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S Rathee

Publications and source records attributed to S Rathee.

At least 19 recordsLinked to original sources
PubMed2006

A bench-top megavoltage fan-beam CT using CdWO4-photodiode detectors. I. System description and detector characterization.

We describe the components of a bench-top megavoltage computed tomography (MVCT) scanner that uses an 80-element detector array consisting of CdWO4 scintillators coupled to photodiodes. Each CdWO4 crystal is 2.75 x 8 x 10 mm3. The detailed design of the detector array, timing control, and multiplexer are presented. The detectors show a linear response to dose (dose rate was varied by changing the source to detector distance) with a correlation coefficient (R2) nearly unity with the standard deviation of signal at each dose being less than 0.25%. The attenuation of a 6 MV beam by solid water measured by this detector array indicates a small, yet significant spectral hardening that needs to be corrected before image reconstruction. The presampled modulation transfer function is strongly affected by the detector's large pitch and a large improvement can be obtained by reducing the detector pitch. The measured detective quantum efficiency at zero spatial frequency is 18.8% for 6 MV photons which will reduce the dose to the patient in MVCT applications. The detector shows a less than a 2% reduction in response for a dose of 24.5 Gy accumulated in 2 h; however, the lost response is recovered on the following day. A complete recovery can be assumed within the experimental uncertainty (standard deviation <0.5%); however, any smaller permanent damage could not be assessed.

Equipment Design↗
PubMed2006

A bench-top megavoltage fan-beam CT using CdWO4-photodiode detectors. II. Image performance evaluation.

Megavoltage computed tomography (MVCT) is a potential imaging tool for positioning and dose delivery verification during image guided radiotherapy. The problem with many MVCT detectors, however, is their low detective quantum efficiency (DQE) which leads to poor low contrast resolution (LCR) and high image noise. This makes separating the tumors from the soft tissue background difficult. This manuscript describes the imaging performance of our bench-top MVCT scanner that uses an 80-element detector array consisting of CdWO4-photodiode elements with a DQE of 19% in 6 MV and 26% in Co60 beams [T. T. Monajemi, S. Steciw, B. G. Fallone, and S. Rathee, "Modelling scintillator-photodiodes as detectors for megavoltage CT," Med. Phys. 31, 1225-1234 (2004)] at zero frequency. The imaging experiments presented were carried out mainly in a Co60 teletherapy unit, while the beam hardening characteristics of the system were also presented for a 6 MV beam. During image evaluation, persistent ring artifacts, caused by air gaps at the ends of the eight-element detector blocks, were removed by using a calibration procedure. The measured contrast of a low contrast target with a 20 mm diameter was determined to be independent of dose, between 2.1 and 17 cGy. The measured LCR of a target with a nominal contrast of 2.8% was reduced from 2.3% to 1.2% when the contrast target diameter was reduced from 15 to 5 mm, using 17 cGy for imaging. The signal to noise ratio of this system is shown to be proportional to the square root of dose. Most importantly, a low contrast target with a diameter of 6 mm and a nominal contrast level of 1.5% is resolved with a radiation dose of 2.1 cGy in the Co60 beam. The spatial resolution in the Co60 beam is limited to one line pair per centimeter mainly due to the size of the Co60 source.

Equipment Design↗
PubMed2006

3D interfractional patient position verification using 2D-3D registration of orthogonal images.

Reproducible positioning of the patient during fractionated external beam radiation therapy is imperative to ensure that the delivered dose distribution matches the planned one. In this paper, we expand on a 2D-3D image registration method to verify a patient's setup in three dimensions (rotations and translations) using orthogonal portal images and megavoltage digitally reconstructed radiographs (MDRRs) derived from CT data. The accuracy of 2D-3D registration was improved by employing additional image preprocessing steps and a parabolic fit to interpolate the parameter space of the cost function utilized for registration. Using a humanoid phantom, precision for registration of three-dimensional translations was found to be better than 0.5 mm (1 s.d.) for any axis when no rotations were present. Three-dimensional rotations about any axis were registered with a precision of better than 0.2 degrees (1 s.d.) when no translations were present. Combined rotations and translations of up to 4 degrees and 15 mm were registered with 0.4 degrees and 0.7 mm accuracy for each axis. The influence of setup translations on registration of rotations and vice versa was also investigated and mostly agrees with a simple geometric model. Additionally, the dependence of registration accuracy on three cost functions, angular spacing between MDRRs, pixel size, and field-of-view, was examined. Best results were achieved by mutual information using 0.5 degrees angular spacing and a 10 x 10 cm2 field-of-view with 140 x 140 pixels. Approximating patient motion as rigid transformation, the registration method is applied to two treatment plans and the patients' setup errors are determined. Their magnitude was found to be < or = 6.1 mm and < or = 2.7 degrees for any axis in all of the six fractions measured for each treatment plan.

