Radiology Modalities

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.

Read on to learn more about the various radiology modalities!

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

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).

Radiography

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).

Fluoroscopy

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

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

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)

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.

As you can see, MRI scanners look very similar to CT scanners but there are major differences between a CT scan vs MRI scan.

MRI uses gadolinium-based contrast ag