Radiology modalities, also known as imaging modalities, are the different types of medical imaging techniques used to visualize the body for diagnostic and therapeutic purposes. The four main types of imaging modalities are x-rays (including computed tomography and fluoroscopy), ultrasound, magnetic resonance imaging, and nuclear medicine (including positron emission tomography [PET]).
Each modality has its own benefits and drawbacks, which I’ll discuss below.
X-rays are a form of electromagnetic radiation (a.k.a. photons; light particles) that were discovered in 1895, giving birth to the field of radiology. X-rays lead to the formation of radiology as a medical specialty and are still used today for a variety of diagnostic purposes.
X-rays consist of ionizing radiation generated from an x-ray machine that pass through patients and are used to create images of whatever they pass though (people, carry-on bags, teeth, animals, etc.). Images are created by the x-rays that penetrate travel through the object being imaged and reach a detector on the other side. The images are referred to as “radiographs” or “plain films.”
X-rays are variably absorbed by the structures they pass through with denser structures/items absorbing a larger amount of x-rays. In essence, x-ray images are density maps of people.
Radiographs are best at detecting significant differences in density, such as lungs and bones on chest radiographs, bones on extremity and spine imaging, and bowel gas and kidney stones when imaging the abdomen and pelvis.
- Advantages: X-ray machines are widely available and relatively quick, cheap, and easy to perform.
- Disadvantages: X-rays consist of harmful ionizing radiation that can damage cells and imaging modalities using x-rays have limited soft tissue contrast resolution (it can be hard to differentiate adjacent soft tissues of similar density from one another).
X-ray radiographs are 2-dimensional (2D) images created by passing x-rays through a patient. Radiography is fairly ubiquitous in medicine with x-ray machines found throughout the vast majority of urgent cares, hospitals, emergency rooms, orthopedic surgery clinics, and chiropractic clinics.
Radiographs are a fast and cheap option in evaluating patients with acute symptoms, are used to assess for and follow-up fractures, assess for dense foreign bodies, and much much more. Radiographs are also used as a screening test to see if more advanced imaging (such as CT), is necessary.
People tend to associate x-rays mostly with radiographs, but x-rays are also used in other important modalities including fluoroscopy and computed tomography (CT).
If an x-ray machine is like a camera, then fluoroscopy is like a video camera – x-rays are produced in a pulsed or continuous fashion and generate real-time images of the body. Think of it as “video x-ray.” The images are of much lower quality than conventional radiographs to limit the patient’s overall radiation exposure.
This allows radiologists and physician extenders to see what’s happening real time. It is often used to guide procedures, such as lumbar punctures and injections and determine when to take a true radiograph when evaluating the gastrointestinal (GI) and genitourinary (GU) tracts.
Common fluoroscopy exams: upper GI series, esophagrams, barium swallows in conjunction with speech pathology, enemas, cystograms, sniff tests, and hysterosalpingograms (HSGs).
Common procedures: joint injections, lumbar puncture, various interventional radiology procedures, heart catheterizations, and in some surgical specialties such as orthopedic surgery and urology.
- Advantages: Allows you to see what is happening in real-time and each image uses an incredibly low dose.
- Disadvantages: Radiation dose is cumulative throughout the exam/procedure, image quality is relatively poor to limit radiation to the patients.
Computed tomography (CT scan) is a form of imaging that contains a donut-shaped ring (gantry) of x-ray generators that rotates around the patient, getting information from multiple different angles. This produces a stack of two-dimensional (2D) cross-sectional images of the body that can be combined to create three-dimensional (3D) appearance.
CT scans rapidly obtain high quality images of the body and are capable of scanning the entire body in under a minute. Because of this, CT is a workhorse of medical imaging, particularly in the emergency setting.
CT scans use x-rays to create images and therefore expose patients to harmful ionizing radiation. While protocols are in place to limit the radiation, it is not zero. Fortunately, the radiation risk is relatively low thanks to advances in scanner hardware and software. That being said, a cost vs benefit analysis should be performed whenever ordering CT scans to prevent any unnecessary radiation exposure.
- Advantages: CT scans are widely available, quick, and give a lot of useful information.
- Disadvantages: Expensive and expose patients to higher levels of harmful ionizing radiation.
Ultrasound is a non-ionizing form of radiology that uses sound waves to create images of the inside of the body. The sound waves are created by the ultrasound probe (aka transducer), which enter the body, interact with various tissues, and return to the probe where the sound waves are detected. A computer converts those sound waves into images.
Ultrasound is often used to visualize organs such as the liver, gallbladder, kidneys, spleen, portions of the pancreas, uterus, ovaries, etc. Ultrasound can also be used to visualize blood vessels (flow direction, speed, and even waveform), evaluate hernias and joints, and to assess the health of fetuses (unborn babies).
Ultrasound allows for dynamic imaging where you can watch what is being scanned in real-time. Needles are generally visible by ultrasound as well, making it an excellent modality for image-guided procedures (biopsies, paracentesis, thoracentesis, etc.).
- Advantages: Widely available, relatively inexpensive, safe in pregnancy, and does not use harmful radiation.
- Disadvantages: Operator-dependent (requires a skilled sonographer or radiologist), limited by body habitus/increased subcutaneous fat and bowel gas, susceptible to artifacts, and unable to see through bone or gas.
Magnetic resonance imaging (MRI) is a non-ionizing form of radiology that uses magnetic fields and radio waves to create images of the inside of the body. It’s like ultrasound’s cooler cousin. Of all imaging modalities, MRI is by far one of the most fascinating.
To overly simplify it, MRI is an incredibly strong magnet that images protons (largely water). Protons will exhibit different imaging properties based on whether they are freely moving (water in cysts, cerebrospinal fluid [CSF], T2 bright) or immobile/bound to proteins (T1 bright).
