The use of 188Re to treat in-stent re-stenosis of coronary arteries.

A pilot study has been conducted in which coronary arteries subject to re-stenosis after angioplasty and stenting have been irradiated following further angioplasty. The method of irradiation has been to use radioactive 188Re in an angioplasty balloon. This paper considers all aspects of the procedure including elution of the rhenium from a tungsten/rhenium generator, its concentration, dispensing and safe delivery to the patient using specially designed equipment to reduce staff doses and radioactive spills. In the pilot study of 28 lesions in 26 patients only 1 was recorded as having angiographic re-stenosis in the treated region at 6 months although 4 other patients had edge re-stenosis. This represents less than 18% re-stenosis in a population that would have been expected to exhibit at least 50% re-stenosis at 6 months. A total of 72 patients have been treated either in the pilot study or a subsequent trial. In only one case has a minor spill of radioactivity occurred and in no case has the balloon burst. Radiation doses to staff are approximately 20 microSv per procedure and are therefore not of serious consequence. It is concluded that this procedure is safe, feasible and effective in reducing in-stent re-stenosis.

The dosimetry for a coronary artery stent coated with radioactive 188Re and 32P.

Radiation dose distributions have been calculated for 188Re and 32P activity on a coronary artery stent. The doses have been calculated both as a function of position along the stent and of depth into the artery wall. Comparisons of the dose from identical activities of 188Re and 32P on the stent show that the major differences arise from the different half-lives of the two activities. Coating the activity onto three surfaces of the stent rather than just the outside surface is found to reduce the dose by approximately 8 to 9%. Similarly, the effect of ignoring the attenuation in the stainless steel of the stent is to increase doses by 11 to 17%. Consideration is also given to the effect of the prolonged treatment times associated with a radioactive stent compared with the more common treatment over several minutes. It is shown that extended treatment may require between two and eight times the single dose to achieve the same effect depending on factors such as the radionuclide used, the dose required and the assumed cell survival curve. On the assumption that an instantaneous dose of 18 Gy at a depth of 1 mm into the artery would be required for successful prevention of neointimal hyperplasia, activities required for a stent coated with 188Re and 32P are tabulated.

The effect of contrast medium and balloon shape on dosimetry for arterial irradiation with 188Re.

The radiation dosimetry associated with the use of the beta particle emitter 188Re in an angioplasty balloon is investigated for the case when the balloon has an elliptical rather than circular cross section and when iodinated x-ray contrast medium is included inside the balloon. It is found that the elliptical cross section introduces significant dose corrections when the eccentricity of the ellipse is equal to or greater than 0.7. However, for cases where the artery is nearly circular in cross section, the corrections are likely to be small. As expected, the dose is reduced along the major axis of the ellipse and increased along the minor axis. The corrections are greatest at larger distances from the surface of the balloon. The effect on dose of contrast in the balloon is significant for 33% Omnipaque in saline. Since this is a typical concentration of contrast that is used for imaging the radiation-filled balloon, correction for the effects of contrast medium in the balloon should in general be applied. To enable corrections to be readily applied for other types and concentrations of contrast media, formulas have been derived that allow the dose correction to be calculated for a range of balloon diameters and at various distances from the surface of the balloon. To undertake this calculation, the elemental composition and density of the material in the balloon needs to be known.

Potential ocular hazard from a surgical light source.

A surgical light source has been examined to determine the potential for retinal damage to staff in the operating theatre. It has been shown that under certain circumstances the light source examined can give an irradiance at the cornea which is well in excess of accepted safety standards. Calculation using data on the retinal irradiance required to produce retinal damage indicates that for an accidental exposure at a distance of 500 mm there is a significant possibility of retinal damage. At closer distances the probability of retinal damage is even higher. It is possible that other surgical light sources produce a similar degree of hazard and hospitals should establish suitable safety measures where necessary.

Determination of electron density, mass density and calcium fraction by mass of soft and osseous tissues by dual energy CT.

Techniques of CT analysis of trabecular regions are concerned with bone mineral assessment, with considerable attention being paid to the effect of unknown fat content. Information concerning mass density and electron density might provide a more complete picture but is not normally obtained. A method for the calculation of mass and electron density, as well as the fraction of calcium by mass, is described and requires only a measurement of effective beam energy on the skin in addition to the CT numbers from a dual energy scan. The method uses the six major elements, H, C, N, O, P and Ca as compartments for the analysis and can also be applied to soft tissue by using only the first four. The calculated mass fraction of Ca is found to be sensitive to fat content and difference between surface and internal energies which can lead to serious underestimates below a fraction of about 0.04. Mass and electron density results are independent of fat content and only marginally affected by energy differences. Results were obtained with simple materials confirming mass density can be calculated to the order of 3% and electron density to considerably better than 1%.

A general method for optimizing tissue discrimination in magnetic resonance imaging.

The basic partial differential formulation for small differences has been applied to the equations for the inversion recovery and saturation recovery sequences with spin-echo rephase pulses and the spin-echo sequence itself. The result is a generalized expression which is proportional to tissue discrimination expressed as the signal-difference-to-noise ratio for a pair of tissues. The expression is a function of the fractional differences in the tissue relaxation times and proton densities, T1, T2, and n, respectively, and the sequence parameters TR and TI, or TE. From it can be derived the conditions which lead to optimum tissue discrimination for a given pulse sequence. It is also possible to predict the optimum pulse sequence to use for any pair of tissues of interest. The method requires a knowledge of the fractional differences in T1, T2, and n for both tissues.

The electron density of bone for inhomogeneity correction in radiotherapy planning using CT numbers.

A CT scanner has been used to measure the electron density of a range of bone types in vivo. It is shown that there is a linear relationship between the CT number of bone and its electron density, which is expected theoretically if different bone types are treated as a variable mixture of osseous material and marrow. A method of calibrating any CT scanner using a simply prepared solution is proposed, which should enable electron densities of bone to be estimated from CT numbers with an accuracy of 5%.

Protection of the eye during carotid angiography.

An eye shield was developed for use in neurological examinations in projections in which it does not interfere with the information sought. Use of this shield in A.P., oblique, 30 deg Towne's and 12 deg carotid Towne's projections reduced the corneal dose to approximately 5% of the unshielded value. For lateral projections, doses were reduced to approximately 5-15%. Use of the shield resulted in corneal doses of the order of 0.5 mrad/mAs and 1.0 mrad/mAs for standard and magnified projections respectively, with the exception of the lateral projections for which the corresponding values were approximately 0.3 mrad/mAs and 0.7 mrad/mAs. Corneal dose for a typical carotid angiogram was estimated to be reduced from 6.2 rad to 0.33 rad by the use of the shield.