BCSE Diagnostics

Diagnostic Imaging: Radiography – BCSE Study Guide

📅 March 28, 2026 ⏱ 30 min read

Radiography remains the cornerstone of veterinary diagnostic imaging and is essential for evaluating the thorax, abdomen, and skeletal system.

Overview and Clinical Importance

Radiography remains the cornerstone of veterinary diagnostic imaging and is essential for evaluating the thorax, abdomen, and skeletal system. Understanding radiographic principles, proper positioning, and systematic interpretation is fundamental for the entry-level veterinarian. The BCSE tests your ability to select appropriate radiographic techniques, recognize normal versus abnormal findings, and integrate radiographic findings with clinical presentations.

High-YieldRadiography questions often integrate physics concepts with clinical interpretation. Expect questions that combine understanding of exposure factors (kVp, mAs) with recognition of radiographic abnormalities.

Factor	Function	Effect on Image
kVp (Kilovolt Peak)	Controls energy and speed of electrons; determines penetrating ability of x-ray beam	Higher kVp = more penetration, lower contrast (more gray shades). Lower kVp = less penetration, higher contrast (more black/white).
mA (Milliamperage)	Controls quantity of electrons flowing in x-ray tube; determines number of x-rays produced per unit time	Higher mA = more x-rays produced = increased image density (blackness). Does NOT affect penetration.
Time (seconds)	Duration of electron flow; controls total exposure	Longer time = more x-rays reach detector. Short times (less than 1/60 sec) reduce motion blur.
mAs (mA x seconds)	Product of milliamperage and time; total radiation exposure	Directly proportional to image density. Doubling mAs doubles film blackening.

Section 1: Radiographic Principles and Physics

X-Ray Production and Properties

X-rays are electromagnetic radiation with short wavelengths that allow penetration of body tissues. The shorter the wavelength, the higher the frequency and penetrating power. X-rays travel in straight lines and are produced when high-speed electrons strike a target anode within the x-ray tube. Understanding these fundamental properties is essential for controlling image quality.

[Include Image: Figure 1. X-ray tube components showing cathode, anode, and electron stream] Source: Wikimedia Commons - https://commons.wikimedia.org/wiki/File:X-ray_tube.svg (Public Domain)

Primary Exposure Factors

MEMORY TIP - kVp vs mAs: "kVp controls QUALITY (penetration and contrast), mAs controls QUANTITY (number of x-rays and density)." Think: Quality starts with Q like kVp sounds quality, Quantity is about amount like mAs.

High-YieldThe 15% Rule: A 15% increase in kVp doubles the radiographic density and has the same effect as doubling the mAs. This is crucial for adjusting techniques and is commonly tested.

Santes Rule for kVp Selection

For calculating appropriate kVp based on patient size: kVp = (2 x tissue thickness in cm) + 40 + grid factor. This formula provides a starting point for technique charts. The grid factor is typically +8 to +10 kVp when using a grid.

MEMORY TIP - Santes Rule: "Two Times Thickness, Plus Forty" - 2T + 40 = base kVp. Easy to remember: just double the centimeters and add 40!

Inverse Square Law

The intensity of x-rays decreases proportionally to the square of the distance from the source. If you double the distance, the intensity decreases to one-quarter. Formula: I1/I2 = (D2)squared/(D1)squared. This principle affects both image quality and radiation safety.

MEMORY TIP - Inverse Square Law: "DISTANCE SQUARED - Doubled distance means QUARTERED intensity." Remember: 2x distance = 1/4 intensity, 3x distance = 1/9 intensity.

Image Quality Factors

High-YieldCommon exam question: An underexposed radiograph requires increased mAs to improve density. If contrast is the issue, adjust kVp instead.

Radiation Safety - ALARA Principle

The ALARA principle (As Low As Reasonably Achievable) guides radiation safety in veterinary practice. The three pillars of protection are Time, Distance, and Shielding. Minimize time of exposure, maximize distance from the x-ray source, and use appropriate shielding including lead aprons, thyroid shields, and gloves.

MEMORY TIP - ALARA Protection: "TDS" - Time, Distance, Shielding. Also remember: Personnel who are pregnant or under 18 should not be involved in radiography when possible.

Factor	Definition	Clinical Significance
Density	Overall blackness of the radiograph; controlled by mAs	Underexposed = too light (increase mAs). Overexposed = too dark (decrease mAs).
Contrast	Difference between adjacent structures; primarily controlled by kVp	High contrast (short scale) = few gray shades. Low contrast (long scale) = many gray shades.
Detail (Sharpness)	Clarity of structural edges; affected by motion, focal spot size, and geometry	Motion blur is the most common cause of poor detail in veterinary radiography.
Distortion	Misrepresentation of object size or shape	Magnification increases with increased OFD (object-film distance) and decreased FFD (focal-film distance).

