Volume 23, Issue 11, November 1996
Index of content:
23(1996); http://dx.doi.org/10.1118/1.597666View Description Hide Description
Using a new reference/test, adaptive forced‐choice method, we investigated detectability of low contrast disks in simulated x‐ray fluoroscopyimagesequences. The method minimized effects of undesirable variables, such as changes in subject attention level by alternating conventional fluoroscopy (reference) and low acquisition rate fluoroscopy (test) presentations. To compare absolute target detectability and x‐ray dose requirements of reference and test presentations, the contrast of the test presentation was set equal to that of the reference presentation, and the dose of the test presentation was adapted to give equivalent detectability of disk targets. Comparing pulsed fluoroscopy at 15 acq/s (pulsed‐15) and 7.5 acq/s (pulsed‐7.5) with conventional fluoroscopy at 30 acq/s (pulsed‐30), dose savings were found in 22 of 24 experiments. When averaged over 3 disk sizes and 4 subjects, savings were 22% and 26% for pulsed‐15 and pulsed‐7.5, respectively. In most experiments, the subject was given an unlimited time to view presentations. With unlimited display times, subjects took slightly longer to respond for pulsed‐15 or pulsed‐7.5 than for pulsed‐30. In other experiments, display times were limited. As the restricted display time increased, absolute target detectability increased while the x‐ray dose for equivalent detectability was unchanged. The number of frames, N, in a repeating loop was also varied. Absolute detectability increased as N increased from 1 to 30 but changed little as more frames were added up to 150. The new experimental paradigm gives efficient, robust comparisons of image data and is applicable to a wide variety of medical image perception problems.
23(1996); http://dx.doi.org/10.1118/1.597836View Description Hide Description
An electronic portal imaging device(EPID) for use in radiotherapy with high energy photons has been under development since 1985 and has been in clinical use since 1988. The x‐ray detector consists of a metal plate/fluorescent screen combination, which is monitored by a charge‐coupled device (CCD)‐camera. This paper discusses the physical quantities governing image quality. A model which describes the signal and noise propagation through the detector is presented. The predicted contrasts and signal‐to‐noise ratios are found to be in agreement with measurements based on the EPIDimages. Based on this agreement the visibility of low contrast structures in clinical images has been calculated with the model. Sufficient visibility of relevant structures (4–10 mm water‐equivalent thickness) has been obtained down to a delivered dose of 4 cGy at dose maximum. It is found that the described system is not limited by quantum noise but by camera read‐out noise. In addition we predict that with a new type of CCDsensor the signal‐to‐noise ratio can be increased by a factor of 5 at small doses, enabling high quality imaging, for most relevant clinical situations, with a patient dose smaller than 4 cGy. The latter system would be quantum noise limited.
Point spread functions of photons in time‐resolved transillumination experiments using simple scaling arguments23(1996); http://dx.doi.org/10.1118/1.597748View Description Hide Description
Simple scaling arguments are used to determine spatial resolution achievable in time‐resolved transillumination experiments involving highly diffuse media. These arguments allow us to obtain relationships linking target resolution at different planes inside an optically turbid slab to the gating times of the imaging system. We show that this approach yields the same results as those obtained previously from an approximate and rather complicated analytical derivation. In addition, we are now able to assess the effects of scatteringanisotropy on spatial resolution attainable when gating times are so short that a constant scaling of photon transport scattering length is not appropriate. These results should enable one to devise more accurate and simpler image reconstruction algorithms.
23(1996); http://dx.doi.org/10.1118/1.597898View Description Hide Description
23(1996); http://dx.doi.org/10.1118/1.597749View Description Hide Description
The ratio between scatter dose and scatter kerma for points on the central axis of 15‐MV x‐ray beams has been examined by Monte Carlo calculations. This ratio, β′ S , behaves differently from that between the primary dose and kerma, β′ p . Both the primary and scatter components of β′ undergo an initial rapid buildup, however, β′ S begins from a much higher surface value. In addition, the depth required for longitudinal electron equilibrium is larger for β′ S than for β′ P . The variation of β′ S with field size and depth is attributed to the spatial variation of scatter kerma in the photon beam. The approximation β′ S =1 is accurate enough for clinical dose‐calculation purposes, leading to less than 0.5% error in total dose, while the assumption β′ S =β′ p may cause up to 2.5% error, relative to the maximum dose, near the surface at 15 MV.
