Extension of RPI-adult male and female computational phantoms to obese patients and a Monte Carlo study of the effect on CT imaging dose

Abstract

Although it is known that obesity has a profound effect on x-ray computed tomography (CT) image quality and patient organ dose, quantitative data describing this relationship are not currently available. This study examines the effect of obesity on the calculated radiation dose to organs and tissues from CT using newly developed phantoms representing overweight and obese patients. These phantoms were derived from the previously developed RPI-adult male and female computational phantoms. The result was a set of ten phantoms (five males, five females) with body mass indexes ranging from 23.5 (normal body weight) to 46.4 kg m(-2) (morbidly obese). The phantoms were modeled using triangular mesh geometry and include specified amounts of the subcutaneous adipose tissue and visceral adipose tissue. The mesh-based phantoms were then voxelized and defined in the Monte Carlo N-Particle Extended code to calculate organ doses from CT imaging. Chest-abdomen-pelvis scanning protocols for a GE LightSpeed 16 scanner operating at 120 and 140 kVp were considered. It was found that for the same scanner operating parameters, radiation doses to organs deep in the abdomen (e.g., colon) can be up to 59% smaller for obese individuals compared to those of normal body weight. This effect was found to be less significant for shallow organs. On the other hand, increasing the tube potential from 120 to 140 kVp for the same obese individual resulted in increased organ doses by as much as 56% for organs within the scan field (e.g., stomach) and 62% for those out of the scan field (e.g., thyroid), respectively. As higher tube currents are often used for larger patients to maintain image quality, it was of interest to quantify the associated effective dose. It was found from this study that when the mAs was doubled for the obese level-I, obese level-II and morbidly-obese phantoms, the effective dose relative to that of the normal weight phantom increased by 57%, 42% and 23%, respectively. This set of new obese phantoms can be used in the future to study the optimization of image quality and radiation dose for patients of different weight classifications. Our ultimate goal is to compile all the data derived from these phantoms into a comprehensive dosimetry database defined in the VirtualDose software.

Figure 1

A cross-sectional image of the abdomen illustrating the division of adipose tissues into two main classes: (1) subcutaneous (SAT): off-white colored tissues located outside the dashed circle and (2) visceral (VAT): off-white colored tissues inside the dashed circle.

Figure 2

A visualization of the adipose tissue modeling for the RPI BMI-adjustable male phantoms: (a) abdominal organs (surface rendering mode) and VAT (wireframe rendering mode) which surrounds the abdominal organs, (b) SAT layer beneath the skin, defined as the region between the body surface and internal body cavity. As the fat tissue is not defined in the ICRP’s radiosensitive organ list, SAT and VAT only serve to attenuate photons.

Figure 3

Anterior and lateral views of the RPI BMI-adjustable (a) male and (b) female phantoms. The phantoms have the same height (1.76 m in the male and 1.63 m in the female) but differ in weight. From left to right the weight classifications are normal-weight, overweight, obese level-I, obese level-II and morbidly obese.

Figure 4

Examples of cross-sectional images at (a) mid-chest and (b) mid-abdomen which are used to virtually measure the AP and lateral dimensions (images originally obtained from the data set of the National Library of Medicine’s visible human project, http://www.nlm.nih.gov/research/visible).

Figure 5

Body dimension versus weight for the RPI male and female obese phantoms: (a) AP dimension at mid-chest, (b) lateral dimension at mid-chest, (c) AP dimension at mid-abdomen and (d) lateral dimension at mid-abdomen. Also plotted in dashed lines are fitted sex-averaged trend curves reported in the literature (Ogden et al 2004).

Figure 5

Body dimension versus weight for the RPI male and female obese phantoms: (a) AP dimension at mid-chest, (b) lateral dimension at mid-chest, (c) AP dimension at mid-abdomen and (d) lateral dimension at mid-abdomen. Also plotted in dashed lines are fitted sex-averaged trend curves reported in the literature (Ogden et al 2004).

Figure 6

Comparison of fat masses in the RPI male and female obese phantoms with values based on published prediction formulas (Deurenberg et al 1991, Jackson et al 2002) using the BMI value of each phantom. Agreements between these two sets of data are observed.

Figure 7

Comparison of organ doses for RPI adult normal body weight, overweight, obese phantoms: (a) males and (b) females. The same tube potential (120 kVp) and tube current time (100 mAs) are used.

Figure 8

Plot of the organ dose ratios of the RPI-AMMOwhen tube potential increases from 120 to 140 kVp without changing the mAs setting and the organ dose ratios increase significantly with the tube potential increasing, ranging from 41% to 62%.