As a radiologist we have a guiding principle that is almost a mantra, an amendment to the Hippocratic oath, ALARA. Radiologists are trained to keep radiation exposure As Low As Reasonably Achievable (ALARA). To achieve ALARA a radiologist must always be cognizant of time, distance, and shielding. The goal being to minimize exposure time, maximize distance between the radiation source and humans, and create barriers with lead aprons, thyroid shields, and lined walls in X-ray rooms, etc. In very simplistic terms X-ray beams can be controlled by adjusting the mA and kV. High mA is higher dose with crisp images. Low mA is lower dose with grainy images. High kV allows better penetration through soft tissues, but can decrease the image contrast. Low kV has less dose and more contrast, which is not a problem unless the patient is overweight, muscular, broad shouldered, or “big-boned.” The easiest way to explain how to reduce radiation exposure in medical imaging is by talking about each imaging modality.
Let’s begin with X-ray, the founding modality of my specialty. Most X-ray equipment have automatic sensors to detect the patient size and adjust the mA and kV settings accordingly. These work okay, but radiology is an art from taking the picture to analyzing the image. Many times, technologists select their own settings obtaining better quality images than the automated software. Aside from these settings, the best way to limit X-ray exposure is by reducing radiation dose. I only see the images sent to me. A good technologist can get a great image the first time. Additional X-rays add exposure, so getting the picture right the first time counts. Again, image count adds dose. If a clinical question can be answered with 2 or 3 views of the lumbar spine, then a 5 or 7 view series is not necessary. I’m sorry but I’m going to lump mammography in here. It is specialized X-ray and federally regulated. Mammography will be tackled in a future multi-part series.
Fluoroscopy is basically low dose real-time X-ray. This modality is used for dynamic imaging. Diagnostic radiologists perform barium exams (esophagram, Upper GI series, Barium enema),joint injections for MR arthrograms, and lumbar punctures most commonly. A good radiologist can perform their exams quickly and efficiently. By far, the vast majority of my time in the fluoroscopy suite is spent first introducing myself to the patient, explaining the procedure, drawing up any necessary medication, and making any adjustments I need to adequately position myself and the patient for a successful procedure. Preparation is key and will decrease procedure time and consequently dose. There is a setting for a radiologist to control dose by adjusting the frame rate, usually anywhere between 1 and 8 frames per second (time). When a patient is on the fluoroscopy table the source is under the table with the movable part on top called the tower containing an image intensifier that is the receiver. The closer the receiver is to the patient, the better the image. The further away the more the machine will try to compensate by increasing radiation dose (distance). Lastly, the machine has lead panels that can come in from the tops and sides to crop the image into portrait, landscape, or a smaller square providing a barrier (shielding).
Computed Tomography (CT) scan is super X-ray. It is like taking an X-ray through a single slice in your body and stacking them all up together to make a 3D image of your body. In fact, it used to be called CAT scan with the ‘A’ standing for axial. Axial is just a plane of viewing. An axial section is best described with Star Wars. When Darth Maul was cut in half, he was sliced axially. Now imagine if he was cut into another couple hundred of those slices. That is how a radiologist looks at someone’s body, laying flat on their back cut into a bunch of slices. The scanners are now much better and images can be reconstructed into any plane (axial – cut separates top from bottom; sagittal separates left from right; and coronal separates front from back). CT dose reduction has been radically improved over the past decade with software enhancements from the manufacturers. The machines can not only assess patient body habitus to modify dose, but can do it on a slice by slice basis using more radiation to get through the shoulder area where there is more bone and soft tissues and less at the abdominal level. CT has come a really long way providing less dose in combination with better imaging.
Nuclear medicine is a totally different game. The others we talked about were all external sources of radiation. In nuclear medicine, we administer radiation directly into your body. It is most commonly injected for heart scans, bone scans, thyroid scans, kidney scans, and liver/gallbladder scans. It could also be added to food (commonly scrambled eggs) and ingested. The great thing about nuclear medicine is that it is physiologic imaging. All the other studies in radiology are anatomic, but this is about function. Often the images seem to be of poor quality, but they provide important clinical information and can be used to assess for metastatic disease and organ function. Sorry, there is really no way to reduce dose here, but for bonus dose go for a PET-CT scan. Positron Emission Tomography in combination with CT scan combines anatomy with physiology. It is used most commonly in cancer imaging. Frankly, if you’ve got cancer decreasing radiation exposure is probably not your major concern. More importantly, your PET-CT results can be a strong indicator of response to treatment and is routinely utilized in cancer staging. So suck it up buttercup. This ones worth the extra dose.
Now I want to shift gears. The best way to reduce radiation exposure in medical imaging is to eliminate it. Ultrasound (US) and Magnetic Resonance Imaging (MRI) are two modalities with NO RADIATION. There are times when either of these modalities might provide the clinical information being sought. Just today I reviewed a CT of the brain of a very young child with the clinical indication of large head/macrocephaly. Thankfully it was normal, but the patient could have gotten the same diagnosis without the radiation exposure. In infants with an open fontanelle, US is hands down the preferred modality. Otherwise, I would suggest a rapid MRI scan. These are exams tailored for kids with fast scanning to reduce motion artifact. Now the images aren’t great, but they tell me everything I need to make a diagnosis and that is all that counts in my book.
So, assuming you’ve read this far, if your a patient hopefully that answered some questions, and if you are a referrer I hope you’ve learned about the importance of the radiologist. If you don’t know your radiologist(s), then meet them. It is their job to educate patients and healthcare providers to the risk, benefits, and alternatives in medical imaging. If you can’t find a nice, helpful one, then reach out to me. I’ll be glad to help. My Rad was designed for you.
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