Comparison of scatter doses from a multislice and a single slice CT scanner.

During shielding calculations for a new multislice CT (MSCT) scanner it was found that the manufacturer's data indicated significantly higher external scatter doses than would be generated for a single slice CT (SSCT). Even allowing for increased beam width, the manufacturer's data indicated that the scatter dose per scan was higher by a factor of about 3 to 4. The magnitude of the discrepancy was contrary to expectations and also contrary to a statement by the UK ImPACT group, which indicated that when beam width is taken into account, the scatter doses should be similar. The matter was investigated by comparing scatter doses from an SSCT and an MSCT. Scatter measurements were performed at three points using a standard perspex CTDI phantom, and CT dose indices were also measured to compare scanner output. MSCT measurements were performed with a 40 mm wide beam, SSCT measurements with a 10 mm wide beam. A film badge survey was also performed after the installation of the MSCT scanner to assess the adequacy of lead shielding in the room. It was found that the scatter doses from the MSCT were lower than indicated by the manufacturer's data. MSCT scatter doses were approximately 4 times higher than those from the SSCT, consistent with expectations due to beam width differences. The CT dose indices were similar, and the film badge survey indicated that the existing shielding, which had been adequate for the SSCT, was also adequate for the MSCT.

Recommendations for a technical quality control program for diagnostic X-ray equipment.

This position paper was produced by a working party set up by the Radiology Special Interest Group of the ACPSEM in 2001. It is designed to give the consensus view of College members in Australia and New Zealand on the nature and frequency of tests which should be performed on diagnostic x-ray equipment to maintain adequate quality control of imaging performance and radiation safety. Tests on mammographic equipment have been excluded having been covered in a previous ACPSEM position paper (Australas Phys Eng Sci Med, 24(3):107-131, 2001). Detailed descriptions of test procedures are not given but it is intended that a series of workbooks should be produced giving College recommended test methods for each imaging modality. The recommendations are produced here in an easy-to-read, tabular form giving the nature and purpose of each test and the implications of non-compliance with regard to image quality and radiation safety.

A critical analysis of the pharmacology of AZT and its use in AIDS.

The triphosphorylated form of the nucleoside analogue 3'-azido-3'-deoxythymidine (Zidovudine, AZT) is claimed to interrupt the HIV replication cycle by a selective inhibition of viral reverse transcriptase, thereby preventing the formation of new proviral DNA in permissive, uninfected cells. Given that initial HIV infection of an individual instigates abundant HIV replication from inception until death, and that the life of infected T-cells is only several days, the administration of AZT should lead both in vitro and in vivo (i) to decreased formation of proviral DNA; and thus (ii) to decreased frequencies of 'HIV isolation' (detection of p24 or reverse transcription or both) in stimulated cultures/cocultures of T-cells from seropositive individuals; (iii) to decreased synthesis of HIV p24 and RNA ('antigenaemia', 'plasma viraemia', 'viral load') ultimately resulting in low or absent levels of all three parameters; and (iv) to a perfect and direct correlation between all these parameters. A critical analysis of the presently available data shows that no such evidence exists, an outcome not unexpected given the pharmacological data on AZT. HIV experts all agree that only the triphosphorylated form of AZT (AZTTP) and not the unphosphorylated form administered to patients, nor its mono- or diphosphate, is the active agent. Furthermore, the mechanism of action is the ability of AZTTP to halt the formation of HIV-DNA (chain termination). However, although this claim was posited from the outset, AZT underwent clinical trials and was introduced as a specific anti-HIV drug many years before there were any data proving that the cells of patients are able to triphosphorylate the parent compound to a level considered sufficient for its putative pharmacological action. Notwithstanding, from the evidence published since 1991 it has become apparent that no such phosphorylation takes place and thus AZT cannot possess an anti-HIV effect. However, the scientific literature does elucidate: (i) a number of biochemical mechanisms which predicate the likelihood of widespread, serious toxicity from use of this drug; (ii) in vitro data proving that AZT has significant antibacterial and antiviral properties which confound interpretation of its effects when administered to patients. Based on all these data it is difficult if not impossible to explain why AZT was introduced and still remains the most widely recommended and used anti-HIV drug.

HIV antibodies: further questions and a plea for clarification.

The existence of specific antibody/protein reactions is the crucial assumption underlying proof of HIV isolation, proof of HIV infection and the causative role of HIV in AIDS. However, since 1. antibodies which react with the 'HIV' proteins arise following allogenic stimuli in non-HIV-infected animals and humans, as well as in mice and humans with autoimmune disorders; antibodies to antigens from both mycobacteria and yeasts cross-react with HIV env and gag proteins; 2. individuals belonging to the AIDS risk groups are subjected to allogenic stimuli and have high levels of autoimmune antibodies, while the vast majority of patients in the AIDS risk groups are infected with either or both mycobacteria or yeasts; the evidence for the existence of HIV and its putative role in AIDS must be reappraised.

Factor VIII, HIV and AIDS in haemophiliacs: an analysis of their relationship.

