Biomedical subjects

A Lomax

Publications and source records attributed to A Lomax.

25 records · Page 2Linked to original sources

PubMed1996

Proton dosimetry intercomparison.

BACKGROUND AND PURPOSE: Methods for determining absorbed dose in clinical proton beams are based on dosimetry protocols provided by the AAPM and the ECHED. Both groups recommend the use of air-filled ionization chambers calibrated in terms of exposure or air kerma in a 60Co beam when a calorimeter or Faraday cup dosimeter is not available. The set of input data used in the AAPM and the ECHED protocols, especially proton stopping powers and w-value is different. In order to verify inter-institutional uniformity of proton beam calibration, the AAPM and the ECHED recommend periodic dosimetry intercomparisons. In this paper we report the results of an international proton dosimetry intercomparison which was held at Loma Linda University Medical Center. The goal of the intercomparison was two-fold: first, to estimate the consistency of absorbed dose delivered to patients among the participating facilities, and second, to evaluate the differences in absorbed dose determination due to differences in 60Co-based ionization chamber calibration protocols. MATERIALS AND METHODS: Thirteen institutions participated in an international proton dosimetry intercomparison. The measurements were performed in a 15-cm square field at a depth of 10 cm in both an unmodulated beam (nominal accelerator energy of 250 MeV) and a 6-cm modulated beam (nominal accelerator energy of 155 MeV), and also in a circular field of diameter 2.6 cm at a depth of 1.14 cm in a beam with 2.4 cm modulation (nominal accelerator energy of 100 MeV). RESULTS: The results of the intercomparison have shown that using ionization chambers with 60Co calibration factors traceable to standard laboratories, and institution-specific conversion factors and dose protocols, the absorbed dose specified to the patient would fall within 3% of the mean value. A single measurement using an ionization chamber with a proton chamber factor determined with a Faraday cup calibration differed from the mean by 8%. CONCLUSION: The adoption of a single ionization chamber dosimetry protocol and uniform conversion factors will establish agreement on proton absorbed dose to approximately 1.5%, consistent with that which has been observed in high-energy photon and electron dosimetry.

Calibration↗

PubMed1996

Characterization of dose distribution in radiation therapy plans.

As a method of considering only significant radiation doses to different tissues, the ICRU Report 50 recommends taking the dose given to a significant tissue volume (minimum diameter greater then 15 mm) instead of choosing a single, potentially insignificant, voxel value. In order to find this significant volume, we have adapted an emission imaging analysis method to radiation therapy planning. The resulting method finds and characterizes the dose distribution in the volumes of interest in a way that includes spatial arrangement. The data can be used to signal significant hot or cold volumes in the dose plan and to score the plans based on significant dose to the tissues.

Humans↗

PubMed1996

The calibration of CT Hounsfield units for radiotherapy treatment planning.

Computer tomographic (CT) scans are used to correct for tissue inhomogeneities in radiotherapy treatment planning. In order to guarantee a precise treatment, it is important to obtain the relationship between CT Hounsfield units and electron densities (or proton stopping powers for proton radiotherapy), which is the basic input for radiotherapy planning systems which consider tissue heterogeneities. A method is described to determine improved CT calibrations for biological tissue (a stoichiometric calibration) based on measurements using tissue equivalent materials. The precision of this stoichiometric calibration and the more usual tissue substitute calibration is determined by a comparison of calculated proton radiographic images based on these calibrations and measured radiographs of a biological sample. It has been found that the stoichiometric calibration is more precise than the tissue substitute calibration.

Humans↗

PubMed1996

Initial experience of 3-D video endoscopy in general surgery.

The worldwide explosion of Iaparoscopic surgery within general surgery began in the late 1980sand early 1990s.From its inception, surgeons have expected the image on the video monitor to resemble closely the . image obtained in similar open surgery.The monitor image, however, is presented in two dimensions- height and width. Orientation of the surgeon to the anatomy is more difficult and requires a sound knowl- edge of anatomy, the relationships of organs to each other, and an understanding of the individual patient's unique structures. Adding a third dimension, depth, to imitate natural vision in open surgery may provide surgeons with the confidence to perform more advanced procedures. In addition, trainee surgeons, without the experience of open surgery which their senior colleagues have gained, may more rapidly appreciate orientation of the anatomy and learn the surgical task more quickly.

Journal Article↗

PubMed1995

The 200-MeV proton therapy project at the Paul Scherrer Institute: conceptual design and practical realization.

The new proton therapy facility is being assembled at the Paul Scherrer Institute (PSI). The beam delivered by the PSI sector cyclotron can be split and brought into a new hall where it is degraded from 590 MeV down to an energy in the range of 85-270 MeV. A new beam line following the degrader is used to clean the low-energetic beam in phase space and momentum band. The analyzed beam is then injected into a compact isocentric gantry, where it is applied to the patient using a new dynamic treatment modality, the so-called spot-scanning technique. This technique will permit full three-dimensional conformation of the dose to the target volume to be realized in a routine way without the need for individualized patient hardware like collimators and compensators. By combining the scanning of the focused pencil beam within the beam optics of the gantry and by mounting the patient table eccentrically on the gantry, the diameter of the rotating structure has been reduced to only 4 m. In the article the degrees of freedom available on the gantry to apply the beam to the patient (with two rotations for head treatments) are also discussed. The devices for the positioning of the patient on the gantry (x rays and proton radiography) and outside the treatment room (the patient transporter system and the modified mechanics of the computer tomograph unit) are briefly presented. The status of the facility and first experimental results are introduced for later reference.

Cyclotrons↗

PubMed1994

Polar map or novel three-dimensional display technique for the improved detection of inferior wall myocardial infarction using tomographic radionuclide ventriculography.

Tomographic radionuclide ventriculography has the potential to be a significant improvement over conventional planar imaging. Although tomographic imaging can now be performed with relative ease, it is little used. This is most probably due to a perceived imbalance between potential clinical benefit and the extra complications of imaging. We investigated this matter by examining a series of 30 patients with isolated inferior or anterior myocardial infarction, identified by cardiac catheterization. Using either radionuclide imaging method, a significant wall motion abnormality was defined as matching (and appropriately located) phase and amplitude values outwith of two standard deviations from control values. These values were obtained from a series of 25 controls and represent construction values used to create a conventional polar map display. Overall detection rates for anterior myocardial infarction were 93 and 100% for planar and tomographic imaging, respectively (ns). For inferior myocardial infarction the rates were 7 and 93%, respectively (p < 0.001). Identical results were found using a novel three-dimensional method of displaying wall motion abnormalities. Tomography is therefore superior to planar imaging for the detection of inferior myocardial infarction but similar to planar imaging for the detection of anterior myocardial infarction.

Cardiac Catheterization↗

PubMed1993

Three-dimensional endoscopic imaging for minimal access surgery.

Three-dimensional endoscopic imaging (3DEndoImaging) is a significant technological advance and has the potential to make minimal access surgery (MAS) easier, quicker, less prone to error and more applicable to advanced procedures. Surgeons involved in MAS will need to have a working knowledge of 3DEndoImaging. This article will enable surgeons to compare stereo systems and evaluate which system would best suit their needs. This paper explains why stereo imaging is important and describes the methods by which stereo images can be produced. The technology required is discussed in simple terms. The types of stereo systems are described and important operational and maintenance issues discussed. Task analysis studies showing significant improvement in performance in stereo are presented. These studies simulated accurately positioning an instrument and threading a small diameter solder lug.

Endoscopes↗