A system for interactive film analysis.

An interactive, minicomputer system has been constructed for analyzing dynamic phenomena recorded on movie film in a developmental biology laboratory. The minicomputer interfaces a stop-motion, variable speed projector, a digitizing pen, and real-time graphics display equipment. An analyst uses the pen to digitize features in a film, e.g. by following a cell. A computer-generated animation portraying all data entered is superimposed on the film image and synchronized with it. Noteworthy system features include: image overlays on a large screen, data entry with the projector running, large data capacity, computer control of the projector, and convenient data entry tools.     

Transverse tomography with incoherent optical reconstruction

A thin layer of an object can be imaged by reconstruction from a so-called sinogram. It is produced by an x-ray fan beam rotating around the object while a recording film is moved in a direction perpendicular to the plane of the fan beam. Before reconstruction the sinogram image is convoluted according to a special function to remove artifacts consisting of spurious shadows between different object elements. The reconstruction is done from the convoluted sinogram by means of a back projector, which operates according to a prinicple that is the reverse of the recording of the original sinogram. Tomograms of phantoms, pork chops and the head of a dog show that the process is capable of high spatial resolution but is limited by low contrast.     

Automatic read-out device for ultraviolet-radiation polymer-film dosimeters

An automatic device for measuring the optical absorbance of polymer film used as an ultravioletradiation dosimeter is described. The automated mechanics are based on a commercial slide projector. The ultraviolet radiation is generated by a tungsten quartz-halogen bulb. The optical absorbance is determined by analogue logarithmic circuitry and the resultant value displayed on a digital panel meter. A digital printer or paper-tape punch may be used for hard-copy output. An uncertainty in measurement of 1% is obtained. The device temperature stability is such that this uncertainty can be maintained for analogue circuit temperatures of 20–40°C.     

Fully 3D Monte Carlo reconstruction in SPECT: a feasibility study

In single photon emission computed tomography (SPECT) with parallel hole collimation, image reconstruction is usually performed as a set of bidimensional (2D) analytical or iterative reconstructions. This approach ignores the tridimensional (3D) nature of scatter and detector response function that affects the detected signal. To deal with the 3D nature of the image formation process, iterative reconstruction can be used by considering a 3D projector modelling the 3D spread of photons. In this paper, we investigate the value of using accurate Monte Carlo simulations to determine the 3D projector used in a fully 3D Monte Carlo (F3DMC) reconstruction approach. Given the 3D projector modelling all physical effects affecting the imaging process, the reconstruction problem is solved using the maximum likelihood expectation maximization (MLEM) algorithm. To validate the concept, three data sets were simulated and F3DMC was compared with two other 3D reconstruction strategies using analytical corrections for attenuation, scatter and camera point spread function. Results suggest that F3DMC improves spatial resolution, relative and absolute quantitation and signal-to-noise ratio. The practical feasibility of the approach on real data sets is discussed.     

Development and initial evaluation of a spectral microdensitometer for analysing radiochromic films

Radiation dose deposited on a radiochromic film is considered as a dose image. A precise image extraction system with commensurate capabilities is required to measure the transmittance of the image and translate it to radiation dose. This paper describes the development of a spectral microdensitometer which has been designed to achieve this goal under the conditions of (a) the linearity and sensitivity of the dose response curve of the radiochromic film being highly dependent on the wavelength of the analysing light, and (b) the inherent high spatial resolution of the film. The microdensitometer consists of a monochromator which provides an analysing light of variable wavelength, a film tray on a high-precision scanning stage, a transmission microscope coupled to a thermoelectrically cooled CCD camera, a microcomputer and corresponding interfaces. The measurement of the transmittance of the radiochromic film is made at the two absorption peaks with maximum sensitivities. The high spatial resolution of the instrument, of the order of micrometres, is achieved through the use of the microscope combined with a measure-and-step technique to cover the whole film. The performance of the instrument in regard to the positional accuracy, system reproducibility and dual-peak film calibration was evaluated. The results show that the instrument fulfils the design objective of providing a precise image extraction system for radiochromic films with micrometre spatial resolution and sensitive dose response.     