Algorithms↗
PubMed2005

Novel methods of measuring single scan dose profiles and cumulative dose in CT.

Computed tomography dose index (CTDI) is a conventional indicator of the patient dose in CT studies. It is measured as the integration of the longitudinal single scan dose profile (SSDP) by using a 100-mm-long pencil ionization chamber and a single axial scan. However, the assumption that most of the SSDP is contained within the chamber length may not be valid even for thin slices. We have measured the SSDPs for several slice widths on two CT scanners using a PTW diamond detector placed in a 300 mm x 200 mm x 300 mm water-equivalent plastic phantom. One SSDP was also measured using lithium fluoride (LiF) TLDs and an IC-10 small volume ion chamber, verifying the general shape of the SSDP measured using the diamond detector. Standard cylindrical PMMA CT phantoms (140 mm length) were also used to qualitatively study the effects of phantom shape, length, and composition on the measured SSDP. The SSDPs measured with the diamond detector in the water-equivalent phantom were numerically integrated to calculate the relative accumulated dose D(L)(0)calc at the center of various scan lengths L. D(L)(0)calc reached an equilibrium value for L > 300 mm, suggesting the need for phantoms longer than standard CT dose phantoms. We have also measured the absolute accumulated dose using an IC-10 small volume ion chamber, D(L)(0)SV, at three points in the phantom cross section for several beamwidths and scan lengths. For one CT system, these measurements were made in both axial and helical scanning modes. The absolute CTDI100, measured with a 102 mm active length pencil chamber, were within 4% of D(L)(0)SV measured with the small volume ion chamber for L approximately 100 mm suggesting that nonpencil chambers can be successfully used for CT dosimetry. For nominal beam widths ranging from 3 to 20 mm and for L approximately 250 mm, D(L)(0)SV values at the center of the water-equivalent phantom's elliptic cross section were approximately 25%-30% higher than the measured CTDI100. For small beamwidths, the difference in D(L)(0)SV for L approximately 250 mm and L approximately 14 x beamwidth (CTDI14nT) reached up to 50%. Peripheral point doses at 70 mm depth along the major axis of the phantom for L approximately 250 mm were up to 22% higher than for L approximately 100 mm. The differences between CTDI100 and D(L)(0)SV for L approximately 250 mm were in good agreement with the predictions made from the numerical integration of the measured SSDPs. Due to the considerable dose measured beyond the length of standard CT phantoms, CT dosimetry for longer body scan series should be performed in longer phantoms. Measurements could be made as we have shown, using a small volume chamber translating through the beam using multiple scans.

Fluorides↗
PubMed2005

Three-dimensional IMRT verification with a flat-panel EPID.

A three-dimensional (3D) intensity-modulated radiotherapy (IMRT) pretreatment verification procedure has been developed based on the measurement of two-dimensional (2D) primary fluence profiles using an amorphous silicon flat-panel electronic portal imaging device (EPID). As described in our previous work, fluence profiles are extracted from EPID images by deconvolution with kernels that represent signal spread in the EPID due to radiation and optical scattering. The deconvolution kernels are derived using Monte Carlo simulations of dose deposition in the EPID and empirical fitting methods, for both 6 and 15 MV photon energies. In our new 3D verification technique, 2D fluence modulation profiles for each IMRT field in a treatment are used as input to a treatment planning system (TPS), which then generates 3D doses. Verification is accomplished by comparing this new EPID-based 3D dose distribution to the planned dose distribution calculated by the TPS. Thermoluminescent dosimeter (TLD) point dose measurements for an IMRT treatment of an anthropomorphic phantom were in good agreement with the EPID-based 3D doses; in contrast, the planned dose under-predicts the TLD measurement in a high-gradient region by approximately 16%. Similarly, large discrepancies between EPID-based and TPS doses were also evident in dose profiles of small fields incident on a water phantom. These results suggest that our 3D EPID-based method is effective in quantifying relevant uncertainties in the dose calculations of our TPS for IMRT treatments. For three clinical head and neck cancer IMRT treatment plans, our TPS was found to underestimate the mean EPID-based doses in the critical structures of the spinal cord and the parotids by approximately 4 Gy (11%-14%). According to radiobiological modeling calculations that were performed, such underestimates can potentially lead to clinically significant underpredictions of normal tissue complication rates.

Algorithms↗
PubMed2004

Modeling scintillator-photodiodes as detectors for megavoltage CT.