Section 2: Patient Positioning

Proper positioning is fundamental to obtaining diagnostic-quality radiographs. Poor positioning can obscure pathology, create artifacts, and necessitate repeat exposures. Orthogonal views (two perpendicular projections) are standard for most studies.

Standard Radiographic Projections

MEMORY TIP - Projection Naming: The projection name describes x-ray beam entry point FIRST, exit point SECOND. VD = enters Ventral, exits Dorsal.

Thoracic Positioning Pearls

Right lateral (RL) is the standard lateral view because the cardiac silhouette is most consistent in this position
Both lateral views are recommended as the dependent lung undergoes atelectasis and lesions may be obscured
VD view best for evaluating the caudal lung lobes; DV view best for evaluating cranial lung fields and accessory lobe
Radiographs should be taken at peak inspiration for optimal lung evaluation

High-YieldOn a left lateral radiograph, the cardiac silhouette rotates away from the sternum - this normal finding should NOT be mistaken for pneumothorax!

Abdominal Positioning Pearls

Standard views: Right lateral, Left lateral, and VD constitute a three-view series (current standard of care)
Left lateral view: Gas rises to pylorus and duodenum - excellent for pyloric foreign body detection
Right lateral view: Gas rises to fundus - standard for GDV evaluation (shows dorsally displaced pylorus)
DV view used for GDV evaluation and contrast studies

MEMORY TIP - Stomach Gas Position: "LEFT lateral = gas in PYLORUS (on the LEFT side of the image). RIGHT lateral = gas in FUNDUS (on the RIGHT side of the image)." Remember: Gas rises to the top!

[Include Image: Figure 2. Diagram showing stomach gas position on left vs right lateral recumbency] Source: University of Illinois Imaging Anatomy - https://vetmed.illinois.edu/imaging_anatomy/ (Educational Use)

Abbreviation	Full Name	X-Ray Beam Direction
VD	Ventrodorsal	Beam enters ventral surface, exits dorsal surface (patient in dorsal recumbency)
DV	Dorsoventral	Beam enters dorsal surface, exits ventral surface (patient in sternal recumbency)
RL or RLAT	Right Lateral	Patient lying on right side; beam enters left, exits right
LL or LLAT	Left Lateral	Patient lying on left side; beam enters right, exits left
CrCd	Craniocaudal	Beam enters cranial aspect, exits caudal aspect (limbs)
CdCr	Caudocranial	Beam enters caudal aspect, exits cranial aspect (limbs)

Section 3: Thoracic Radiograph Interpretation

Systematic evaluation of thoracic radiographs is essential. The approach should compartmentalize findings into: extrathoracic structures, pleural space, pulmonary parenchyma, and mediastinum (including cardiac silhouette). This organized approach ensures no pathology is overlooked.

Vertebral Heart Score (VHS)

The VHS provides an objective measurement of cardiac size by comparing the sum of the heart long and short axis measurements to vertebral body lengths. This method normalizes cardiac size to patient body size and reduces subjective interpretation error.

High-YieldBreed variation affects VHS - Bulldogs, Cavalier King Charles Spaniels, and Boxers have naturally higher VHS values. Short thoracic vertebrae in brachycephalic breeds can falsely elevate VHS measurements.

MEMORY TIP - VHS Values: "Dogs: 10 is the MAGIC NUMBER" (normal up to about 10.5). "Cats: 8 is GREAT" (normal is less than 8.1).

[Include Image: Figure 3. Lateral thoracic radiograph demonstrating VHS measurement technique] Source: PMC Open Access - https://pmc.ncbi.nlm.nih.gov/articles/PMC8529508/ (CC BY-NC-ND)

Pulmonary Patterns

Pulmonary patterns describe characteristic radiographic appearances associated with different types of lung pathology. Recognition of these patterns helps narrow the differential diagnosis list. Remember that many diseases present with mixed patterns.

MEMORY TIP - Air Bronchogram Recognition: "Air bronchograms = ALVEOLAR pattern." The bronchi contain air, but surrounding alveoli are filled with fluid, making the bronchial lumen visible. If you see air bronchograms, think pneumonia or edema!