A prototype quantitative film scanner for radiochromic film dosimetry

We have developed a high resolution, quantitative, two-dimensional optical film scanner for use with a commercial high sensitivity radiochromic film (RCF) for measuring single fraction external-beam radiotherapy dose distributions. The film scanner was designed to eliminate artifacts commonly observed in RCF dosimetry. The scanner employed a stationary light source and detector with a moving antireflective glass film platen attached to a high precision computerized X-Y translation stage. An ultrabright red light emitting diode (LED) with a peak output at 633 nm and full width at half maximum (FWHM) of 16 nm was selected as the scanner light source to match the RCF absorption peak. A dual detector system was created using two silicon photodiode detectors to simultaneously measure incident and transmitted light. The LED light output was focused to a submillimeter (FWHM 0.67 mm) spot size, which was determined from a scanning knife-edge technique for measuring Gaussian optical beams. Data acquisition was performed with a 16-bit A/D card in conjunction with commercial software. The linearity of the measured densities on the scanner was tested using a calibrated neutral-density step filter. Sensitometric curves and three IMRT field scans were acquired with a spatial resolution of 1 mm for both radiographic film and RCF. The results were compared with measurements taken with a commercial diode array under identical delivery conditions. The RCF was rotated by 90 deg and rescanned to study orientation effects. Comparison between the RCF and the diode array measurements using percent dose difference and distance-to-agreement criteria produced average passing rates of 99.0% using 3%/3 mm criteria and 96.7% using 2%/2 mm criteria. The same comparison between the radiographic film and diode array measurements resulted in average passing rates 96.6% and 91.6% for the above two criteria, respectively. No measurable light-scatter or interference scanner artifacts were observed. The RCF rotated by 90 deg showed no measurable orientation effect. A scan of a 15 x 15 cm2 area with 1 mm resolution required 22 min to acquire. The LED densitometer provides an accurate film dosimetry system with 1 mm or better resolution. The scanner eliminates the orientation dependence of RCF dosimetry that was previously reported with commercial flatbed scanners.     

Blood film review by biomedical scientists

This study aims to provide evidence of biomedical scientists' competence in blood film cytomorphology and to improve continuing assessment and training. Twenty photomicrographs are prepared from historical teaching slides, and a slide projector timer is designed and built to trigger slide advance automatically every 30 sec. Haematology staff in local district general hospitals take part in the study. During each visit, the test is explained and participants mark their observations on individual results sheets. Two sets (accepted and rejected) of observations are generated, which are validated by peer review. The accepted set is scored. No evidence is found to prove that members of each department were drawn from separate groups (P=0.36). Seniors and routine cytomorphologists are compared for each department and a non-zero difference in score was found (P=0.00). A comparison is made between the number of times a scored or non-scored observation is recorded, resulting in a probability that approaches zero. Thus, it is highly unlikely that the observations are drawn from one pool, and the test score and ability in cytomorphology would appear to be directly related. This study challenges the notion that unrestricted time, clinical data and a microscope are required for haematology blood film cytomorphology tests. Introduction of this type of test would provide biomedical science with a valuable assessment tool.     

Metal screen-film detector MTF at megavoltage x-ray energies

The MTF of metal screen film detectors used in radiation treatment verification has been measured at 4 and 8 MV x-ray energies. The results show that lead screens provide better resolution than copper screens, and a single-emulsion film offers considerable advantage over the traditional double-emulsion film. A rear lead screen was found to seriously degrade the resolution properties of a front lead screen single-emulsion film detector. The detector MTF was found to be energy dependent. In general, both the low and the high spatial frequency response decreased with increasing x-ray energy. This, in part, accounts for the noticeable image quality difference between 4 and 8 MV radiographs.     

Quantitative evaluation of film-screen combinations for x-ray diagnosis

The properties of screen/film combinations for radiographs set a lower limit for the x-ray exposure of the patient and an upper limit for the quality of the x-ray picture. Sensitivity, slope and resolution of different screen/film combinations were determined using a measuring phantom which was developed in the PTB. For all screens used the measurements show the same relation between screen sensitivity and resolution. This allows quantitative evaluation of image quality. A classification scheme derived from these results facilitates the selection of screen/film combinations for practical use. In addition for quality assurance gross differences in material properties and conditions of film development can be detected with the aid of the measuring phantom.     

Use of multiple layers of Gafchromic film to increase sensitivity.

