Introduction to Radiology for Physician Assistants

Updated: 06/09/2016

In this post, I hope to introduce you all to the differences between imaging modalities, including indications and contraindications. My goal is also to provide a basic understanding of the concepts surrounding density and normal x-ray findings. Future posts will investigate the findings for pneumonia, pneumothoraces, effusions, atelectasis, fractures/dislocations, arthritis, intracranial, GI and hepatic pathology, bowel, lines/tubes, and heart disease.

Conventional Radiography (Plain Films)

Images produced through use of ionizing radiation
No contrast material (barium or iodine)

Why it’s important: Large doses of radiation can produce cell mutations leading to cancer or anomalies. Even low levels of radiation are teratogenic (avoid in pregnancy).

Relatively inexpensive, obtained easily (portable, mobile), most widely used
Common uses: chest x-ray (CXR), abdominal x-ray, and for fractures or arthritis

Computed Tomography (CT or CAT Scans)

Uses a gantry with a rotating x-ray beam and multiple detectors with sophisticated algorithms to process the data
Expensive equipment, lots of space, and high computer processing power required

Ultrasound (US)

Uses acoustic energy above the frequency of human hearing to produce images
Uses a transducer (produces and records signal), processed by a computer
Inexpensive compared to CT/MRI, widely available, portable/handheld
Very safe with no major side effects
No use of ionizing radiation - great for women of childbearing age, pregnant women, and children
Common uses: Used to image soft tissues and delineating solid from cystic structures, image-guided biopsies, non-invasive means to study blood flow

Magnetic Resonance Imaging (MRI)

Utilizes potential energy stored in body’s hydrogen atoms (manipulated by magnetic fields and radiofrequency pulses) to produce localizing and tissue-specific energy
2 or 3-D images
Not as widely available as CT, expensive, and high-operating costs
No ionizing radiation, produce higher contrast between different types of soft tissue compared to CT
Cannot use with objects within the body (pacemakers), ferromagnetic objects in MRI-scanner field (O2 tanks)
Common uses: neurologic imaging, soft tissues (muscles, tendons, ligaments)
Adverse effects from some MRI contrast agents

Terminology Conventions

Most images are viewed as if you are viewing the patient face-to-face (no matter the position of the patient when the image was exposed)

Ex. the patient’s R-side is on your L-side and vice-versa

Study: collection of images to examine a particular part of the body or system
Contrast material or agent: administered to a patient to make structures more visible

Liquid barium (oral for upper GI exams and barium enemas)
Iodine (IV for contrast-enhanced CT scans)
Gadolinium (IV solution for MRI) and ultrasound (gas-filled microbubbles)
Dye is a lay-term for contrast (only used if talking to a patient)

Horizontal vs. Vertical X-rays

Horizontal x-ray beams are parallel to the floor (duh!), also called upright chest examinations

Why is this important? An air-fluid or fat-fluid level will only be visible if the x-ray beam is horizontal (regardless of the patient’s position)
Any study with the terms: erect, upright, cross-table, or decubitus are by default horizontal

Vertical x-ray beams are between the tube and cassette (supine radiographs)

Examples	Orientation	Implications
Upright Abdomen	Horizontal	Air-fluid levels Free-air under the diaphragm
Left lateral decubitus of Abdomen	Horizontal	Air-fluid levels Free-air over the liver
Supine Abdomen	Vertical	Free-air only if large amounts NO AIR FLUID LEVELS
Upright Chest	Horizontal	Pneumothorax at apex of lung Air-fluid levels
Supine Chest	Vertical	Pneumothorax only if large NO AIR FLUID LEVELS

The Five Basic Densities

From least to most dense

Density	Appearance
Air (black)	Absorbs the least (least dense)
Fat	Gray, somewhat blacker than soft tissue
Fluid or Soft Tissue	Both fluid (blood) and soft tissue (muscle) have the same densities on XR Note: cannot differentiate between blood/heart muscle on XR
Calcium (White)	Most dense, naturally occurring material (bones)
Metal (Whitest)	Absorbs all x-rays (bullets, barium), most dense

White and Black

	Terms for “White”	Terms for “Black”
Conventional Radiographs	Increased density or opaque	Decreased density or lucent
CT	Increased (high attenuation) Hyperintense or hyperdense	Decreased (low attenuation) Hypodense
MRI	Increased (high) signal intensity Bright	Decreased (low) signal intensity Dark
Ultrasound	Increased echogenicity Sonodense	Decreased echogenicity Sonolucent
Nuclear Medicine	Increased tracer uptake	Decreased tracer uptake
Barium studies	Radiopaque	Nonopaque Radiolucent

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Vasculature is seen as white lines on an x-ray (cannot differentiate pulmonary arteries and veins)
Bronchi are invisible because they are thin-walled, contain air, and surrounded by air
Pleura, composed of 2 layers and an inner pleural space, contains several mm of fluid but no air → neither are visible on XR (if they are, they are no thicker than a thin line)

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2-Views (the Upright and Left Lateral XR)

Left Lateral (left side against film): imagine you are looking at the patient’s right side, spine should be on your left

Helps to determine location, confirm the presence, or rule out disease based on frontal (upright) XR findings
Helps to visualize hilar densities (enlarged pulmonary arteries) or lymph nodes, which cast a distinct lobulated mass-like shadow
Major (Oblique) and Minor (Horizontal) Fissures

