Potential and role of a prototype amorphous silicon array electronic portal imaging device in breathing synchronized radiotherapy.

Current electronic portal imaging devices (EPID) are limited in their ability to provide direct and quick verification and monitoring of patients during both setup and treatment of breathing synchronized radiotherapy (BSRT, including breathing gated, voluntary and forced breath-hold radiotherapy treatment.) These limitations are largely due to their slow image capture rate and poor image quality. An amorphous silicon array flat panel electronic portal imaging device (si-EPID) is emerging to meet the challenge. The purpose of this study is threefold: (1) to characterize the performance of a prototype si-EPID; (2) to compare image quality against that of digitized films; and (3) to evaluate the device in terms of verification of patient setup and monitoring during BSRT. In this study a Varian prototype si-EPID detector array and Clinic accelerator at the University of California Davis Cancer Center were used for imaging. Three quality assurance phantoms: a Lutz PVC phantom, a modified "Las Vegas" phantom, and a RMI model 1151 phantom, were used to characterize the imaging system. A Rando head phantom was used for anthropomorphic imaging tests. Images were obtained with the si-EPID and a Fuji RX film in a Kodak X-Omatic cassette. To investigate the clinical application, two sets of si-EPID images were collected from a lung cancer patient during a 22 s breath-hold and normal breathing. The quality of images obtained with the fast mode was found to be comparable to that obtained with the digitized films. The images with the standard mode were found to be better than the digitized film images. With this prototype si-EPID, it is possible to collect the images at the beginning, middle, and end of each breath-hold for those patients who can hold their breath for longer than 15 s. The si-EPID images can provide a quick verification of the initial patient setup and subsequent treatment position throughout the daily fractionation.

Heating patterns of the Helios ultrasound hyperthermia system.

Thermal dosimetry studies of the Helios ultrasound system were performed in a tissue-equivalent phantom and in porcine tissues in vivo. SAR distributions in the phantom demonstrate the adjustability of the power deposition pattern from the 50 cm diameter annular array of 30 transducers. Examples are given of well-localized SAR patterns (< 3 cm diameter) in the phantom, and of peripherally enhanced (doughnut-shaped) SAR patterns up to 10 cm in diameter at the focal plane. In vivo trials in porcine thigh muscle demonstrate the ability of the Helios system to produce regions of therapeutic heating (42-45 degrees C) as small as 20 cm3 and as large as 200 cm3 at depths of greater than 10 cm at a frequency of 0.5 MHz.

Potential for localized, adjustable deep heating in soft-tissue environments with a 30-beam ultrasonic hyperthermia system.

Initial heating rates (degrees C/min) along parallel tracks at depths of 1-14 cm in a static, muscle-like phantom were determined from time-temperature profiles obtained with 'Helios', a 30-beam ultrasonic hyperthermia system developed by Varian Associates. Data were taken at a single operating frequency of 556 kHz, for different sets of focal plane ring diameters of the four-ring array applicator, different levels of transducer driving power and two different focal plane depths, 6 cm and 9 cm. In each experiment, at each point of temperature measurement, analysis of temperature versus time data over a 2 min heating interval permitted separation of the desired phantom heating from artefactual heating resulting primarily from absorption of transverse (shear) waves produced at phantom-metal probe catheter interfaces. The results of the studies conducted suggest that in a non-translating carriage mode, Helios can produce axially and laterally localized deep heating in soft tissues for tissue volumes of lateral dimension up to a minimum of 4 cm and tissue depths of at least 11 cm. The results obtained also suggest that Helios can produce laterally localized heating to tissue depths of at least 11 cm without excessive heating of superficial soft-tissue layers, for tissue volumes of lateral dimension up to a minimum of 8 cm. The methodology used in the phantom studies was applied to the production of localized heating in the right lobe of the liver of adult pigs. Temperature versus time profiles obtained in the in vivo studies indicated that, for the set of system parameters employed, concentration of ultrasonic power at greater depths in the liver (e.g. 10.5 cm versus 5 cm) could be achieved, suggesting that Helios should be able to produce localized heating of targeted hepatic volumes when its operating parameters are selected in accordance with effective treatment planning techniques.

Early experience with the Varian Six Second body scanner in the diagnosis of hepatobiliary tract disease.

Thirty-four patients with liver disease were studied with a research model of the Varian Six Second body scanner. Useful information was gained in patients with a variety of hepatic disorders. This scanner permits resolution of normal hepatic parenchymal detail which has not been reported previously and has reduced the streak artifacts originating from high- and low-CT objects which have been a major source of image degradation with other units.

Stop-action cardiac computed tomography.

Computed tomographic (CT) cardiac imaging in vivo has been hampered by motion of the heart during its cardiac cycle. A technique of post data-acquisition correlation of the angular projection data using the electrocardiogram as a reference signal is described. This method produced seven "stop action" images of the heart and resulted in delineating morphological detail not recognizable on the conventional CT scan.