This note investigates the increase of the sensitivity of radiochromic film by layering the film together. Investigations with up to five layers of radiochromic film placed together as a single dosimeter have allowed accurate two-dimensional dosimetry at low radiotherapy doses. The sensitivity of the response of the layered film increases with the number of layers, with the five-layer dosimeter increasing the change in optical density by 4.3 +/- 0.2 times that for a single layer film at 670 nm readout wavelength. Measurements in the penumbral region of a 6 MV x-ray beam show that the spatial resolution is slightly reduced with the layered film. The distance across the region from 20 to 80% of the maximum dose at the beam edge (20/80% penumbra) measured at 1.5 cm depth was measured as 3.1 +/- 0.3 and 3.5 +/- 0.3 mm for one and five layers, respectively. Thus no major reduction in spatial resolution was found by layering the film to five layers but the sensitivity was increased by 430% at 670 nm readout wavelength.     

The performance of a CCD digital autoradiography imaging system

Autoradiography is a widely used technique for imaging trace quantities of radioactivity within biological samples, conventionally using photographic film. This method produces images with high spatial resolution, but it suffers from very low sensitivity and poor dynamic range. Digital autoradiography systems with greatly improved sensitivity and linearity are commercially available, but the spatial resolution is usually much less than that achieved using film. We report here the design, construction and characterization of a novel digital autoradiography system based on scientific-grade charged coupled devices (CCDs). Images of x-ray and beta emissions from radionuclides commonly used in autoradiography show that the system can perform high-speed quantitative imaging with a spatial resolution of approximately 30, microm. Using a frame by frame acquisition method the dynamic range is shown to be at least three orders of magnitude. The absolute detection efficiency is comparable to the best of the currently available digital systems. CCD images of 125I and 14C radioisotope distributions in tissue samples are superior to the equivalent film images and have been acquired in 1-10% of the time.     

The spatial resolution in dosimetry with normoxic polymer-gels investigated with the dose modulation transfer approach

The verification of dose distributions with high dose gradients as appearing in brachytherapy or stereotactic radiotherapy for example, calls for dosimetric methods with sufficiently high spatial resolution. Polymer gels in combination with a MR or optical scanner as a readout device have the potential of performing the verification of a three-dimensional dose distribution within a single measurement. The purpose of this work is to investigate the spatial resolution achievable in MR-based polymer gel dosimetry. The authors show that dosimetry on a very small spatial scale (voxel size: 94 x 94 x 1000 microm3) can be performed with normoxic polymer gels using parameter selective T2 imaging. In order to prove the spatial resolution obtained we are relying on the dose-modulation transfer function (DMTF) concept based on very fine dose modulations at half periods of 200 microm. Very fine periodic dose modulations of a 60Co photon field were achieved by means of an absorption grid made of tungsten-carbide, specifically designed for quality control. The dose modulation in the polymer gel is compared with that of film dosimetry in one plane via the DMTF concept for general access to the spatial resolution of a dose imaging system. Additionally Monte Carlo simulations were performed and used for the calculation of the DMTF of both, the polymer gel and film dosimetry. The results obtained by film dosimetry agree well with those of Monte Carlo simulations, whereas polymer gel dosimetry overestimates the amplitude value of the fine dose modulations. The authors discuss possible reasons. The in-plane resolution achieved in this work competes with the spatial resolution of standard clinical film-scanner systems.     

Temporomandibular joint pantomography using charge-coupled device,photostimulable phosphor,and film receptors: A comparison

Our objective was to compare the accuracy and practicality in use of three available imaging receptors for temporomandibular joint (TMJ) imaging; namely, two computer-assisted and one traditional analog x-ray film system. A standardized tissue-equivalent encased human skull specimen was imaged using lateral and posteroanterior (PA) pantomographic projections with the Orthopantomograph OP 100 (Instrumentarium Imaging, Tuusula, Finland) and three different receptor modalities: (1) Ektavision film with Ektavision screens (Eastman Kodak, Rochester, NY); (2) DenOptix photostimulable phosphor screens (Dentsply/Gendex, Chicago, IL); and (3) the charge-coupled device (CCD) receptor, DigiPan (TREX/Trophy Radiology, Marne-la-Vallée, France). The effective focal trough was found for each receptor using lead resolution grids placed at fractional millimeter distances along empirically determined beam projection angulations. The time to acquire and process images was also established. We found that the CCD system permitted real-time display, whereas the use of traditional film took 2 minutes to load the cassette in a darkroom and perform the exposure, and then a further 2 minutes to unload and process. The storage phosphor took 3 minutes to unload the cassette and process the image and a further 20 seconds to clear the plate following laser scanning. Film produced the greatest maximum resolution followed by the storage phosphor and the CCD. In conclusion, CCD-based TMJ pantomography provided an instant image. The photostimulable phosphor system used was the least satisfactory in terms of the time expended to obtain an image, but provided better spatial resolution than the CCD. Ektavision film/screens provided the best spatial resolution in this investigation.     