Both may be visible as fine, white lines and differentiate the upper and lower lobes on left, upper, middle, and lower lobes on right
Major crosses obliquely through 5th thoracic vertebra to point on diaphragmatic surface of pleura a few cm behind sternum
Minor fissure crosses at level of 4th anterior rib (R-side only) and is horizontal
Only minor fissure seen on frontal view because of oblique plane of major fissure

Degeneration can lead to narrowing of the disk space and development of small, bony spurs (osteophytes) at margins
Right hemidiaphragm is visible in entire length (front-back) and is slightly higher than left
Left hemidiaphragm is sharply posterior and is silhouetted by muscle of heart anteriorly
Air in the stomach or splenic flexure of the colon appears immediately below the left hemidiaphragm
Posterior costophrenic sulcus (angles) - each hemidiaphragm produces a rounded dome that indents the central portion of the base of the lung producing a sulcus (depression) surrounding the periphery of each lung

Air in the stomach or splenic flexure appears below the left hemidiaphragm
Note on frontal XR, these are called lateral costophrenic angles (sulci)
Only requires 75 cc or less to blunt posterior angle on lateral films and 250-300 cc to blunt lateral angles on frontal

Five Key Areas on Lateral CXR

Region	Normal Findings
Retrosternal Clear Space	Lucent crescent between sternum and ascending aorta Note: do not mistake patient’s arms (superimposed soft tissue) for ‘filling in’ of clear space; occasionally patients are too weak to raise arms above their heads → if you see the humerus, it's an arm!	If obscured, think anterior mediastinal lymphadenopathy (most common)
Hilar Region	No discrete mass present	Look for a lobulated mass in region of the hila as shown above
Fissures	Major and minor fissures should be pencil thin (if at all)	Become thick with fluid (added Kerley B lines; think pleural effusions) or fibrosis (no other signs)
Thoracic Spine	Rectangular vertebral bodies with parallel end plates Disk spaces maintain height from top to bottom, each becomes slightly taller than or remains the same height as the one above it	Compression fracture = loss of body height
Diaphragm and Posterior Costophrenic Sulci	R-hemidiaphragm slightly higher than left Sharp posterior costophrenic sulci	Pleural effusions blunt the costophrenic angles

The 5 Technical Factors - Is My X-ray Adequate?

Underpenetration (inadequate): will appear too light and you won’t be able to see the spine through the heart shadow

Left hemidiaphragm may not be visible on frontal XR, left lung base opaque, which can mask true pneumonia or pleural effusions → get a lateral XR to confirm
Pulmonary markings may appear more prominent, leading you to think patient has CHF or pulmonary fibrosis → get lateral XR and look for increased markings, airspace disease, or effusion

Overpenetration (too dark), lung markings appear decreased or absent

Leads you to think patient has emphysema or pneumothorax → look for other radiographic findings to support your claim or repeat XR

Degree of Inspiration - count number of posterior ribs above the diaphragm on frontal XR

Posterior ribs are immediately apparent and oriented horizontally, each attaching to a thoracic vertebral body
Anterior ribs are harder to see and are oriented downward toward the feet, attaching to the sternum
Poor inspiration: compresses and crowds lung markings, especially at the bases leading you to think a lower lobe pneumonia exists → confirm with lateral XR

Significant Rotation - may alter contour of heart, vasculature, hila, and hemidiaphragms based on the position of the patient

Medial ends of clavicles are anterior and spinous process is posterior; should be equidistant from medial ends of clavicle on frontal XR
If patient’s spinous process is closer to left clavicle → rotated toward R-side and vice-versa
Marked rotation can make hilum appear larger → confirm with lateral XR or compare with previous XR
Hemidiaphragm may appear higher on side rotated away from cassette → compare to previous study
Remember: objects that are farther from the cassette are magnified compared to closer objects, always compare discrepancies to older XR or order a lateral

Magnification - to assess the size of the heart

The closer the object to imaging surface, the truer its actual size; farther it is, the more magnified
Most CXRs are posteroanterior (PA) views - heart is closer (anterior) and truer to actual size

This means the x-ray beam enters at P (posterior) and exits at A (anterior)

Take Away: In AP image, heart is farther away and slightly magnified

Angulation

X-ray beam normally passes horizontally (parallel to floor) in upright XR, but hospitalized patients may not be able to sit up in bed
Excessive angulation: clavicles will project at or above the posterior first ribs distorting the normal ‘S’ shape of the clavicle (makes them straight)

May also make heart enlarged, mimicking cardiomegaly

Sharp border of left hemidiaphragm may be lost, mimicking a pleural effusion or left lower lobe pneumonia → recognize artifacts and understand distortion, consult radiology

	Normal Findings	Abnormal Findings
Penetration	Spine visible through the heart	Ex. Underpenetration
Inspiration	At least 8-9 posterior ribs visible
Rotation	Spinous process should be equidistant between medial ends of clavicles	Hemidiaphragm appears higher on side rotated away (L) from cassette
Magnification	AP films (most) will magnify heart
Angulation	Clavicle has ‘S’ shape and superimposes on rib 3 or 4