Gerald Cohen

The use of contrast -- detail -- dose evaluation of image quality in a computed tomographic scanner.

Limiting-perception measurements of hole patterns in computed tomography reconstructions of a low-contrast phantom are used to generate contrast-detail-dose diagrams for various contrasts, hole diameters, dose, and system modulation transfer functions (MTF). These curves are then divided into MTF-dominated, transition, and noise-dominated regions. The low-contrast perceptibility and image noise are parameterized and compared. A limiting-perception factor is defined and is shown to be nearly independent of contrast, hole diameter, dose, and MTF in the noise-dominated region.

Contrast-detail-dose analysis of six different computed tomographic scanners

Low-contrast visibility of various rotate-translate, rotating detector, and stationary detector computed tomography (CT) scanners is measured at contrasts ranging from 0.3 to 3% using a phantom based on the partial volume of water and polystyrene. The results are plotted on log-log curves, as a function of entrance dose, to provide a family of contrast (C)-detail (d)-dose (D) visibility curves. The transition region, ∼ 1 to 10% contrast, depends on modulation transfer function (MTF), contrast, and dose (noise), while the noise region, ≤ 1% contrast, is independent of MTF. The slopes of the curves in the noise region are consistent with a value of −1, as opposed to a theoretically expected value of −3/2 for ideal CT noise. Relative dose efficiency of various scanners is examined by plotting CdD1/2 versus contrast for both peak dose (Dp) and summation dose (DΣ). The rotating detector systems are shown to be, on the average (at 0.5% contrast), 3.4 (Dp) and 4 (DΣ) times more dose efficient than the stationary array system examined, and 2.4 (Dp) and 3.2 (DΣ) times more efficient than the rotate-translate units examined.

Digital radiography using a computed tomographic instrument

A prototype computed radiography (CR) system was evaluated for its efficacy as an independent diagnostic modality. Preliminary measurements of high contrast resolution, low contrast perceptibility, and dose were obtained. Clinical examinations including skull, abdomen, liver, gallbladder, biliary system, spine, and extremities were performed as an adjunct to either computed tomography or CR. The data suggest that CR can be an effective diagnostic imaging modality by itself. Advantages over conventional radiography include high scatter rejection, low patient dose, wide dynamic range, and good low contrast sensitivity for large objects; disadvantages, its long exposure time and relatively poor high contrast spatial resolution.

Absorbed Doses to Patients and Personnel from Endoscopic Retrograde Cholangiopancreatographic (ERCP) Examinations

Radiation doses to the skin, bone marrow, and gonads were determined during endoscopic retrograde cholangiopancreatographic examinations. Average patient entrance doses were 7.5, 4.9, 17.5 and 2.7 rads to the abdomen, back, and left lateral and right lateral positions, respectively. Mean active bone marrow dose was 0.4 rads and average gonadal dose was 0.2 rads. Spot radiographs and fluoroscopy provided equal contributions to patient dose. Patient entrance dose was less than that from abdominal angiography, and comparable to that from a typical upper-gastrointestinal series. The pancreatic dose was comparable to that from a radionuclide scan. Dose to personnel was negligible.

Dose efficiency and the effects of resolution and noise on detail perceptibility in radiographic magnification

The detail signal-to-noise ratio model of radiographic imaging is quantitatively analyzed in terms of its accuracy in describing observer threshold perceptibility of radiographic detail. The model is found to adequately describe the effects of magnification, scatter radiation, and system resolution on observer threshold perceptibility. However, it is shown that the model does not apply in screen/film radiography for very low contrasts and high scatter conditions due to insufficient optical density contrast. The dose-to-information conversion efficiency of a radiographic imaging system is defined and the effects of magnification, scatter, resolution, image processing, detector efficiency, grids, patient table support, field size, and geometry on the dose efficiency of the imaging system are investigated.

Quantitative assessment of field uniformity for gamma cameras.

Objective criteria for judging the uniformity of field floods are developed and the effects of the energy window and scatter material on uniformity are examined. There is a highly significant correlation (p = 0.99) between a 14% difference in average count among adjacent photomultiplier tube regions and observer detection of detuned areas. Similarly, a 10% average count difference is not significantly recognized as a nonuniform area. Results indicate that there is a complex interaction between uniformity, energy window (15-30% symmetrical) and 0-7.6 cm of tissue-equivalent scatter medium interposed between the flood source and the collimator. Quantitation of the flood should be performed to indicate which regions need retuning.

The Effects of the Film/Screen Combination on Tomographic Image Quality

Several film/screen combinations were studied to evaluate their effect on thin-section tomograms of the inner ear. Hypocycloidal images of a head phantom were subjectively ranked for image quality, and contrast, spatial frequency spectra, and Wiener noise spectra were measured. Resolution appears to be significantly improved when medium-speed screens are used instead of fast, unsharp screens; observers could perceive increased detail through the higher noise of the medium-speed screens. The authors conclude that medium-speed screens yield more diagnostic information than fast, unsharp screens in thin-section tomography of the inner ear.

Resolution in radiographic magnification

Radiographic resolution and image sharpness are analyzed as a function of magnification for a few screen/film systems and a microfocal spot x-ray tube. Resolution and sharpness are described in terms of both MTF and effective aperture. The analysis demonstrates advantages of magnification for any combination of radiographic screen/film system and x-ray focal spot, and, in addition, allows one to compare different screen/film systems each used at different magnifications. Such an analysis is applicable to any radiographic system and may serve as a guide in selecting screen/film systems for use in magnification procedures.

Energy dependence of contrast-detail-dose and object-detectability-dose curves for CT scanners

The energy dependence of contrast-detail-dose (CdD) and object-detectability-dose (OdD) curves for computed tomographic scanners is investigated. The effects of changes in beam energy on perceptibility are shown to be due to changes in signal-to-noise ratio resulting from changes in contrast and photon statistics. Energy-dependence analysis of OdD curves is shown to depend on the atomic composition of the phantom used to generate the curves, while such an analysis of CdD curves is independent of the atomic composition of the phantom. It is also shown that any OdD curve can be generated from CdD curves and that use of this fact rectifies any potential energy-dependent interpretation of CdD curves.