LOR-OSEM: statistical PET reconstruction from raw line-of-response histograms

Iterative statistical reconstruction methods are becoming the standard in positron emission tomography (PET). Conventional maximum-likelihood expectation-maximization (MLEM) and ordered-subsets (OSEM) algorithms act on data which have been pre-processed into corrected, evenly-spaced histograms; however, such pre-processing corrupts the Poisson statistics. Recent advances have incorporated attenuation, scatter and randoms compensation into the iterative reconstruction. The objective of this work was to incorporate the remaining pre-processing steps, including arc correction, to reconstruct directly from raw unevenly-spaced line-of-response (LOR) histograms. This exactly preserves Poisson statistics and full spatial information in a manner closely related to listmode ML, making full use of the ML statistical model. The LOR-OSEM algorithm was implemented using a rotation-based projector which maps directly to the unevenly-spaced LOR grid. Simulation and phantom experiments were performed to characterize resolution, contrast and noise properties for 2D PET. LOR-OSEM provided a beneficial noise-resolution tradeoff, outperforming AW-OSEM by about the same margin that AW-OSEM outperformed pre-corrected OSEM. The relationship between LOR-ML and listmode ML algorithms was explored, and implementation differences are discussed. LOR-OSEM is a viable alternative to AW-OSEM for histogram-based reconstruction with improved spatial resolution and noise properties.     

The influence of noise in full Monte Carlo ML-EM and dual matrix reconstructions in positron emission tomography

Monte Carlo (MC) simulations in positron emission tomography (PET) play an important role in detector modeling and algorithm testing. Whereas the simulations are widely used in a forward projection manner to accomplish this task, ideally they should be included into the reconstruction process itself. It is therefore desirable to investigate the convergence properties and the propagation of MC noise of these kinds of reconstruction algorithms. MC simulations were integrated into the maximum likelihood expectation maximization (ML-EM) algorithm in two different ways. In the full matrix approach the system matrix was calculated by running MC simulations, including scatter. This matrix was used in both the projector and the backprojector. In the dual matrix (DM) approach, MC simulations were used to incorporate scatter in the projector, whereas the backprojector only comprised attenuation. Repeated reconstructions with different MC seeds allowed a statistical analysis of the error at each iteration step and made it possible to investigate separately the propagation of the MC noise that was introduced by the sinogram, by the projector, and by the matrix. Both approaches resulted in similar images, but the DM approach with unmatched projector and backprojector yielded a faster initial convergence when compared to the ideal full matrix approach. The analysis of the noise sources for the modeled single ring scanner in full matrix reconstruction showed that the noise introduced by the matrix became comparable to the noise introduced by the sinogram when using a matrix that was simulated with 10,000 emissions/voxel.     

Contrast information of digital imaging in laser film digitizer and display monitor

A 10-bit or 12-bit gray scale is provided in commercial laser film digitizers. The true contrast resolution on the digitized image within a laser spot area of 200 microns in diameter is limited by both the quantum mottle and instrumentation noise. In this report, we investigated the mean value, standard deviation, and adjacent pixel correlation coefficient on a calibrated step wedge film with two laser digitizers. The results were disappointing, because we found that the evaluated contrast information is inferior to the manufacturers' specifications. On the output side, the brightnesses of different gray levels from a clinical monitor were measured with a narrow angle luminance probe and evaluated by a brief human perception study. In addition, the implications for teleradiology applications are discussed.     

Quality assurance of picture archiving communication systems with laser film digitizers

A comprehensive quality assurance (QA) program should be implemented for all teleradiology and picture archival and communications (PACS) systems. In this report, we summarize our QA experience with a teleradiology system that includes a laser digitizer for x-ray film. A key component required for the evaluation of laser film scanners is an appropriate test pattern; digitizers should be evaluated with enhanced test patterns specifically designed for this purpose. The phantom pattern should measure high-contrast resolution, low-contrast discrimination, gray scale linearity, geometric distortion, and noise. In addition, a uniformly exposed sheet of film (approximately 0.3 optical density) serves as a good phantom for testing screen nonuniformities of viewing station monitors. It is also suggested that clinical images should be included in a QA program. Finally, it is recommended that any discrepancies in the interpretation of teleradiology images should be monitored and investigated.     

