The performance characteristics of a photostimulable phosphor based computed radiographic (CR) system were studied. The modulation transfer function (MTF), noise power spectra (NPS), and detective quantum efficiency (DQE) of the Kodak Digital Science computed radiography (CR) system (Eastman Kodak Co.-model 400) were measured and compared to previously published results of a Fuji based CR system (Philips Medical Systems-PCR model 7000). To maximize comparability, the same measurement techniques and analysis methods were used. The DQE at four exposure levels (30, 3, 0.3, 0.03 mR) and two plate types (standard and high resolution) were calculated from the NPS and MTF measurements. The NPS was determined from two-dimensional Fourier analysis of uniformly exposed plates. The presampling MTF was determined from the Fourier transform (FT) of the system's finely sampled line spread function (LSF) as produced by a narrow slit. A comparison of the slit type ("beveled edge" versus "straight edge") and its effect on the resulting MTF measurements was also performed. The results show that both systems are comparable in resolution performance. The noise power studies indicated a higher level of noise for the Kodak images (approximately 20% at the low exposure levels and 40%-70% at higher exposure levels). Within the clinically relevant exposure range (0.3-3 mR), the resulting DQE for the Kodak plates ranged between 20%-50% lower than for the corresponding Fuji plates. Measurements of the presampling MTF with the two slit types have shown that a correction factor can be applied to compensate for transmission through the relief edges.

The slit camera was analyzed in order to establish its utility and limitations as an MTF measurement tool for characterizing radiographic imaging systems. Commercial slit cameras are attractive for MTF measurements because the beveled edges significantly reduce their alignment sensitivity as compared to the conventional parallel jaw slit. Radiation passing through the beveled edges increases the effective width of the slit camera so that a correction based on the nominal slit width would leave residual error in the MTF measurement. Experimental and Monte Carlo simulated MTF measurements were made on a slit camera (10 microm nominal slit width) in order to estimate its sensitivity in alignment, quantify the error in MTF due to transmission through the beveled jaws, and provide a correction factor. The alignment tolerances of the slit camera were found to be about 12 times larger than for the parallel jaw slit at small HVLs (approximately 1.3 mm Al) of the incident beam and 9 times larger at higher HVLs (approximately 7 mm Al). The magnitude of the residual error in MTF was dependent on the quality of the incident spectrum. For incident spectra with high kVp and HVL (> or = 120 kVp, > or =5 mm Al HVL), transmission through the beveled edges produced errors in MTF up to 15% at 5 cycles/mm and 30% at 10 cycles/mm. By assuming a rectangular slit profile with an effective width based on the kVp, HVL, and filtration material of the incident beam, an MTF correction factor was determined. Application of this correction factor reduced the errors to less than 4% up to 10 cycles/mm. At low beam energies and spatial frequencies, the correction is less critical. Ease of alignment and greater availability make a commercial slit camera useful for MTF measurements. Accurate MTF measurements can be made if appropriate correction factors are applied.

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Purpose: Head and Neck Image Guided Radiation Therapy (IGRT) is of vital importance particularly with the advent of Intensity Modulated Radiation Therapy (IMRT). The responsibility of IGRT rests with the treating radiation therapy team, therefore it is essential that they undertake IGRT in a uniform manner. This study represents an assessment of head and neck IGRT analysis across multiple radiation oncology departments, inclusive of radiation therapist seniority, image modality and anatomical variation within the treatment volume. Methods: Site visits were undertaken at three radiation oncology departments. At each site kV and MV image datasets were analysed by one senior radiation therapist and one junior radiation therapist. The influence of differing sites, radiation therapist seniority, image modality and cervical vertebra position on head and neck IGRT was then assessed. Results: Statistical analysis of the diagnosed field placement errors indicated that head and neck IGRT was undertaken consistently regardless of site, radiation therapist seniority, image modality and the position of the cervical vertebra. Conclusion: In the era of sophisticated treatment planning it is vital that the treatment delivery mechanism (IGRT) is performed consistently. Head and neck IGRT is the responsibility of the treating radiation therapist regardless of site, seniority, image modality and positioning challenges.