The use of cadmium tungstate (CdWO4) and cesium iodide [CsI(Tl)] scintillation detectors is studied in megavoltage computed tomography (MVCT). A model describing the signal acquired from a scintillation detector has been developed which contains two steps: (1) the calculation of the energy deposited in the crystal due to MeV photons using the EGSnrc Monte Carlo code; and (2) the transport of the optical photons generated in the crystal voxels to photodiodes using the optical Monte Carlo code DETECT2000. The measured detector signals in single CdWO4 and CsI(Tl) scintillation crystals of base 0.275 x 0.8 cm2 and heights 0.4, 1, 1.2, 1.6 and 2 cm were, generally, in good agreement with the signals calculated with the model. A prototype detector array which contains 8 CdWO4 crystals, each 0.275 x 0.8 x 1 cm3, in contact with a 16-element array of photodiodes was built. The measured attenuation of a Cobalt-60 beam as a function of solid water thickness behaves linearly. The frequency dependent modulation transfer function [MTF(f)], noise power spectrum [NPS(f)], and detective quantum efficiency [DQE(f)] were measured for 1.25 MeV photons (in a Cobalt-60 beam). For 6 MV photons, only the MTF(f) was measured from a linear accelerator, where large pulse-to-pulse fluctuations in the output of the linear accelerator did not allow the measurement of the NPS(f). A two-step Monte Carlo simulation was used to model the detector's MTF(f), NPS(f) and DQE(f). The DQE(0) of the detector array was found to be 26% and 19% for 1.25 MeV and 6 MV photons, respectively. For 1.25 MeV photons, the maximum discrepancies between the measured and modeled MTF(f), relative NPS(f) and the DQE(f) were found to be 1.5%, 1.2%, and 1.9%, respectively. For the 6 MV beam, the maximum discrepancy between the modeled and the measured MTF(f) was found to be 2.5%. The modeling is sufficiently accurate for designing appropriate detectors for MVCT.

Computer-Aided Design↗
PubMed2004

Lag measurement in an a-Se active matrix flat-panel imager.

Lag and residual contrast have been quantified in an amorphous selenium (a-Se) active-matrix flat-panel imager (AMFPI) as a function of frame time, kilovoltage (kV) and megavoltage (MV) x-ray photon energies and amount of radiation incident on the detector. The AMFPI contains a 200 microm thick a-Se layer deposited on a thin film transistor (TFT) array of size 8.7 cm x 8.7 cm with an 85-microm pixel pitch. For all energies, the lag (signal normalized to the signal due to exposure) for the first (n = 1) and second (n = 2) frame after exposure ranges from 0.45% to 0.91% and from 0.29% to 0.51%, respectively. The amount of lag was determined to be a function of the time after the x-ray exposure irrespective of frame time or the magnitude of exposure. The lag for MV photon energies was slightly less than that for kV photon energies. The residual contrast for all energies studied ranges from 0.41% to 0.75% and from 0.219% to 0.41% for the n = 1 and n = 2 frames, respectively. These results show that lag and residual contrast in kV and MV radiographic applications are always less than 1% for the detection system used and only depend on the time after x-ray exposure.

Equipment Failure Analysis↗
PubMed2003

Dosimetric IMRT verification with a flat-panel EPID.

A convolution-based calibration procedure has been developed to use an amorphous silicon flat-panel electronic portal imaging device (EPID) for accurate dosimetric verification of intensity-modulated radiotherapy (IMRT) treatments. Raw EPID images were deconvolved to accurate, high-resolution 2-D distributions of primary fluence using a scatter kernel composed of two elements: a Monte Carlo generated kernel describing dose deposition in the EPID phosphor, and an empirically derived kernel describing optical photon spreading. Relative fluence profiles measured with the EPID are in very good agreement with those measured with a diamond detector, and exhibit excellent spatial resolution required for IMRT verification. For dosimetric verification, the EPID-measured primary fluences are convolved with a Monte Carlo kernel describing dose deposition in a solid water phantom, and cross-calibrated with ion chamber measurements. Dose distributions measured using the EPID agree to within 2.1% with those measured with film for open fields of 2 x 2 cm2 and 10 x 10 cm2. Predictions of the EPID phantom scattering factors (SPE) based on our scatter kernels are within 1% of the SPE measured for open field sizes of up to 16 x 16 cm2. Pretreatment verifications of step-and-shoot IMRT treatments using the EPID are in good agreement with those performed with film, with a mean percent difference of 0.2 +/- 1.0% for three IMRT treatments (24 fields).

Canada↗
PubMed2002

Modulation transfer function of digitally reconstructed radiographs using helical computed tomography.

We have measured the modulation transfer function (MTF), at a distance from CT isocenter, in digitally reconstructed radiographs (DRR) using a bar pattern phantom for axial and helical data acquisitions. Spatial resolution in DRR increases for thinner slice thicknesses (43% at 1.6 lp/cm for 2 mm versus 16% for 5 mm). For the three slice thicknesses studied, the axial scanning mode provided better spatial resolution in DRR than helical scans performed at pitches > or = 1.5 (41% at 1.6 lp/cm for axial, 3 mm slice versus 18% for pitch = 2), but is similar to helical scans with pitch of 1. The reconstruction of overlapping slices from helical acquisitions of pitch = 1.5 results in spatial resolution of the DRR that is similar to that resulting from axial scans with contiguous reconstruction, but also results in the fine streaks known as "zebra" artifacts in the DRR.