MEMORY TIP - Pulmonary Vessel Location: "Arteries are ABOVE, Veins are VENERAL (ventral)" - On VD/DV views, pulmonary arteries are dorsal (lateral) to bronchi, veins are ventral (medial) to bronchi.

High-YieldPulmonary metastases must be greater than 4-5 mm to be visible radiographically. Tiny 1-3 mm mineral foci in older dogs (especially Collies and Shelties) are usually osseous metaplasia, NOT metastasis!

[Include Image: Figure 4. Lateral thoracic radiograph showing alveolar pattern with air bronchograms] Source: Wikimedia Commons - https://commons.wikimedia.org/wiki/Category:Veterinary_radiology (Various Licenses)

Pleural Space Evaluation

Normal pleural space is not visible - only potential space
Pleural effusion signs: retraction of lung lobes from thoracic wall, rounding of costophrenic angles, interlobar fissure lines, loss of cardiac silhouette definition
Pneumothorax signs: elevation of heart from sternum, lung lobe retraction, absence of pulmonary vessels in periphery, increased radiolucency dorsally
Best view for pneumothorax: horizontal beam lateral with patient in lateral recumbency (gas rises to non-dependent hemithorax)

Species	Normal VHS Range	Measurement Method
Dog	8.4 - 10.5 vertebrae (mean 9.7 plus/minus 0.5)	Long axis + Short axis, measured starting at T4 on right lateral view
Cat	Less than 8.1 vertebrae (mean 7.5 plus/minus 0.3)	Same method; cardiac silhouette normally 2-2.5 intercostal spaces wide

Section 4: Abdominal Radiograph Interpretation

Abdominal radiograph interpretation relies heavily on recognizing serosal detail, organ size and position, and gas patterns within the gastrointestinal tract. A systematic approach evaluating each organ system ensures thorough assessment.

Serosal Detail (Peritoneal Detail)

Serosal detail refers to the ability to visualize organ margins within the abdomen. This detail depends on contrast between fat and soft tissue structures. Normal abdominal fat provides the contrast needed to see organ borders.

MEMORY TIP - Effusion vs Emaciation: "Round = Fluid, Tucked = Thin." Effusion causes the abdomen to bulge; emaciation causes it to appear tucked or concave.

Gastrointestinal Assessment

Small Intestinal Dilation Criteria

Intestinal dilation suggests obstruction. The key measurements help differentiate normal from abnormal:

High-YieldThe "rule of thumb" for maximum small intestinal diameter: In dogs, SI diameter should not exceed the width of a rib; in cats, it should not exceed 12 mm.

Gastric Dilatation-Volvulus (GDV)

GDV is a surgical emergency requiring rapid radiographic diagnosis. Classic findings include compartmentalization of the stomach with a bipartite appearance and dorsal displacement of the pylorus.

Right lateral view is standard for GDV evaluation - gas-filled pylorus appears dorsally displaced ("double bubble" or "Popeye arm" sign)
Soft tissue band (shelf sign) may divide stomach into compartments
Associated findings: microcardia, small caudal vena cava (hypovolemia), splenomegaly, dilated esophagus
Gas in gastric wall = gastric necrosis (grave prognosis)

MEMORY TIP - GDV Recognition: "Double Bubble on the Right" - The GDV creates a compartmentalized stomach best seen on RIGHT lateral. The pylorus (normally ventral) moves DORSALLY in GDV.

[Include Image: Figure 5. Right lateral abdominal radiograph showing classic GDV with compartmentalization] Source: MSPCA-Angell Educational Resources - https://www.mspca.org/angell_services/abdominal-radiography/

Linear Foreign Body Signs

Plication (accordion-like bunching) of intestines
Geometric gas patterns: comma-shaped, crescent-shaped, or triangular gas bubbles
Often anchored at base of tongue (cats) or pylorus (dogs)
Tortuous, bunched duodenum curving away from normal path

MEMORY TIP - Linear Foreign Body Gas Patterns: "Paisley, Comma, Crescent" - These GEOMETRIC shapes of gas trapped between plicated bowel loops suggest linear foreign body. Normal obstruction creates ROUND, dilated loops.

Pneumoperitoneum

Free gas in the peritoneal cavity is abnormal and usually indicates GI perforation. Best detected on horizontal beam lateral radiograph with patient in lateral recumbency - gas rises and outlines the diaphragmatic crura and serosal surfaces.

High-YieldFree abdominal gas highlights structures NOT normally visible: the diaphragmatic crura become visible, serosal surfaces of liver and kidneys are outlined. This is the opposite of effusion which obscures detail.

