Edward L. Nickoloff

A simplified approach for modulation transfer function determinations in computed tomography

In order to determine the modulation transfer functions (MTFs) for x-ray computed tomography (CT) scanners, a measurement must be performed to obtain either the point spread function (PSF) or the line spread function (LSF). Thereafter, the usual procedure is to interpolate between the measured points and to determine the Fourier transforms numerically in order to obtain the MTF. Since this must usually be done many times to evaluate various reconstruction kernels and scan modalities, the process is tedious. Fortunately, it can be greatly simplified by utilizing a mathematical function to describe the PSF or LSF. Measured data for five CT scanners indicates that the PSF can usually be described by a Gaussian function. Hence, the MTF can be written in a generalized form eliminating the necessity of performing Fourier transformations each time. The MTF is determined directly from a single performance characteristic related to the full width at half maximum. The accuracy of the approach is compared with detailed MTF calculations for five CT scanners and it is shown to agree favorably with this data.

Normal thermographic standards for the cervical spine and upper extremities.

Although thermography has been used for a variety of abnormal conditions, extensive data on large, relatively asymptomatic populations has heretofore not been available. More specifically, no data deal with the upper extremities and, more particularly, no analyses are based on simultaneous thermograms of the posterior neck and shoulders. The current study undertook this task.The results confirm the existence of thermal symmetry in the overwhelming majority of 100 normal relatively asymptomatic, actively employed factory workers. Conversely, if persistent, statistically significant thermal asymmetry exists, as outlined and correlates with patient symptomatology, an organic basis for it should be sought.

Mammographic resolution: Influence of focal spot intensity distribution and geometry

The influence of focal spot intensity distribution and geometry upon mammographic image quality were evaluated. The modulation transfer functions (MTFs) for eight different intensity distributions were determined and plotted in a manner to eliminate the effects of magnification and focal spot dimension. The results indicated that the total cross-sectional area is important for focal spots with uniform intensity distributions and equivalent diameters. For equivalent focal spot dimensions, intensity distributions with edge bands were shown to have less spatial resolution than uniform intensity distributions. Focal spots with greater intensities towards their centers provided better resolution than either uniform intensity distributions or distributions with edge bands for equivalent sizes. The type of intensity distribution was also shown to affect the accuracy of star pattern measurements of focal spot size; this method of measurement is only precise for a uniform square intensity distribution. Errors obtained with several other intensity distributions were tabulated. The variations of the effective focal spot size with position along the anode-cathode axis were shown to be of a factor of approximately two to three. The combined effects of geometric blur and film/screen blur were present for various heights above the cassette tray on several different mammographic systems.

Influence of Flat-Panel Fluoroscopic Equipment Variables on Cardiac Radiation Doses

PurposeTo assess the influence of physician-selectable equipment variables on the potential radiation dose reductions during cardiac catheterization examinations using modern imaging equipment.MaterialsA modern bi-plane angiography unit with flat-panel image receptors was used. Patients were simulated with 15–30 cm of acrylic plastic. The variables studied were: patient thickness, fluoroscopy pulse rates, record mode frame rates, image receptor field-of-view (FoV), automatic dose control (ADC) mode, SID/SSD geometry setting, automatic collimation, automatic positioning, and others.ResultsPatient radiation doses double for every additional 3.5–4.5 cm of soft tissue. The dose is directly related to the imaging frame rate; a decrease from 30 pps to 15 pps reduces the dose by about 50%. The dose is related to [(FoV)−N] where 2.0 < N < 3.0. Suboptimal positioning of the patient can nearly double the dose. The ADC system provides three selections that can vary the radiation level by 50%. For pediatric studies (2–5 years old), the selection of equipment variables can result in entrance radiation doses that range between 6 and 60 cGy for diagnostic cases and between 15 and 140 cGy for interventional cases. For adult studies, the equipment variables can produce entrance radiation doses that range between 13 and 130 cGy for diagnostic cases and between 30 and 400 cGy for interventional cases.ConclusionsOverall dose reductions of 70–90% can be achieved with pediatric patients and about 90% with adult patients solely through optimal selection of equipment variables.

Pediatric high KV/filtered airway radiographs: comparison of CR and film-screen systems

The imaging of pediatric airways presents a challenge because of the superimposition of the airway over the bone of the spine on the AP view. In recent years, some radiology departments have replaced conventional X-ray films by computed radiography (CR). The effect of the various changes upon image quality and radiation doses has not been clearly demonstrated. The goal of this paper was to investigate and identify potential improvements and/or degradations to pediatric airways imaging from the application of new technology, in particular to high KV/filtered radiographs; a new filter was designed. Two modern film-screen combinations and a CR system were evaluated for a range of tube potentials from 60 to 140 kVp. The spatial resolutions were measured for different geometrical magnifications. Relative radiation doses were also determined. Clinical airway images of children taken with the different imaging methods were subjectively compared. Our study confirmed that the visualization of the pediatric airways is enhanced by using high X-ray tube potentials with proper X-ray beam filtration. For CR systems, the selection of the cassette size, cassette type, focal spot, and geometrical magnification impact upon the image quality. Despite the increased dynamic range and image processing advantage with CR systems, CR techniques need to be improved to be more comparable with high kVp filtered magnification radiographs using film screens and small X-ray tube focal spots. With appropriate X-ray beam filtration and high kVps, CR image receptors can provide adequate image quality for pediatric airway imaging. However, the transition to digital radiography involves certain caveats. In general, radiation doses with CR systems are greater than typical doses with film-screen systems.