Image Processing, Computer-Assisted↗
PubMed2000

Size and positioning reproducibility of an 192Ir brachytherapy stepping source.

In this paper we describe techniques for measuring the dimensions and position reproducibility of an 192Ir brachytherapy stepping source. Measurements were carried out using a 0.25x10x152 mm3 collimator placed in front of a detector of our own design. The brachytherapy source was translated past the collimator in 0.025 mm increments using a stepper motor. The source was found to be 3.58 mm long and 0.69 mm wide, which is in good agreement with the manufacturer's values of 3.5x0.6 mm2. The source position was reproducible to within 0.12 mm.

Biophysical Phenomena↗
PubMed1999

Commissioning, clinical implementation and quality assurance of Siemen's Virtual Wedge.

This report presents the results of commissioning, clinical implementation and quality assurance of Siemens Virtual Wedge. Our measurements show that: (1) wedge factors are within 2% of unity, (2) percentage depth doses are within 1% of open beam data, and (3) wedged beam profiles can be modeled similarly to a physical wedge and follow a well defined equation to facilitate modeling of an arbitrary wedge angle. The gantry angle dependence of wedge profiles is similar to open beam profiles. The output of wedged fields is linear with delivered monitor units within 1%. Quality assurance results indicate the wedge profiles are very stable over time. Day to day variations of two points measured along the wedge gradient direction are within 1.5%.

Algorithms↗
PubMed1997

Practice of standards in female sterilisation.

During training of trainers (TOT) courses organised for medical personnel of Haryana Civil Medical Services (HCMS) by COE Medical College, Rohtak, 55 doctors involved in female sterilisations were interrogated regarding practices in counselling, informed decision, asepsis, surgical procedure, operative and postoperative care and follow-up of the clients accepting sterilisation as contraception. Counselling was the responsibility of auxiliary nurse cum midwife (ANM) lady health volunteer (LHV)/other paramedical workers as viewed by 89.1% participants whereas 85.4% thought that the registration clerk should take the informed consent. Eligibility criteria were always adhered to by 10.9% participants. Asepsis and sterilisation of instruments, etc, were maintained by operation theatre (OT) attendant or OT nurse as answered by 90.9% doctors. Skin preparation was done by a solution containing cetrimide and chlorhexidine alone by 70.8% doctors. The ligation and excision was the method practised by all. Catgut suture was used by only 43.6% doctors. Twenty-six maternal deaths were reported by 20 participants during their whole career. There were 7 deaths on the table, all with laparoscopic sterilisation. Peritonitis with septicaemia was the major cause of death in majority of cases. To ensure high quality and safety of voluntary surgical contraception, programmes must establish a system to ensure that standards are maintained.

Adult↗
PubMed1995

Influence of intraumbilical oxytocin on the third stage of labour.

In this study the effect of intraumbilical oxytocin on duration and amount of blood loss in third stage of labour was studied. Pregnant women were randomized into 2 groups of 50 each. Group I was managed actively with 10 units of oxytocin diluted in 20 ml saline given through umbilical vein immediately after cord claming and Group II managed traditionally with oxytocin infusion 10 units in 250 ml of dexrose saline at rate of 125 ml/hr given after delivery of baby. In the study group there was a statistically significant decrease in duration of third stage of labour <1.48 min vs 3.27 min>, fall in haemoglobin <1.2 g/dl vs 1.96 g/dl> and fall in haematocrit <3.88% Vs 7.20%<. It was concluded that intraumbilical oxytocin appears to be a useful, safe and practical method for active management of third stage.

Adult↗
PubMed1995

Vesicovaginal fistulae: a 12-year study.

Repairs in 49 cases of vesicovaginal fistulae were performed during the last 12 years. Obstetric cause for vesicovaginal fistulae was noted in 81.63% cases and the most important being obstructed labour (73.46% cases). Repair was successful in 71.42% cases. Factors favourable for success were a non-obstetric fistula, small size fistula (< 10 mm) and absence of fibrosis. Repair operations were performed mainly by vaginal flap technique. The fistulous track was not excised. Polyglactin suture gave the best results.

Adolescent↗
PubMed1992

Detection of Chlamydia trachomatis antigen by enzyme immunoassay in patients with pelvic inflammatory disease.

In 27 out of the 60 women clinically suspected to have pelvic inflammatory disease (PID) studied, the C. trachomatis antigen titre was found to be higher than the cut-off value. Since the presence of C. trachomatis antigen was detected in 45 per cent of women, it is suggested that all the PID patients may be investigated by ELISA to confirm the role of C. trachomatis as the etiological agent.

Adult↗