23(1996); http://dx.doi.org/10.1118/1.597750View Description Hide Description
The electron contamination in photonbeams has been investigated by means of contaminating lepton depth doses and dose profiles in different geometries with two 20 MV beams. Different components of this contamination have been investigated separately by systematically adding contamination to a ‘‘clean’’ reference field. At 20 MV, the air generated electrons were found to be almost negligible compared to the electrons originating from the accelerator head when measurements were performed in standard fields at SSDs between 80 and 120 cm. The total electron part of the depth dose curve was then almost the same, i.e., independent of SSD, when the collimator opening was held fixed. However, when different accessories such as a shaping block and different attenuating plates were located in the beam path below the collimators, a large SSD dependence of the electron contamination was noticed. A comparison was also made between two machines, one equipped with a multileaf collimator, with similar beam qualities at 20 MV. These measurements indicate that the interior view of the treatment head seen by the detector (mainly the flattening filter, monitor chamber, or other electron generating material) influences the magnitude of the electron contamination. When the collimator opening is decreased the electron contamination will also decrease as parts of the electron source will be shielded by the collimator blocks.
23(1996); http://dx.doi.org/10.1118/1.597747View Description Hide Description
Recently, a new model of radiochromic film has been developed for medical applications to provide a higher sensitivity and better uniformity of response than existing models (i.e., MD‐55). Dosimetric characteristics including sensitivity, linearity, reproducibility, uniformity, and dependence on energy and time have been studied experimentally. The characteristics of the new films were compared with those of model MD‐55. For these investigations, the two films were exposed to ionizing radiation in the dose range from 1–72 Gy, using γ‐rays from a 60Co teletherapy unit and 6‐ and 18‐MV x rays from a linear accelerator. The response of the exposed film was measured with a helium–neon laser densitometer. The results indicated that the sensitivity of the improved film was about 40% greater than that of MD‐55 film. Moreover, the response of the improved film was found to be uniform within 4% only in one direction of the film. The orthogonal direction indicated a nonuniformity of up to 15%, similar to that of model MD‐55. Less than 5% energy dependence in the megavoltage photon range was observed for the new film. Complete dosimetric characteristics of the new film are presented.
23(1996); http://dx.doi.org/10.1118/1.597751View Description Hide Description
The continuing development of probes for use with beta (positron and electron) emitting radionuclides may result in more complete excision of tracer‐avid tumors. Perhaps one of the most promising radiopharmaceuticals for this task is 18F‐labeled‐Fluoro‐2‐Deoxy‐D‐Glucose (FDG). This positron‐emitting agent has been demonstrated to be avidly and rapidly absorbed by many human cancers. We have investigated the use of ion‐implanted‐silicon detectors in intraoperative positron‐sensitive surgical probes for use with FDG. These detectors possess very high positrondetection efficiency, while the efficiency for 511 keV photondetection is low. The spatial resolution, as well as positron and annihilation photondetection sensitivity, of an ion‐implanted‐silicon detector used with 18F was measured at several energy thresholds. In addition, the ability of the device to detect the presence of relatively small amounts of FDG during surgery was evaluated by simulating a surgical field in which some tumor was left intact following lesion excision. The performance of the ion‐implanted‐silicon detector was compared to the operating characteristics of a positron‐sensitive surgical probe which utilizes plastic scintillator. In all areas of performance the ion‐implanted‐silicon detector proved superior to the plastic scintillator‐based probe. At an energy threshold of 14 keV positron sensitivity measured for the ion‐implanted‐silicon detector was 101.3 cps/kBq, photon sensitivity was 7.4 cps/kBq. In addition, spatial resolution was found to be relatively unaffected by the presence of distant sources of annihilation photon flux. Finally, the detector was demonstrated to be able to localize small amounts of FDG in a simulated tumor bed; indicating that this device has promise as a probe to aid in FDG‐guided surgery.
Pelvic irradiation of the obese patient: A treatment strategy involving megavoltage simulation and intratreatment setup corrections23(1996); http://dx.doi.org/10.1118/1.597752View Description Hide Description
This study involves a fractionated course of external radiation therapy for a 42 year old female weighing 150 kg, diagnosed with stage IIb cancer of the cervix. The patient could not be simulated in the conventional sense due to weight restrictions on the simulator couch, and body casts or molds were impractical. Using an on‐line portal imaging device, treatment fields were established during the first session, and intratreatment verification was used before every subsequent treatment to measure and, when necessary, to correct the patient setup. Two courses of treatment were prescribed with a total dose of 60 Gy delivered by a four field box technique (A/P, P/A, and two lateral fields). Out of a total of 108 treatment fields monitored, 12 anterior fields and 1 posterior field were corrected (exclusive of the first, or simulation fraction). Without corrections, 10% of the initial setup displacements would have had displacements greater than 10 mm, 21% greater than 7 mm, and 41% greater than 5 mm. With the application of intratreatment corrections, only 2% of the displacements were greater than 10 mm, 11% were greater than 7 mm, and 32% were greater than 5 mm. It was also found that the second field treated in a parallel opposed pair (i.e., anterior/posterior or left/right lateral) had lower setup displacements and did not require verification or correction.