Evaluation of a cinevideodensitometric method for measuring vessel dimensions from digitized angiograms.

A cinevideodensitometric method for measuring the dimensions of small vessels by computer analysis of digitized cineangiograms was developed and validated in radiographic phantom models. With this method, which is based on full-width-at-half-maximum analysis of videodensitometric profile curves, the diameters of contrast-filled plexiglass cylinders ranging from 1.78 mm to 4.14 mm in diameter were measured to within 2% mean error. The theoretical basis for this method of cinevideodensitometric analysis is provided.

A quality-control phantom for digitization of radiographs

Quality control is fundamental to the clinical application of digital radiography. A 14×17-in phantom radiograph was designed to test digital image quality by measurement of five parameters: high-contrast spatial resolution, low-contrast discrimination, linearity of gray-scale response, high-frequency noise, and geometric distortion. The phantom was used to evaluate the AT&T-Philips CommView picture archival and communications system (AT&T Bell Laboratories, West Long Branch, NJ; Philips Medical Systems, Shelton, CT). High-contrast resolution was found to be greater along the diagonal axis of the system than along either the horizontal or vertical axis. Problems with low-contrast discrimination and linearity of gray-scale response were identified. This phantom provides a simple tool for daily quality assurance testing and an objective standard for comparison of image quality between different digital radiography systems.

Proposition: a pregnant resident physician should be excused from training rotations such as angiography and nuclear medicine because of the potential exposure of the fetus.

It has been reasonably well documented that a pregnant resident physician can assume radiology rotations, including higher-exposure rotations such as angiography and nuclear medicine, without exposing the fetus to radiation levels that exceed national and international guidelines. Hence, many medical physicists support the contention that rotations should not be altered because a resident is pregnant. On the other hand, many if not most physicists subscribe to the ALARA (as low as reasonably achievable) principle, especially in cases of fetal exposure where increased radiation susceptibility is combined with an inability to decide for one-self. In addition, altered rotations usually can be accommodated by swapping rotations with other residents, with the pregnant resident taking high exposure rotations after delivery of the child. Policies on this issue vary among institutions, possibly because medical physicists have not come to closure on the issue. This issue of Point/Counterpoint is directed toward that objective.

SU-GG-I-07: Monitoring Radiation Dose in Cerebral CT Perfusion Exams

PURPOSE: This paper is to study how to monitor and manage the radiation dose in cerebral CT perfusion exams with various CTscanners including a 10 slice MDCT, a 64 slice VCT and a 320 detector row volume scanner. METHODS & MATERIALS: Several cerebral CT perfusion protocols have been studied with various tube current settings. Radiation dose measurements were done on an anthropormorphic head phantom using an optically stimulated luminescence(OSL) dosimeter device (MicroStar, Landauer) and an ion chamber dosimeter (Accu‐Pro, RadCal). All the measurements were done at the skin surface. Several OSLs were used to cover the X‐ray slice beam width. The readout of OSLs was performed right on site with a portable reading device. RESULTS: The accuracy of the OSLs was verified through a side‐by‐side comparison with an ion chamber dosimeter. It is shown that the OSL sensor is angle independent that makes it advantageous over those solid state personal dosimeters for real‐time CT dose measurements. For our study, 80 kVp was utilized for the cerebral CT perfusion exams. The tube current varied from 80 mA to 200 mA. The radiation dose varied directly with the selection of tube voltage and current settings. Considerable variations were also observed among different types of CTscanners. Conclusion: Since there is a potential risk for the skin dose to be accumulated beyond the threshold for determinant effects such as skin erythema and epilation in CT perfusion exams, it is important to optimize the protocol and monitor the patient dose closely. This paper verifies a tool to do that. It can be utilized during the initial set‐up of the protocol. Also, it will be useful for individual patient dose monitoring from case to case. It adds the safeguard to high dose CT scans such as cerebral CT perfusion exams.

SU‐FF‐I‐09: Pediatric Patient Dose Management From a 64 Slice VCT

Purpose: As the clinical utilization of CT grows, it becomes more important to manage patient dose without compromising image quality, especially in children. A special effort should be made to reduce pediatric patient dose through age‐ and size‐specific protocols. Methods & Materials: A pediatric 64 slice VCT scanner(GE Lightspeed) was tested with a group of cylindrical acrylic phantoms with diameters ranging from 6 – 32 cm. In addition, anthropomorphic phantoms (CIRS adult and pediatric dosimetry verification phantoms) were employed to correlate the CTDI values with the skin doses measured by a solid state dosimeter (Unfors PSD). The dose affecting factors included: kVp, mAs, beam filtration, beam collimation, pitch, patient size, detector configuration and dose reduction techniques such as mA modulation and post‐processing. Various techniques and their combinations were included in this study. Finally, clinical protocols for pediatric applications were evaluated and adjusted based upon the measured patient dose and image quality. Results: The automated CTDI values displayed on the system agreed with our measurements when the standard phantom sizes were used, i.e., 16 cm in diameter for head, 32 cm for adult body and 16 cm for pediatric body. However, the measured dose differed from the automated CTDI by a factor of 1.72 for a reduced head phantom size of 6 cm in diameter and a factor of 3.2 for a reduced body phantom size of 10 cm in diameter. Patient age also played an important role in estimating effective dose. The changing beam filtration caused a variation of up to 42% in the in‐air dose output. Concurrently, noise from the phantom images was evaluated. Conclusion: The clinical protocols were established based upon the dose level corresponding to the patient size and age as well as the tolerable noise level corresponding to the specific clinical applications.