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Biomedical subjects

H Shackford

Publications and source records attributed to H Shackford.

6 recordsLinked to original sources
PubMed1996

A dynamic match-line wedge.

PURPOSE: To implement match-line wedges at the abutting edges of x-ray fields using dynamic collimation. METHODS AND MATERIALS: Experiments were made using a computer-controlled linear accelerator equipped with developmental software that allows for collimator jaw motion while the beam is on. The jaws defining the abutting field edges were programmed to move from 1.5 cm inside to 1.5 cm outside the prescribed field during irradiation. Films were taken in plastic phantoms to assess the resulting edge gradient and to evaluate the sensitivity of this technique to setup errors. RESULTS: The measured edge gradient for a single field was 30% per cm. Parallel-opposed lateral fields produced a gradient of 28% per cm along their midline. A simulated central nervous system irradiation with cranial and spinal fields kept dose variations in the field-match region to less than 10% with setup errors of 3 mm. CONCLUSION: The use of collimator motion during irradiation is an effective and simple means of reducing the dose variation in a field-match region due to setup errors and system tolerances. Treatment time is not increased and labor savings can be achieved when compared to feathering techniques commonly used.

Humans↗
PubMed1995

Dynamic universal wedge.

A computer-controlled equivalent of the universal wedge was designed by moving one collimator with step-wise constant velocity to produce the same primary-dose profile as a 20x20-cm2 conventional 58-deg wedge. It was used for smaller fields without changing the profile and combined with open beams to alter the wedge angle. Output factors in air and wedge factors in air and water were determined experimentally for the dynamic wedge and compared to predictions based on the assumption that the primary dose is proportional to the number of monitor units that the point of interest is in the open portion of the beam. This model was found to be accurate within about 2% and the deviations caused by head-scatter changes and collimator transmission after the moving collimator has passed. Measurements of the wedge factor in water indicated that the scatter factors for large wedged beams slightly exceeded those for open beams.

Air↗
PubMed1995

X-ray source and the output factor.

When the collimator setting of a linear accelerator is made sufficiently small, the output factor in air, R, is greatly reduced because the collimators obstruct the periphery of the x-ray source. This has been utilized to examine the size of the source by varying the width y of a narrow field and determining how R(y) varies. The sources diameters in the two principal directions were clearly influenced by the design of the accelerators. The x-ray sources of two accelerators with bending magnets were found to be noncircular while that of a linear accelerator without a magnet showed circular symmetry. The position of the source relative to the axis of collimator rotation was determined by measuring R for offset narrow fields. For one of the accelerators, the source was initially moving and off the central axis by about 2 mm for the first five monitor units. The results correlated well with sharpness in portal-film images. The technique can serve to evaluate the major source characteristics in acceptance testing and quality control.

Equipment Design↗
PubMed1994

Attenuation in high-energy x-ray beams.

Attenuation factors in water have been measured by a narrow-beam technique in various portions of x-ray beams with nominal energies of 6 and 25 MV, with and without a wedge in the beam. The results were analyzed in terms of an attenuation coefficient mu for small water thicknesses and a beam-hardening coefficient eta that describes the change in attenuation per unit depth. The variation of mu within the field was significant, about 0.5% per centimeter at 6 MV and 0.8% per centimeter at 25 MV for open beams. The heavy wedge used in these experiments caused significant (about 10%) beam hardening at 6 MV, softened the beam somewhat at 25 MV, and increased the variation of mu within the field to 3%-5%. These effects should be taken into account in dose calculations, and correction factors can be designed based on the variation of mu with off-axis radius for open beams and with off-axis position for wedged beams. The experimental technique, based on two measurements with the beam going through a water tank with either 26- or 50-cm path length, was simple and highly reproducible. The beam hardening with depth in water, i.e., the value of eta, was readily determined but found to be clinically insignificant.

Algorithms↗
PubMed1991

Detection of coronary artery disease by vasodilator thallium imaging of the heart with amyl nitrite inhalation: a pilot study.

Thallium imaging of the heart using dipyridamole-induced coronary arteriolar vasodilation has proven to be an effective means of detecting significant coronary stenosis. However, intravenous dipyridamole has not yet been made available for general use. We therefore examined the feasibility of substituting amyl nitrite inhalation as an arteriolar vasodilator prior to thallium imaging. Seventeen patients, all of whom had catheterization-proven coronary stenosis, inhaled amyl nitrite for 2-5 min. Thallium was injected after 45-60 s of inhalation. Completion of inhalation was followed immediately by planar imaging. Of 6 patients who inhaled amyl nitrite for at least 4 min, 5 had moderate or severe image defects on immediate scans which completely resolved on delayed scans. Only 3 of 11 who inhaled amyl nitrite for 2 min or less prior to scanning had similarly positive tests. Overall sensitivity for significant stenosis was 8 of 17 (47%). Inhalation was well tolerated with only one episode of angina and hypotension. We conclude that amyl nitrite inhalation for at least 4 min may offer an effective and readily available alternative to intravenous dipyridamole for vasodilator imaging of the heart.

Administration, Inhalation↗