Techniques to Reduce Bone Densitometry Radiation Dose
In this article we talk about bone densitometry radiation dose and the techniques that radiologic technologists can apply to reduce it. The two key principles in patient radiation protection are justification and optimization. Justification implies that any X-ray exposure is justified clinically. It means the examinations that will not affect the patient’s care are avoided. Once an examination is justified, optimization consists of activities that reduce bone densitometry radiation dose. This means delivery of the smallest possible dose to the patient.
Steps for Reducing Radiation Exposure
Preparing the patient before the DXA scan is an important step in bone densitometry radiation dose reduction. The radiologic technologist should carefully check the patient for any clothing, jewelry, keys, or coins that could produce artifacts. This can lead to suboptimal image quality and the need to repeat imaging. Repeat imaging is associated with additional exposure to radiation.
When imaging children, appropriate interaction with the patient and caregiver is essential to ensure no movement occurs during imaging. Moreover, the radiologic technologist should ensure that artifacts, such as a cast on a fracture, are not included in the imaged area.
It is worth noting that protective devices are not necessary for patients during a DXA scan. These devices do not protect adjoining organs outside the region of interest from internal scatter. There is no clinical indication for their use. However, if a patient requests a protective device despite being informed to the contrary, it is sometimes wise to accommodate the request. This is especially true when the scan is a screening study.
Bone Densitometry Radiation Dose Reduction Techniques
Image Length
It is important to minimize the image length of the DXA as appropriate to the size of the patient. As much as possible, the primary beam should avoid radiosensitive organs. Modern-day MDCT systems permit rapid imaging on account of multiple arrays of detectors. It should be noted that if the image length is increased to include more than one skeletal site, it will result in a higher DLP and delivery of a higher effective dose to the patient. Optimization of image length is important to minimize bone densitometry radiation dose.
Z-Overscanning
Radiologic technologists should be aware of Z-overscanning. Here, tissue outside the boundary of the volume of interest is exposed to X-rays. This unnecessarily increases the bone densitometry radiation dose. In a 16-slice CT scanner, Z-overscanning can increase the patient’s effective dose by as much as 36 percent. Carefully collimating the beam and selecting slice width and pitch can limit unnecessary exposure on account of Z-overscanning.
Tube Voltage
By varying the tube voltage, radiologic technologists can substantially alter the bone densitometry radiation dose as well as the quality of the image. Images acquisition at 80 kVp for single-slice QCT of the spine delivers a considerably lower radiation dose compared to images acquired at a higher voltage (120 kVp). Therefore, tube voltage merits investigation as a dose reduction technique. This is especially true in small-sized patients where it can help achieve substantial reduction in patient dose.
The automatic exposure control (AEC) adjusts parameters based on the size and attenuation of the imaged area to achieve the desired image quality at the lowest possible dose. Therefore, AEC is a useful tool for reduction of bone densitometry radiation dose by 15 to 16 percent compared to fixed mA systems.
Radiogenic Risks of Bone Imaging
DXA and other diagnostic X-ray examinations are valuable clinical tools, but they are expose patients and workers to a potential risk of carcinogenesis. That is why it is important to apply techniques that reduce bone densitometry radiation dose. Radiation dose data is used to estimate cancer risk from exposure to diagnostic X-rays.
The linear no-threshold model (LNT model) is used for risk estimation. This model makes an assumption that health risks and radiation dose have a linear relationship. The LNT model can be used to justify medical exposure and also to compare other risks to diagnostic exposure. The BEIR Committee has estimated risk factors attributable to radiation and specific to age and gender. However, it is noteworthy that the radiogenic risks of bone evaluation techniques are negligible compared to the anticipated benefits.
DXA Scanning and Cancer Risk
Debate continues about low-level radiation, such as that associated with DXA scans. It is controversial whether bone densitometry radiation dose and increases the risk of developing cancer by any significant degree. Evaluation of radiogenic risk is further limited by the fact that the biological effects that radiation exposure produces in the body have only been evaluated indirectly. Information on radiogenic risk relies on patients treated with radiotherapy, survivors of atomic bombs, and radiation workers. Individuals in these groups receive radiation doses that far exceed the doses delivered by diagnostic X-ray studies, especially low-dose techniques such as DXA.
Nonetheless, studies on survivors of atomic bombings have found that radiation exposure comparable to diagnostic radiological examinations led to a significantly increased cancer risk. An individual’s potential risk from bone densitometry radiation dose is low. However, due to the uncertainty about the true risk of radiation exposure, it is prudent to apply principles of radiation protection in daily clinical practice.
Self-Referred Patients
With increasing awareness of osteoporosis, one dilemma that radiation personnel in a DXA unit may face is women wishing to have their BMD measured as a precaution. Many patients may arrive at the unit without any referral from a physician. DXA workers must then determine whether the bone densitometry radiation dose can be medically justified.
The DXA unit should develop a simple protocol to determine whether DXA scans are justified in patients seeking BMD testing without a physician referral. It is also important to educate patients about bone densitometry radiation dose. This will ensure that in each patient the benefits outweigh the risks. It will also protect the person in charge of the DXA unit in terms of legal responsibility.
Number of DXA Scans
Because bone densitometry radiation dose exposure is relatively low, there is no upper limit on the number of DXA examinations a person can undergo. This is provided each scan is medically justified in terms of risk versus benefit. Again, the least significant change should be calculated because DXA is not sensitive to small changes in BMD over a short period of time. It is also worth noting that in terms of DXA scans as part of research studies, the justification criteria for volunteers is different than that for patients. A research ethics committee typically examines the risks and benefits of the study. Volunteers are given clear information about potential risks and asked to sign an informed consent.
Continuing Education for DXA Operators
We offer a range of e-courses for ARRT® CE credits that are appropriate for DXA operators, mammographers, fluoroscopists, and general radiographers. There are also some offers for free continuing education from time to time on our website. To earn CEUs and complete your ARRT® structured education requirements, please purchase one of our online courses. Once you have read the online e-course and passed the post-test, you will instantly receive a certification of completion. Our bone densitometry e-course is worth 23 CEUs, so it’s a quick and easy way to earn most of the required credits.
Further Reading
DXA Osteoporosis Test: Does My Patient Need It?
Quantitative Computed Tomography
Get more information about radiologic technologist continuing education courses here.