A pre-clinical phantom comparison of tissue harmonic and brightness mode imaging for application in ultrasound guided prostate brachytherapy.

PURPOSE: The current practice of prostate brachytherapy utilizes the brightness (B) mode ultrasound imaging for volume definition and needle guidance. However, tissue harmonic (H) mode available with new scanners has shown the improved image quality. The aim of this study was to perform a pre-clinical phantom evaluation of harmonic imaging as an alternative to B mode in prostate brachytherapy. METHODS: Performance characteristics viz. dead zone, depth of penetration, geometric accuracy, spatial resolution, tissue to clutter ratio (TCR) and signal to noise ratio (SNR), were compared between two modes using an in-house phantom. Images were acquired under the same settings except the gain; which is higher for the H mode than that of B mode. A qualitative comparison between two modes was also performed using commercial CIRS053 phantom. RESULTS: Dead zone, depth of penetration and geometric accuracy were respectively <1 mm, >8 cm and <1% for both modes. Relative TCR, SNR and the spatial resolution were improved in H mode compared with B mode. Images with CIRS053 phantom in H mode demonstrate sharper boundaries for prostate and urethra, freedom from background clutter, and better identification of the brachytherapy needles. CONCLUSIONS: This study indicates the superiority of H over B mode, in terms of spatial resolution, relative contrast, and overall image quality. Thus H mode has the potential benefit in prostate brachytherapy. This study provides the basis to move forward to investigate whether the superior image quality observed in the laboratory can be translated into a higher treatment quality for the patient.

Measurement of time delay for a prospectively gated CT simulator.

For the management of mobile tumors, respiratory gating is the ideal option, both during imaging and during therapy. The major advantage of respiratory gating during imaging is that it is possible to create a single artifact-free CT data-set during a selected phase of the patient's breathing cycle. The purpose of the present work is to present a simple technique to measure the time delay during acquisition of a prospectively gated CT. The time delay of a Philips Brilliance BigBore (Philips Medical Systems, Madison, WI) scanner attached to a Varian Real-Time Position Management (RPM) system (Varian Medical Systems, Palo Alto, CA) was measured. Two methods were used to measure the CT time delay: using a motion phantom and using a recorded data file from the RPM system. In the first technique, a rotating wheel phantom was altered by placing two plastic balls on its axis and rim, respectively. For a desired gate, the relative positions of the balls were measured from the acquired CT data and converted into corresponding phases. Phase difference was calculated between the measured phases and the desired phases. Using period of motion, the phase difference was converted into time delay. The Varian RPM system provides an external breathing signal; it also records transistor-transistor logic (TTL) 'X-Ray ON' status signal from the CT scanner in a text file. The TTL 'X-Ray ON' indicates the start of CT image acquisition. Thus, knowledge of the start time of CT acquisition, combined with the real-time phase and amplitude data from the external respiratory signal, provides time-stamping of all images in an axial CT scan. The TTL signal with time-stamp was used to calculate when (during the breathing cycle) a slice was recorded. Using the two approaches, the time delay between the prospective gating signal and CT simulator has been determined to be 367 +/- 40 ms. The delay requires corrections both at image acquisition and while setting gates for the treatment delivery; otherwise the simulation and treatment may not be correlated with the patient's breathing.

Quantification of low dose signal in EPR tooth dosimetry--a novel approach.

For radiation exposures below 100 mGy, the dosimetric signal in tooth enamel is too small to be measured by using the traditional dose reconstruction procedure. This is because low amplitude zero-added-dose signal can not be identified in an EPR spectrometer. A technique is presented wherein, zero-added-dose signal. when amplified by a proper known dose, can be measured in the EPR spectrometer. Mathematically, the accidental dose x is modified by a known amount of exposure, y (large enough so that the signal is now visible), and total exposure becomes x' = x + y, which is the modified-zero-added dose. The exposure x' is then quantified using the conventional backward extrapolation method and the accidental dose can be measured. In a laboratory controlled experiment, the feasibility of dose reconstruction in the 100 mGy range has been demonstrated. This may enable measurements of dose even due to suspected low exposure in tooth enamel.