defined for unconstrained conditions pertaining to dose uniformity or minimized dose gradient over the tumor bed, and for constrained conditions relating to isodose curve levels or anatomic points of interest. The optimization strategy followed in the case of single and multiple beam variables is described and illustrated in a clinical case study.

A new method of calculating absolute thyroid activity in intravenous I-123 uptake tests

A technique for determining the absolute activity of I-123 in early thyroidal uptake tests is presented. The method is independent of geometry, tissue attenuation, an extrathyroidal neck activity (ENA). It utilizes the efficiency factors (ExT and E gamma T) derivable for the thyroid gland using coincidence counting techniques, and a quantity, alpha, characteristics of the vascularity of the individual patient. Although ENA is not determined in absolute terms (e.g., percent of injected dose), count rates due to ENA are determined. After subtracting the ENA from the total count rates thyroidal uptake can be determined as absolute activity by means of the efficiency factors. The validity and usefulness of the technique have been demonstrated in phantom and patient studies. Thyroid uptakes of less than 0.1% of injected dose can be measured accurately in our patient studies.