Diagnostic Imaging: A Primer for the Nurse Practitioner
Let’s review the basics!
First, we should have a solid understanding of how to apply sensitivity & specificity, AND positive and negative predictive values, when ordering a a test. Further, we should always consider our pre-test probability, and whether a test will affect our post-test probability.
Remember to ask the following 5 questions, when ordering a test:
What is the probability that this is [X condition] (low, medium, or high)?
Have I reached the threshold for treatment?
Will any test change my plan for treatment?
Could ordering a test possibly do more harm than good? (e.g. exposure to risks of the test, incidental findings, cost)
What will I do with a positive finding? What about a negative finding?
Diagnostic imaging is complex—ordering the right test depends on the clinical scenario, your pre-test probability, patient factors, test availability, and more. Refer to the diagnostic imaging blog for specific guidance on diagnostic imaging with various clinical scenarios.
BOTTOM LINE: Before ordering a test, consider your pre-test probability and how the result will change management. Ask: What will I do if the test is positive? What will I do if it’s negative? This frames test performance (sensitivity, specificity, predictive values, likelihood ratios) in a practical context.
Radiographs (x-ray)
Plain films
What is it? a form a short wavelength electromagnetic energy. The density of a structure determines its ability to attenuate or “weaken” the x-ray beam (air <fat <water <bone <metal). Structures that have high attenuation (e.g. bone) appear white on imaging (1).
Produces a 2-D image.
Examples of Applications (1):
Detecting bone fractures and dislocations.
Diagnosing and monitoring bone and joint conditions (e.g. arthritis, osteoporosis).
Identifying lung conditions (e.g. pneumonia, lung cancer).
Detecting dental problems (e.g. cavities).
Guiding medical procedures (e.g. placing catheters or stents).
Locating foreign objects (metal, glass, dense plastics - not good for wood).
Cancer detection (e.g. mammography)
Contraindications: pregnancy (relative).
✅ Advantages: quick, inexpensive, widely available, portable, reproducible, easily read.
❌ Limitations: radiation exposure (minimal), 2-D view only, limited soft tissue detail.
Fluoroscopy
What is it? a continuous x-ray that shows organs, tissues, or other internal structures moving in real time. Structures that are radiolucent appear bright, structures that are radiopaque on plain film appear dark (2).
Examples of Applications: for guiding angiogaphic and interventional procedures, in contrast examinations of the GI tract, or for certain surgical procedures (1, 2).
Contraindications: severe allergic reaction to iodinated contrast, pregnancy (1).
✅ Advantages: real-time visualization of structures.
❌ Limitations: increased radiation dose compared to plain films.
BOTTOM LINE: X-rays are quick, cheap, and great for foreign body detection (but not wood), bones and lungs, but give limited soft tissue detail and involve radiation.
Computed Tomography (CT)
What is it? uses X-rays and computer technology to create detailed, cross-sectional images (slices) of the body (similar to looking at a loaf of bread sliced into individual pieces) (2). We can order a contrast CT (iodine or barium commonly used), or non-contrast CT.
Non-Contrast CT:
Dense structures (bone, blood, calcifications) show up well without contrast (3).
Examples of Applications:
Acute hemorrhage (e.g., stroke, trauma).
Bone fractures and spine evaluation.
Kidney stones (contrast may obscure them).
Lung disease (initial nodule detection, pneumothorax).
Contraindications: pregnancy.
✅ Advantages: fast, widely available, inexpensive (compared to MR), excellent for bone, hemorrhage, and kidney stones, no risk of contrast reaction or nephrotoxicity.
❌ Limitations: more radiation than X-rays, limited soft tissue detail, may miss vascular pathology, tumours, and infection.
Contrast CT
Contrast highlights blood vessels and differentiates soft tissue structures (3).
Examples of Applications:
Tumour detection and staging.
Infection/abscess.
Vascular imaging (CT angiography for PE, aortic dissection).
Abdominal emergencies (appendicitis, diverticulitis, bowel ischemia).
Contraindications: pregnancy (relative), severe allergic reaction to contrast agents, chronic or worsening kidney disease or acute kidney injury, radioactive iodine treatment for thyroid disease (3).
✅ Advantages: fast, widely available, enhances visualization of tumours, abscesses, vascular structures, and abdominal pathology. Essential for CT angiography (PE, aortic dissection, ischemia).
❌ Limitations: more radiation than x-rays, may obscure stones in urinary tract, risk of allergy or nephrotoxicity.
BOTTOM LINE: CT without contrast is best for bones, bleeding, and stones, while CT with contrast is best for vessels, tumours, and infection.
Ultrasound (US)
What is it? high-frequency sound waves are transmitted from a transducer and passed through tissues; reflections of the sound waves are picked up by the transducer and transformed into images (1). Reflection or “echo” occurs when the sound waves reflect off tissue interfaces of different acoustic densities. Structures are described based on their echogenicity (1). Hyperechoic structures appear bright on US, hypoechoic structures appear dark (1).