In this review, the association between the Acquired Immune Deficiency Syndrome (AIDS) and haemophilia has been carefully examined, especially the data that have been interpreted as indicating transmission of the human immunodeficiency virus (HIV) to the recipients of purportedly contaminated factor VIII preparations. In our view, the published data do not prove the hypothesis that such transmission occurs, and therefore HIV cannot account for AIDS in haemophiliacs.

A critical analysis of the HIV-T4-cell-AIDS hypothesis.

The data generally accepted as proving the HIV theory of AIDS, HIV cytopathy, destruction of T4 lymphocytes, and the relationship between T4 cells, HIV and the acquired immune deficiency clinical syndrome are critically evaluated. It is concluded these data do not prove that HIV preferentially destroys T4 cells or has any cytopathic effects, nor do they demonstrate that T4 cells are preferentially destroyed in AIDS patients, or that T4 cell destruction and HIV are either necessary or sufficient prerequisites for the development of the clinical syndrome.

Regional pulmonary distribution of krypton-81m gas delivered by different breathing systems.

Regional pulmonary distribution of 81mKr gas delivered by three breathing systems was determined. Data from 18 patients were analyzed. Posterior images were obtained using each breathing system in turn. Distribution of Kr gas was determined in terms of penetration and zonal indices. For penetration indices each lung was divided into a central, intermediate, and peripheral region and these indices, defined as the ratio of counts/cell in the intermediate or the peripheral region over those in the central region, were calculated. For the zonal indices each lung was divided equally into upper and lower zones and the percentage ratio of the counts in each zone to the total counts in both lungs was calculated. For all patients, in addition, the size, height, and width of each lung were determined from computer images. These parameters were compared between the breathing systems using a paired t-test. It was found that there were no statistical differences among the three breathing systems, either in the regional pulmonary distribution of the 81mKr gas or in the overall shapes of the lungs.

Regional specific mean expiratory gas flow from 81mKr equilibrium inhalation data.

A new method of analysing the data available from routine 81m Kr equilibrium inhalation investigations has been developed. The data for analysis are acquired from a gamma camera in the form of a sequential series of images from which multiple breath activity-time curves are generated for eight regions in the lung. The method is based on a description of the behaviour of the radioactive gas in the lung using a mathematical model. Values of specific mean expiratory gas flow, that is mean expiratory gas flow per unit lung volume, are calculated from the application of the model to the expiratory phase only of a single breath activity-time curve which is generated from the multiple breath activity-time curve using post-acquisition gating. This method overcomes the problem of non-uniform inspiratory concentration of tracer gas experienced in previously reported techniques of analysing inhalation data obtained using poorly soluble radioactive gases. The model is shown, in simulation studies, to be an adequate description of the behaviour of radioactive gas in the lung and the analysis technique is shown, in clinical studies, to be both reproducible and sensitive to disease state.

Measurements of the rubidium isotopic activities and the elution parameters of 81mKr generators using a standardized 114mIn source.

A method is described for measuring a number of parameters associated with an inorganic ion-exchange krypton generator. These are the activities of rubidium isotopes in inorganic ion-exchange krypton generators, the 81mKr extraction rate, the 81mKr activity delivered to patients during ventilation studies, the elution efficiency, and the radionuclide purity of the eluted gas. The method is based on the calibration of detectors, Ge(Li) and NaI(Tl), with a standardized 114mIn source at matching photon energies. The average activities present in our generators at the end of bombardment (EOB) were 14.6 +/- 3.8 mCi (81Rb), 6.2 +/- 1.6 mCi (82mRb) and 53 +/- 9.4 microCi (83Rb). The 81mKr extraction rate 2 h post-EOB was 10.2 +/- 2.3 mCi/min at an air flowrate of 1 l/min. The 81mKr activity delivered to patients during a ventilation study was 91 +/- 16 mCi. The elution efficiency of the generators averaged 50% +/- 7% at an air flowrate of 1 l/min. The eluted gas contained Rb radioisotopic impurities in trace quantities of approximately 0.06 microCi/l.

Quantitation and the elimination of errors in bile reflux tests using a gamma camera.

Entero-gastric reflux may be assessed quantitatively using 99mTc HIDA and a gamma camera. We have devised a computer program which applies corrections for several sources of error. The technique was validated using naso-gastric aspiration and phantom experiments. In 23 patient studies, 13 patients considered not to show reflux by visual assessment had a mean percentage injected dose (PID) in the stomach of 1.9% before correction and less than 1% after correction. The 10 patients with reflux showed a fall of PID from 5.8% to 1.7% as a result of the correction. In 14 patients the mean PID in the stomach after computer correction (3%) was not significantly different from that (3.5%) measured by aspiration. Computer correction of bile reflux data is essential when attempting to quantify the amount of entero-gastric reflux using a gamma camera.

A quantitative evaluation of breathing systems used with Kr-81m generators.

A quantitative evaluation of breathing systems currently in use with Rb-81 leads to Kr-81m generators is presented. Four systems were evaluated: a reservoir unit, a disposable oxygen face mask unit, and two types of nasal oxygen cannula units. These systems were used on 30 patients. It was found that the reservoir breathing system (a) delivered approximately 10% more Kr-81m gas to the lungs, and (b) reduced the image collection time by a factor of two or more, compared with the other three systems.