While the physics of MRI feels like magic, it allows for some pretty incredible imaging benefits and accounts for MRI’s superior soft tissue contrast resolution (i.e., it’s very good at differentiating soft tissues of similar density from one another). This makes MRI the gold standard in evaluating the spinal cord, bone infection (osteomyelitis), and incidentally detected masses within various organs in the abdomen and pelvis.
MRI uses gadolinium-based contrast agents similar to CT’s iodinated contrast. This makes pathology stand out and easier to diagnose. While early MRI contrast agents had a serious risk of nephrogenic systemic fibrosis (NSF) in patients with late stage chronic renal failure, we’ve since moved on to newer and better agents that don’t appear to have this association. Current MRI contrast agents are incredibly safe!
- Advantages: Superior soft tissue contrast resolution making MRI the best medical imaging option for the spinal cord, preferred choice over other imaging modalities for evaluating masses and infection in the brain and masses in the abdomen and pelvis, and lack of ionizing radiation.
- Disadvantages: MRI is very sensitive to various artifacts (breathing, gas, motion, metal), expensive, exams may be long (upwards of 60 minutes of laying still in the scanner) and incredibly difficult for patients with significant claustrophobia, will be contraindicated in some patients with implantable devices or with a magnetic foreign body, and are still somewhat operator-dependent.
Nuclear medicine, occasionally referred to as “unclear medicine,” is one of the more interesting imaging modalities out there (I’m biased – I did extra training in nuclear medicine). Fortunately, it’s actually very straightforward!
Nuclear medicine involves giving patients a radioactive medication via IV or by mouth. That radioactive drug will go through a physiological pathway in the body and “trace” that pathway. Hence, radioactive drugs are frequently referred to as “radiotracers.”
Nerd Alert: Patients become walking x-ray machines (technically gamma rays, but aside from how they’re created, they’re identical – light particles/photons). What’s even cooler? PET Scans use ANTIMATTER and image the annihilation reactions between positrons and electrons (E=mc2).
Since a gamma ray is analogous to an x-ray, they also fall into the harmful ionizing radiation category (they interact with other elements and convert them into ions by knocking off an electron).
- HIDA Scan: An injected radiotracer is absorbed by the liver and excreted into the bile. The radiotracer will go wherever the bile goes. Patients with acute cholecystitis have a blocked cystic duct and the gallbladder will not fill with radiotracer. If there is a bile leak, the radiotracer will also leak out.
- Gastric Emptying Study: Patients eat radioactive eggs (or substitute such as oatmeal) and we literally watch how long it takes the stomach to empty to see if it’s rapid, normal, or delayed.
- F-18 FDG PET/CT: Inject radioactive sugar. Cancer cells use higher amounts of sugar than most noncancer cells and cancer cells literally “light up.”
See, pretty straightforward.
Nuclear medicine is split into two categories: single photon imaging and dual photon imaging.
Conventional Nuclear Medicine – Single Photon Imaging
Bread and butter nuclear medicine consists of single-photon imaging and can be found in the majority of hospitals. 99m-technetium is the radioactive element of choice thanks to its excellent imaging properties and comes in an easy-to-use and store generator.
All radioactive elements in this category decay by emitting a single photon (gamma ray) per element (hence “single photon imaging”).
Examples include: HIDA scans (cholecystitis, bile leak), ventilation/perfusion scans (pulmonary embolism), myocardial perfusion/stress test, lymphoscintigraphy (sentinel lymph node detection in breast cancer and melanoma patients), gastric emptying scan (gastroparesis), Octresoscan (images neuroendocrine tumors), and many more.
Positron emission tomography (PET) is a type of nuclear medicine that uses radioactive elements that create positrons (the antimatter equivalent to an electron) and images the light (photons) produced when a positron annihilates with an electron (E=mc2 – the mass of each particle turns into a light particle, aka photon).
18-fluorine is the most commonly used PET radiotracer and 18F-FDG (radioactive sugar) is the most commonly used PET agent and is used for staging/restaging many cancers.
Because annihilation reactions create 2 photons per interaction, PET scans have more photons reach the detector and the image quality is far superior to images created by single-photon emitters.
Switch out a photon emitting element for a radioactive element that emits more harmful particles (electrons, alpha particles) in a radiotracer that seeks out cancer cells and you have a cancer treatment!
What’s even better, most of these radioactive compounds isolate within the cancer cells and the body gets rid of any harmful radiotracer not localized to the cancer cells. This means you have a cancer treatment with far fewer side effects!
- Advantages: Excellent at assessing physiologic processes and instrumental in cancer staging and restaging. Newer PET agents are appearing, drastically changing/improving cancer management.
- Disadvantages: Expensive, newer PET agents are currently in somewhat limited supply with growing demand, some agents result in fairly high patient exposure/dose, and not all agents are widely available and/or have to be ordered day(s) in advance.
The best radiology modality for a particular patient will depend on a number of factors, including the type of condition being diagnosed, the patient’s age, and the availability of equipment.
If you ever find yourself unsure of what to order, simply give your friendly neighborhood radiologist a call – we’re here to help!
- Broken bone: X-ray
- Pregnancy: Ultrasound
- Pelvic pain (female): Ultrasound
- Right upper quadrant pain: Ultrasound
- Appendicitis/Diverticulitis: CT (contrast preferred)
- Appendicitis in pregnancy: Noncontrast MRI
- Indeterminate abdominal mass on CT: MRI
- Stroke: CT (rule out hemorrhage that would preclude treatment), MRI (diagnosis)
- Spinal cord compression: MRI