I am often asked by non-radiologist colleagues to recommend the best radiological examination for evaluating a given clinical scenario. In this discussion, I will give a general outline of the best test to order for your patients, as well as pitfalls to avoid.
First, the pitfalls:
The first priority in ordering a radiological exam is to avoid harming the patient. Unfortunately, the risk of harm is a reality and can even result in death, so safety issues are of the utmost importance.
Radiology examinations utilize a variety of energy forms to create an anatomical image. Contrast agents, both oral and intravenous, are utilized to better distinguish normal from abnormal soft tissue structures. Dangers can occur both from the type of energy exposure, both x-ray and MRI, and from the contrast agents used for an exam.
X-ray and CT scans focus electrons through tissues, some of which are stopped by the body, others of which pass through. The varying number of electrons passing through a given region render differing densities or shades of gray on the image. Nuclear medicine links x-ray emitting isotopes to metabolites that are either injected or ingested, and the differing densities of emission from body structures creates an image.
Since these processes use ionizing radiation, some tissue damage can occur. As a rule, radiology exams should be avoided during pregnancy, especially during organogenesis that occurs during the first trimester. Unfortunately, early pregnancy is often unknown to the patient. So please, if there is any chance whatsoever that a sexually active woman is pregnant, get a serum pregnancy test to see if the study is safe for the patient.
Ionizing radiation is also potentially carcinogenic, so the lowest dose and fewest number of x-ray studies should be obtained with optimal patient diagnosis and care in mind.
CT scans often require an intravenously injected iodinated contrast agent that highlights the relative vasculature of a soft tissue structure and differentiates it from surrounding soft tissues. This process renders better detection of abnormalities. However, the contrast agent is potentially allergenic, even in patients who have never been exposed to it, and the allergic reaction can range from minor itching to full blown anaphylaxis.
In addition, iodinated contrast agents can further damage kidneys in patients with compromised renal function. So please, ask the patient about prior contrast agent reactions and ask and check for any renal function abnormality prior to ordering contrast for a CT scan. Often a CT scan can be done without contrast. Alternatively, an MRI or ultrasound can be substituted to avoid exposing the patient to unnecessary harm.
Patients on Metformin and other oral anti-diabetic agents, which are excreted by the kidney, must refrain from taking their medication prior to administration of the iodinated contrast agent. Renal function needs to be rechecked 24 hours after the administration of the iodinated contrast as well. In the rare occurrence that renal compromise develops and the patient continues metformin, a fatal ketoacidosis can occur.
Magnetic Resonance Imaging utilizes magnetism and radiofrequency to create an image. This occurs because radiofrequency can alter a magnetic field, and a magnetic field can generate radio waves. The degree of this effect is specific to the type of tissue being examined. The strong magnetic field in the MRI machine can dislodge implanted medical devices containing iron, ruin their electronics, and – in the case of pacemakers, defibrillators and nerve stimulators – cause them to discharge. Orthopedic hardware, newer stents and metallic suture can degrade MRI images by warping the precision of the magnetic field, but generally they are safe.
In the event that a stent was implanted overseas or is more than ten years old, it is best to consult with the radiologist. When ordering an MRI, always ask your patients if they have an implanted device of any kind in their body – aneurysm clips, cochlear implants, cardiac or other vascular stents, pacemakers, defibrillators, nerve stimulators or insulin pumps. In addition, as MRI uses microwave radiofrequency, there is mild warming of body tissues throughout, relatively contraindicating scanning during the first trimester of pregnancy.
Magnetic Resonance Imaging often requires the administration of Gadolinium contrast to better differentiate normal from abnormal soft tissue structures. In patients with renal compromise, nephrogenic systemic sclerosis (NSF) can occur following gadolinium administration and is often fatal. So again, prior to ordering gadolinium, please be sure that the patient’s renal function is normal.
Ultrasound creates images using reflected sound waves to create an image and is safe across the board.
Now, guidelines for choosing the best exam:
For evaluation of the brain, face and neck, an MRI is often the modality of choice. Exceptions include acute trauma, evaluation of the sinuses or bone detail such as that required for the ossicles of the ear. In this case, a CT is best. Palpable masses of the neck or evaluation of the thyroid, neck nodes and salivary glands are best evaluated and can be potentially biopsied with ultrasound. Nuclear medicine has specialized but increasingly less commonly utilized applications, especially of the thyroid.
For evaluation of the lungs: X-ray is the first exam of choice, and for questions that remain unanswered, CT renders better detail of both acute and chronic disease. Ultrasound cannot be used to evaluate the lungs for other than pleural effusion.
For evaluation of the heart: Echocardiography, performed by cardiologists in their offices, is the best first anatomical and functional study, followed by CT with contrast, which better defines the coronary arteries and great vessels, and can examine both anatomy and function. Increasingly MRI is gaining ground for both anatomical and functional studies, although it still lacks the ability to optimally evaluate the coronary arteries, for which catheter angiography remains the gold standard. For peripheral vascular disease, ultrasound is the first modality of choice, but as it cannot see through the lungs to evaluate the thoracic great vessels. For this, CT or MRI is best.
For evaluation of the abdomen: Ultrasound is the best first study for evaluation of the solid organs and gallbladder, and can often image appendicitis. However, CT is optimal for evaluation of bowel disease, renal or ureteral calculi or malignancy. MRI is increasingly being used subtle evaluation of soft tissues, especially masses of the solid organs and for both contrast and non-contrast evaluation of the aorta and renal arteries as well as the biliary system. Newer fast scan techniques offer promise for studying the bowel.
For evaluation of the pelvis: In general, in females, ultrasound is the first modality of choice, in males, CT. CT will better evaluate bowel disease, such as diverticulitis or other inflammatory bowel disease or malignancy. MRI is increasingly being used for evaluation of infertility and disease states in the female pelvis, and for prostate cancer in males.
For evaluation of the musculoskeletal system and spine: MRI is the modality of choice, followed by CT. Ultrasound can often best evaluate superficial, palpable structures, but cannot see into or through bone.
For evaluation of the breast: Mammography is the first exam of choice, but in patients with dense breast tissue where mammography may not detect malignancies, or in patients who refuse mammography, ultrasound is proving to be a valuable means of detecting breast cancer. However, with our current technology, ultrasound cannot detect micro-calcifications seen only with mammography, which can be the earliest sign of intra-ductal cancer. Increasingly, MRI is being used for screening and staging for breast cancer, but again does not visualize calcifications.
What about contrast? I think it is best to order an exam with contrast per radiologist discretion, but to be sure that the patient can tolerate the contrast if administered, and does not have renal compromise, allergies to contrast, or in the case of MRI, implanted medical devices. Remember to mention on your order for CT contrast that the patient is on metformin
If in doubt, ask. Most radiologists will be happy to field a question so that the patient gets the correct exam. I am always open to any questions that you may have and can be reached at: 425-454-1700.