(a) Stepwedge radiographic exposure at 3 m from the x-ray tube focus (left) to the front of the CR cassette (in the vertical stand). The solid-state dosimeter was placed on top of the vertical CR cas- sette.
(b) Regions of interest used for analysis of the step- wedge radiographs.
Figure B.1 Experimental setup and analysis of the aluminium stepwedge used for investigation of
the computed radiography pixel intensity and noise characteristics.
B.3
Hand phantom
A hand phantom was used to simulate human anatomy so that the noise and pixel intensi- ties of the Kodak CR images could be studied without having to expose children’s hands to multiple x-rays. By designing areas of homogeneity within the phantom (see Appendix C) it was possible to determine the relationship between the x-ray factors and the noise in certain parts of the image.
The hand phantom was x-rayed using the same x-ray generator voltage as is locally used clinically for hand-wrist radiography (52 kVp), and a x-ray tube focus to CR plate distance of 100 cm. The experimental configuration is illustrated in Figure B.2, along with the Im- ageJ regions of interest used in the analysis.
All measurements were made using the x-ray tube fine focus and the images were pro- cessed using the Kodak CR body part of “Hand AP” and the low exposure optimisation, edge enhancement, and ‘EVP’ modes enabled; this was equivalent to clinical use of the system.
(a) (b)
Figure B.2 Experimental setup and analysis of the hand phantom images. (a) The radiographs
were taken using a focus-to-cassette distance of 100 cm, x-ray tube light beam diaphragm centred on the cassette, and an Unfors solid-state dosimeter at the top of the hand (anode end of the x-ray tube). The cassette was realigned before each exposure by using a thin perspex template attached to the x-ray table. (b) Radiograph of the phantom showing the regions of interest used for analysis of the pixel intensities and noise. This radiograph shows the aluminium plate used to simulate overlapping carpal bones.
Appendix C
Hand-wrist phantom
A hand phantom was designed and developed by the author1 to simulate the hand of a young adult undergoing radiography for bone age assessment. The phantom was neces- sary because the Kodak computed radiography system includes anatomical-based image processing that is only enabled when the user selects the body part being examined. In the case of the hand-wrist radiograph, the user selects the body part ”Hand AP” on the Kodak CR 900 system and image processing such as tone scaling, and edge enhancement is performed based on both the image data and preset parameters [Bogu95].
The purpose of the phantom design was to simulate the attenuation properties of the hand but without the structural complexities of bones. A simple geometry was used so that uniform areas were present in the radiograph to allow pixel intensity measurements to be made without the complication of bone structure. The materials consisted of perspex to simulate the soft-tissue of the hand, and aluminium to simulate bone. The x-ray spectrum simulation program Spectra (National Radiation Laboratory, New Zealand) was used to calculate the attenuation properties of perspex and aluminium for an x-ray spectrum based on an x-ray exposure using 52 kVp at 100 cm focus-to-cassette distance and a simulated 3 mm aluminium inherent filtration (this being typical of general purpose x-ray tubes). Under these x-rays conditions it was found that soft tissue is approximately equivalent to perspex of the same thickness. A constant soft tissue thickness of 12 mm was simulated us- ing two sheets of 6 mm perspex glued together using chloroform. No attempt was made to simulate the wedge-shaped soft-tissue thickness of the hand because of the desire to have uniform areas within the radiograph. A cortical bone thickness of 13 mm with an overly- ing 12 mm of soft tissue equated to 10 mm of aluminium and 12 mm of perspex combined. The equivalence of these simulation materials is only approximate because the calculations
1
The author would like to thank Grant Wylie, Mechanical Workshop, Medical Physics and Bioengineering Department, Christchurch Hospital, for assistance with machining parts of the phantom.
12 mm thick perspex 10 mm diameter aluminium rods 16 mm diameter aluminium rods 10 mm thick aluminium blocks (a) (b) 2.5 mm, 5.0 mm, and 7.5 mm rebates in 10 mm thick block 2.5 5.0 7.5
Figure C.1 Details of the phantom used to simulate a hand for hand-wrist radiography. (a) The
phantom positioned on a Kodak CR cassette. The phantom was attached to a cardboard template to allow repositioning on the cassette entrance face; (b) Radiograph of the phantom with an en- larged section showing the objects used to simulate edges and contrast of bones in the carpus. This radiograph has very low noise because it was taken using a large x-ray exposure (20 mAs).
were based solely on the x-ray radiation dose at the entrance to the CR plate and did not compensate for changes in performance of the CR plate due to energy spectrum changes caused by the simulation material.
Details of the phantom construction and a radiographic image of the phantom are illus- trated in Figure C.1. The carpal bones were simulated using a triangular block to give sharp bone contour angles, and a square block with rebates to give different bone contrasts. Overlapping carpal bones were simulated using a rectangular sheet of 5 mm of aluminium positioned over the blocks.