A method for verifying treatment times for simple high‐dose‐rate endobronchial brachytherapy procedures23(1996); http://dx.doi.org/10.1118/1.597753View Description Hide Description
An empirical method for verifying the total treatment time for either a one‐ or a two‐catheter high‐dose‐rate procedure has been developed. The method can be performed quickly and allows for easy verification of the accuracy of the treatment time arrived at by a computerized planning system. The method is designed to confirm the treatment time to within 10%.
23(1996); http://dx.doi.org/10.1118/1.597754View Description Hide Description
We report on the repositioning accuracy of patient setup achieved with a noninvasive head fixation device for stereotactic radiotherapy. A custom head mask which attaches to our stereotactic radiosurgery head ring assembly is fabricated for each patient. The position and orientation of a patient in the stereotactic space at the time of treatment are determined from analyzing portal films containing images of radio‐opaque spheres embedded in a custom mouthpiece. From analysis of 104 setups of 12 patients, we find that the average distance between the treated isocenter and its mean position is 1.8 mm, and that the standard deviations of the position of the treated isocenter in stereotactic coordinate space about its mean position are less than 1.4 mm in translation in any direction and less than 1° of rotation about any axis.
23(1996); http://dx.doi.org/10.1118/1.597755View Description Hide Description
A method is described for the determination of radioactivity (μCi or MBq) at an organ site within an object or patient. Using both anatomic image data (CT or MRI scans) and planar gamma cameraimages, activity at depth is determined using a matrix inversion method based on least squares. The result of the inversion analysis was the unknown set of n linear (uniform) activity densities representative of each organ within the phantom or patient. The problem was overdetermined since the number of unknown activity densities (μCi/cm) was much less than the number of analysis points (N) within the nuclear image. This method, defined as the CT assisted matrix inversion (CAMI) technique, was accurate to within 15% for a three ‘‘organ’’ plastic phantom, wherein the organs were right circular cylinders having activities of 74 to 508 μCi (or 2.74 MBq to 18.8 MBq). This accuracy included image quantitation effects, particularly assumptions concerning attenuation correction. The average absolute percent error of the estimated activity in four distinct radioactive volumes in the phantom was 9.8%. It was found that the background activity within the phantom was estimated to be too high if sampling regions near strong sources were used in the analysis (scatter effect). This was minimized by going at least 2 cm away from such sources. By applying the method to a monoclonal antibody clinical study, activities within the patient’s major organs such as liver, spleen, and kidney could be estimated, even in cases where the organ could not be visualized. Here, the CAMI algorithm gave internally consistent results for the patient’s left and right lung linear activity concentrations. The CAMI technique resolves the problem of tissue superimposition using depth information from 3‐D CT and is applicable in cases where a number of organs overlap in the gamma cameraimage. Thus, the method should be generally useful to nuclear image quantitation and the estimation of absorbed radiation doses in patients. One particular application is the estimation of radiation doses in radioimmunotherapy (RIT).
23(1996); http://dx.doi.org/10.1118/1.597838View Description Hide Description
Capable of simultaneous x‐ray and nuclear‐medicine imaging, the emission–transmission computed tomography (ETCT) system is a unique approach to the problems of quantitation and localization in radionuclide measurements. Several potential improvements to the ETCT front‐end are assessed in terms of their benefit for quantitative measurements. First, a ‘‘triple‐mode’’ readout circuit has been designed to improve scatter rejection without sacrificing flexibility. The circuit can be operated in three modes: a slow pulse‐counting mode for detection of radionuclide events with excellent energy resolution, a fast pulse‐counting mode for simultaneous acquisition of emission and transmission data or for collection of dual‐energy x‐ray data at high rates with moderate energy resolution; and current‐mode operation for acquisition of x‐ray transmission data. Next, cadmium‐telluride and cadmium‐zinc‐telluride detectors are evaluated. These materials offer room‐temperature operation, large size, and good stopping efficiency; however, they also exhibit limited energy resolution and poor charge collection. It is shown that their most serious limitation is low photopeak efficiency. Thus, methods for reduction of charge‐trapping losses are proposed. Finally, the benefits of excellent scatter rejection are assessed. Although scatter introduces a measurable error, the effects of attenuation and collimator blur are more significant. In addition, at an energy resolution of several keV, the error due to scatter is small compared to the statistical uncertainty in quantitative measurements.