Duplex scan: Uses ultrasound to create a black-and-white (grey-scale) image and measures how fast blood is moving by detecting the change in sound as it bounces off moving blood (1).
Colour doppler: Adds color to the ultrasound image to show the direction of blood flow—usually red means blood is moving toward the probe, and blue means it’s moving away (1).
Examples of Applications:
Real-time imaging can guide biopsies and other procedures (e.g. central line placement, abscess drainage).
Assessment of soft tissue and fluid-filled structures.
Abdomen/pelvis: gall stones, liver lesions, kidney pathology, ascites, pregnancy, pelvic pathology.
Vascular: DVT, carotid stenosis, blood flow assessment (doppler).
MSK: tendon tears, joint effusions, soft tissue masses.
Thyroid and neck: thyroid nodules, lymph nodes, parathyroid pathology.
Cardiac: echocardiography (e.g. heart failure, valvular disease).
Contraindications:
✅ Advantages: it is safe, non-invasive, portable, low-cost, offers multiple imaging planes, can assess blood flow with doppler images, differentiates cystic from solid structures (1).
❌ Limitations: can be limited by air in the bowel, body habitus, provides a smaller field of view, poor for bone evaluation, and is operator-dependent (1).
BOTTOM LINE: Ultrasound is a safe, real-time, non-invasive tool best for soft tissue, fluid, and blood flow assessment, guiding procedures, and evaluating organs, vessels, and joints—though it’s limited by body habitus and operator skill.
Magnetic Resonance Imaging (MR)
MR without Contrast
What is it? is a non-invasive imaging technique that uses a strong magnetic field and radio waves to create detailed cross-sectional images of the body. Unlike X-ray or CT, it does not use ionizing radiation (1, 4).
Examples of Applications (4):
Brain (detecting stroke [especially acute ischemic stroke with diffusion-weighted imaging] multiple sclerosis plaques, brain tumours [initial evaluation], demyelinating disease).
Spinal cord (disk herniation, spinal stenosis, spinal cord lesions).
Musculoskeletal injuries (ligaments, meniscus, cartilage).
Abdomen/pelvis (liver, kidneys, and other organs for structural abnormalities - though contrast may be needed for vascular lesions or tumours).
✅ Advantages:
Superior natural soft tissue contrast.
Excellent for brain, spine, joints, cartilage, ligaments.
No radiation.
Useful for detecting chronic disease (e.g., old strokes, MS plaques).
❌ Disadvantages:
Longer scan time, less available, more expensive.
Motion sensitive—patients need to stay still.
Claustrophobia and metal implant issues.
May not fully distinguish active vs inactive lesions.
MR with contrast
What is it? MR that uses a contrast dye (usually gadolinium) to highlight active disease, blood flow, and breakdown of the blood-brain barrier (1).
Examples of Applications (4):
Tumour detection and characterization (brain/spine, liver, breast). Contrast helps differentiate tumour tissue from surrounding normal tissue.
Infection or abscess (osteomyelitis, meningitis, encephalitis, spinal abscess).
Inflammatory and demyelinating disease activity (e.g., MS plaques enhancing, inflammatory lesions).
To assess vascular pathology (aneurysms, vascular malformations, and areas of ischemia or infarct).
✅ Advantages:
Highlights tumours, infection, inflammation, active demyelination.
Excellent for vascular imaging (MR angiography/MR venography).
Shows blood-brain barrier breakdown.
❌ Disadvantages:
Contraindicated in severe renal dysfunction.
Small risk of gadolinium allergy.
Same limitations as non-contrast MR (cost, time, motion sensitivity).
BOTTOM LINE: MR without contrast is best for anatomy, tissue structure, and chronic disease, while MR with contrast highlights tumours, infection, inflammation, blood vessel pathology and vascular lesions.
Check out the diagnostic imaging blog for more reading and case-based learning!
References Toronto Notes. (2017). Toronto Notes 2017. Toronto, Ontario, Canada: Toronto Notes for Medical Students Inc.Shalom NE, Gong GX, Auster M. Fluoroscopy: An essential diagnostic modality in the age of high-resolution cross-sectional imaging. World J Radiol. 2020 Oct 28;12(10):213-230. doi: 10.4329/wjr.v12.i10.213. PMID: 33240462; PMCID: PMC7653184.Rawson JV, Pelletier AL. When to Order a Contrast-Enhanced CT. Am Fam Physician. 2013 Sep 1;88(5):312-6. PMID: 24010394.Ashby K, Adams BN, Shetty M. Appropriate Magnetic Resonance Imaging Ordering. [Updated 2022 Nov 14]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK565857/