Research Description

The Comparison of the Pediatric Effective Dose to the Weighted CT Dose Index

Introduction

Computed Tomography has revolutionized the diagnostic radiology community, since its start in the 1970’s, its use has increased rapidly. It was the first tomographic imaging technique to produce axial images of high quality and opened the way to an explosion of tomographic and three-dimensional imaging techniques. Its success has largely depended on the development of fast and accurate image reconstruction algorithms. The basics of CT is simple, a motorized table moves the patient through the CT imaging system. At the same time, a source of x-rays rotates within the circular opening, and a set of x-ray detectors rotate in synchrony on the far side of the patient. The x-ray source produces a narrow, fan-shaped beam, with widths ranging from 1 to 20 mm. The data is processed by computer to produce a series of image slices representing a three-dimensional view of the target organ or body region. A consequence of being able to produce 3-D images is that CT delivers a larger radiation dose than the more common, conventional x-ray imaging procedures making the way for controversy about its use.

Problem

The diagnostic radiology community tends to exploit technical advances in CT by expanding the scope and complexity of its use, and thus increasing the resulting radiation exposure. CT represents by far the largest contribution to the radiation exposure of the population from diagnostic medical sources. There has become a tendency among clinicians to increase the amount of image information collected by increasing scan length, decreasing the slice thickness, and/or performing over-sampled scans to monitor fast real-time processes such as breathing. Consequently, CT exposure levels for a particular exam have in fact tended to increase over the years.

Proposed Solution

The radiation dose delivered by CT scanning can be measured so as to give a generic estimate of the overall harm to the patient. Two measurements can be used to describe the exposure: Weighted CT dose index and the effective dose.

The weighted CT dose index represents the average dose within a single scan slice during a complete exam and allows for the fact that slices up and downstream from a particular slice will contribute dose to the point in question through scattered radiation. This is measured in a clinical setting with an ionization chamber that integrates the dose of a single slice of an axial scan over a length of 100 mm. Measurements are made in the center and at four equally spaced peripheral locations around the center circumference of a cylindrical phantom. The peripheral measurement points are at a depth of 1 cm from the surface.

The effective dose, in turn, is used for dose distributions that are not homogenously deposited in the body (which is always the case with CT) and is computed by weighting the equivalent dose values for various organs by the organ weighting factors and then summing over the exposed volume so as to obtain an equivalent whole-body dose. A precise effective dose depends on the imaging modality, the energy of the radiation, the age and sex of the patient, and the anatomical particulars of the exam. Therefore, several trials must be done to accommodate all these factors when measuring the effective dose. This will be measured using a pediatric anthropomorphic phantom (human like in the sense that that the phantom is created with tissue that mimics, but is not, human tissue) with dosimeters placed specifically in every organ to be measured.

Methodology

Both the effective dose and the weighed CT dose index will be compared to provide a generic estimate of the total dose exposure to a pediatric patient. In measuring the CT dose index a cylindrical acrylic phantom/dosimeter of 16 cm in diameter for a pediatric patient will be used.

In measuring the effective dose a 1 yr. old pediatric anthropomorphic phantom will be used, which will mimic a pediatric body. The phantom is designed to radiographically mimic human tissue at diagnostic photon energies. The tissue that makes up this anthropomorphic phantom is manufactured with an epoxy resin base in which phenolic microspheres are used to adjust the mass density in order to closely resemble as much as possible the human tissue. The dosimeters (which measure the radiation dose) are called fiber optic coupled dosimeters (FOCD), they will be placed at the centroid of the organs of interest (smaller organs), and the larger organs will require multiple measurement points, in order to find an average organ dose.

These measurements will be done in the Shands Orthopedic Clinic using a Siemens 16 slice CT.

Conclusion

A study regarding the pediatric effective dose will be beneficial to the Medical community. With the equipment available for measurements, accurate data can be acquired and the soon released for improved public knowledge and awareness.