Measurement of high energy x-ray beam penumbra with Gafchromic EBT radiochromic film

High energy x-ray beam penumbra are measured using Gafchromic EBT film. Gafchromic EBT, due to its limited energy dependence and high spatial resolution provide a high level of accuracy for dose assessment in penumbral regions. The spatial resolution of film detector systems is normally limited by the scanning resolution of the densitometer. Penumbral widths (80%/20%) measured at Dmax were found to be 2.8, 3.0, 3.2, and 3.4 mm (+/- 0.2 mm) using 5, 10, 20, and 30 cm square field sizes, respectively, for a 6 MV linear accelerator produced x-ray beam. This is compared to 3.2 mm +/- 0.2 mm (Kodak EDR2) and 3.6 mm +/- 0.2 mm (Kodak X-Omat V) at 10 cm x 10 cm measured using radiographic film. Using a zero volume extrapolation technique for ionization chamber measurements, the 10 cm X 10 cm field penumbra at Dmax was measured to be 3.1 mm, a close match to Gafchromic EBT results. Penumbral measurements can also be made at other depths, including the surface, as the film does not suffer significantly from dosimetric variations caused by changing x-ray energy spectra. Gafchromic EBT film provides an adequate measure of penumbral dose for high energy x-ray beams.     

Comparative Studies on Tree Pollen Allergens

Visualization by autoradiography of specific IgE binding in crossed radioimmunoelectrophoresis (CRIE) and other 125I-immunoautoradiography (IAR) techniques is done in two different ways; either by traditional direct autoradiography (D-ARG), where the film is exposed to the 125I-anti-IgE incubated sample at room temperature, or by indirect autoradiography (ID-ARG), applying intensifying screen, low-temperature exposure and, eventually, pre-exposure. This study confirmed that D-ARG provided the benefits of simplicity and better image resolution with the disadvantage of prolonged exposure periods. ID-ARG reduced the exposures needed to produce film image densities of 0.01 and 0.1 A540 nm (i.e. autoradiographic sensitivity (AR sigma) and autoradiographic speed (ARs] to 1/18 and 1/55 respectively of the corresponding exposures in D-ARG. The lowest detection limits for 125I in 24 h were 1.2 cpm mm-2 with the indirect and 6.8 cpm mm-2 with the direct systems investigated. The major drawbacks of ID-ARG were inferior image resolution and higher background levels, especially when pre-exposure was included.     

Photon energy dependence of the sensitivity of radiochromic film and comparison with silver halide film and LiF TLDs used for brachytherapy dosimetry

There is a new radiochromic film, a highly uniform, thin (100-microns) detector whose sensitive layer (6 microns thick) changes from colorless to blue by dye polymerization without processing, upon exposure to ionizing radiation. Because the dose gradients around brachytherapy sources are steep, the high spatial resolution offered by film dosimetry is an advantage over other detectors such as thermoluminescent dosimeters (TLDs). This compares the photon energy dependence of the sensitivities of GafChromic film, silver halide verification film (Kodak X-Omat V Film), and lithium fluoride TLDs (Harshaw), over the photon energy range 28 keV to 1.7 MeV, which is of interest in brachytherapy. Sensitivity of the radiochromic film is observed to decrease by about 30% as effective photon energy decreases from 1710 keV (4-MV x rays) to 28 keV (60-kV x rays, 2-mm A1 filter). In contrast, the sensitivity of verification film increases by 980% and that of LiF TLDs increases by 41%. The variation of the sensitivity of radiochromic film with photon energy is considerably less than that for silver halide film and similar to that for LiF TLDs, but in the opposite direction. Radiochromic film, like LIF TLDs, does not exhibit the drastic sensitivity changes below 127 keV that silver halide film exhibits. Dose distribution in the immediate vicinity of a high activity (370 GBq) brachytherapy 192Ir source has been mapped using radiochromic film and is presented to illustrate the applicability of this new technology to brachytherapy dosimetry.