- The Importance of Exposure Reduction
- Exposure Reduction Methods
- How Much Can Exposure Be Reduced?
- What You Can Do
The number of schools conducting scoliosis screening using non-radiographic methods has been increasing. Over one-half of the States conduct scoliosis screening programs, in some or all of their school jurisdictions, on children ages 9 to 14 years. An estimated 120,000 adolescents identified through the screening programs will require follow-up surveillance or treatment employing periodic x-ray examinations (often several times a year) until the child reaches skeletal maturity. Females comprise roughly two-thirds of the positive scoliosis population who will require follow up and possibly treatment.
Radiation-induced breast cancer in women has been well-documented in the scientific literature. In addition, several bioeffects studies indicate that women who were exposed to ionizing radiation during adolescence have a greater risk of developing breast cancer than those who were exposed later in life.
A wide range of radiation exposures to the breasts during scoliosis radiography has been reported. Studies published in the scientific literature indicate that exposures to the skin, from one spine projection, can range from a low of 30 milliroentgens (mR) to a high of 1100 mR. For comparison, the mean exposure to the breast for a one-craniocaudal view mammogram is 600 mR.
These four factors: (1) potentially high exposures to the breasts, (2) high frequency of examination, (3) the young age group which is being examined, and (4) the radiosensitivity of the breast tissue, emphasize the importance of keeping the dose to the breast tissue as low as possible.
1. Use a fast or rare-earth film/screen combination
Since the primary purpose of scoliosis radiography is to evaluate the degree of spinal deformity, fine detail (resolution) may not be as critical a factor as in other skeletal procedures. The use of very fast conventional or rare-earth film/screen combinations to drastically reduce overall patient exposure is therefore possible. More resolution is maintained when rare-earth intensifying screens are used because of the special properties of its phosphors.
2. Use a compensating filter
It is difficult to obtain a properly-exposed radiograph of the entire spine on one film since a typical body trunk consists of a variety of thicknesses and densities. Compensating filters (sometimes called "wedges" or "dodgers") can be used to even out the densities of the spine by absorbing excess radiation transmitted through the thinner or less dense body areas before it reaches the film. Because the compensating filters are positioned in front of the patient, they also tend to significantly reduce radiation exposure to the breasts which are located in the less dense portion of the trunk. Differently-shaped filters, constructed of leaded acrylic, aluminum, and a variety of other metals, are positioned on the x-ray machine's collimator for different projections. (These filters are not to be confused with gradient screens which will also assist in obtaining a uniform density radiograph but will do nothing to reduce patient exposure.)
3. Use breast shielding
The breasts can be directly protected either by positioning lead or metal shields on the collimator, thus, casting a "shadow" on the patient (shadow shield), or by positioning lead shields directly on the patient during exposure (contact shield). These types of shields can be used for the AP and lateral projections. The PA projection can also be considered as a type of breast "shield" because the patient's body absorbs a considerable fraction of the primary x-ray beam before the breast tissue is irradiated.
Other exposure reduction techniques can be used such as employing a lower grid ratio and/or using a higher kVp (remembering in both cases that mAs must be adjusted to obtain proper film density). However, the impact on exposure reduction is not as great as with the three techniques outlined above. In any case, the x-ray beam should be collimated as close to the spine as diagnostically practicable to avoid unnecessarily exposing other body areas. Finally, practitioners should also review the clinical indications and needs for each examination and projection.
The Center for Devices and Radiological Health and the Naval Hospital: Bethesda, recently evaluated several of these exposure reduction methods. The use of a compensating filter for the AP projection reduced patient exposure at the breast area from 127 to 22 mR; at the umbilicus, exposure was reduced from 141 to 109 mR. During the lateral projection, exposure was reduced from 240 to 45 mR at the breast level. When a fast rare-earth film/screen combination was used, exposure was reduced by a factor of 2. The average exposure to the breast during one AP examination using both the compensating filter and the rare-earth film/screen systems was 11 mR.
The actual exposure reduction obtained in practice will depend on the speed of the film/screen system selected, the radiographic techniques, the types of the compensating filters, and the breast shielding employed. Exposure reduction will be greater if a combination of techniques are employed. Approximate exposure reduction factors are given in Table 1.
|METHOD||EXPOSURE REDUCTION FACTOR|
(or PA projection)
|Combined Methods||Approx. 12-360 (Multiplicative Net Reduction)|
A significant reduction in breast exposure during scoliosis radiography can be accomplished relatively easily without compromising diagnostic quality of the image. The Center for Devices and Radiological Health encourages the use of one or more of the following techniques:
- Fast or rare-earth film/screen combination
- Compensating filters
- Breast shielding
- shadow shields
- contact shields
- PA projection