The Development of a Chest Phantom for Use in Radiologic Dosimetry

Abstract

Our interest in the fabrication of a physically and anatomically realistic chest phantom was first stimulated by the work being done on supervoltage (2 mv) chest roentgenography by one of the authors (11, 12). In the course of investigations of the value of supervoltage technics in the detection of thoracic lesions, it became apparent that such a phantom was needed for two basic reasons: first, to permit the measurement of integral dose received by a patient at 2 mv compared to that absorbed during a similar examination at the usual diagnostic energy levels; second, for use in the comparative evaluation of diagnostic technics. Previous work by Weatherwax (14), Quimby (9), Granke (4), and Nahon (8) involving chest phantoms for dose measurements has contributed greatly to an understanding of intrathoracic dosage. None of the earlier phantoms, however, were suitable for use at energy levels below 200 kvp, and in none was there sufficient anatomic detail to permit the investigation of diagnostic technics. The purpose of this paper, therefore, is to describe a chest phantom which, with respect to effective atomic number, mass density, and anatomic detail, closely resembles the human thorax. The materials and methods used in its construction, and its applications in diagnostic and therapeutic radiology will be discussed. Evaluation Of Phantom Materials A. Materials to be Used for the Soft Tissues of the Chest Wall: The criteria for choosing materials to be tested for use in simulating the soft tissues of the chest wall were an effective atomic number (Z) and density comparable to those of tissue, 7.33 (10) and 1.00 (2), respectively. Water approximates these values very closely and therefore, as a final measure of the suitability of the various substances, their radiographic density and homogeneity were compared to those of an equal thickness of water. Exposures were made at various energy levels and the shadows compared visually and photodensitornetrically. Additional factors considered were adaptability, ease of handling and fabrication, durability, strength, uniformity (reproducibility), and availability of the materials. 1. Paraffin and dental wax meet the secondary criteria, but are deficient in effective atomic number (5.41) and mass density (0.92). 2. Jones and Raine (6) described a compound called “Mix D,” which has an effective atomic number of 7.32 and a mass density of 0.99, composed of paraffin, polyethylene, magnesium oxide and titanium dioxide. While the Z and mass density are ideal, the mixture is difficult to prepare and is not handled easily. Polyethylene solidifies rapidly around 140 0 C., precluding the moulding and shaping required to represent adequately the distribution of the absorbing masses of the chest wall. At room temperature, the mixture is quite hard and is not readily shaped without the use of power tools.