Pattern	Radiographic Appearance	Common Causes
Alveolar	Fluffy, cotton-like opacity; air bronchograms (air-filled bronchi visible against fluid-filled lung); silhouette sign (loss of border with heart/diaphragm); lobar distribution	Pneumonia, pulmonary edema (cardiogenic or non-cardiogenic), hemorrhage, aspiration, near drowning
Bronchial	"Rings and lines" or "tram lines" - thickened bronchial walls visible end-on (rings) and in profile (parallel lines); generalized distribution	Chronic bronchitis, feline asthma, bronchiectasis, aging changes (common incidental finding in older animals)
Interstitial - Structured	Discrete nodules or masses; well-defined margins when surrounded by aerated lung; variable size and number	Metastatic neoplasia (nodules must be greater than 4-5 mm to be visible), primary lung tumors, granulomas, fungal disease, parasites
Interstitial - Unstructured	Hazy, ground-glass opacity; increased lung opacity without discrete margins; vessels still visible but less distinct	Early edema, fibrosis, viral pneumonia, hemorrhage, underexposure artifact, expiratory film artifact
Vascular	Enlarged pulmonary arteries and/or veins; arteries extend dorsally on VD view, veins extend ventrally	Pulmonary arteries: heartworm, PTE, pulmonary hypertension. Pulmonary veins: left-sided CHF. Both: left-to-right shunts (PDA, VSD)

Section 5: Skeletal Radiograph Interpretation

Skeletal radiography requires minimum of two orthogonal views as single projections cannot display the complex three-dimensional geometry of most fractures or bone lesions. Remember that approximately 50% of bone mineral must be lost before lysis becomes radiographically visible.

Fracture Classification

MEMORY TIP - Oblique Fracture Length: "SHORT oblique = LESS than 2x diameter, LONG oblique = MORE than 2x diameter." Think: a SHORT person is LESS than average height.

Salter-Harris Classification (Physeal Fractures)

The Salter-Harris classification describes fractures involving the growth plate in immature animals. Higher types have progressively worse prognosis for normal growth.

MEMORY TIP - Salter-Harris Mnemonic: "SALTR" - Type I: Separation (Slipped). Type II: Above (most common). Type III: Lower (below through epiphysis). Type IV: Through/Total (everything). Type V: Ruined/Rammed (crushed).

High-YieldSalter-Harris Type II is the MOST COMMON physeal fracture and has the best prognosis. Type V may not be visible initially - suspect with history of crush injury and subsequent growth disturbance.

[Include Image: Figure 6. Diagram of Salter-Harris fracture classification Types I-V] Source: Wikimedia Commons - https://commons.wikimedia.org/wiki/File:Salter-Harris_classification.svg (CC BY-SA)

Aggressive vs Non-Aggressive Bone Lesions

Distinguishing aggressive from non-aggressive bone lesions is crucial for prioritizing differentials and determining urgency of further diagnostics. Aggressive lesions suggest neoplasia or osteomyelitis; non-aggressive lesions include fracture callus, cysts, and degenerative changes.

MEMORY TIP - Aggressive Lysis Patterns: "Geographic = Good, Moth-eaten = Medium, Permeative = Poor." The more holes and the smaller they are, the more aggressive the lesion. Geographic is a SINGLE hole with clear edges.

MEMORY TIP - Periosteal Reaction: "Smooth = Slow, Spiky = Scary." Continuous periosteal reaction that you can trace smoothly indicates a slow, non-aggressive process. Interrupted, spiculated reactions indicate aggressive disease.

High-YieldIf unsure whether a lesion is aggressive, repeat radiographs in 2-4 weeks. Aggressive lesions will show rapid progression; non-aggressive lesions will show minimal change or healing.

Fracture Healing Assessment - The 4 As

Post-operative radiographic evaluation uses the systematic 4 As approach:

Alignment: Are joints above and below anatomically aligned?
Apposition: Is there adequate fragment contact or proximity?
Apparatus: Are implants appropriate size, position, and number?
Activity: Is there evidence of bone healing (callus formation)?

MEMORY TIP - 4 As of Fracture Healing: "AAAA" - Alignment, Apposition, Apparatus, Activity. The first three are evaluated immediately post-op; Activity is evaluated on follow-up radiographs.

Cause of Poor Serosal Detail	Differentiating Features
Peritoneal effusion (ascites, hemorrhage, urine, exudate)	Abdomen appears rounded; ventral bulge on lateral view; organs "float" and are difficult to distinguish
Lack of abdominal fat (emaciation)	Abdomen appears tucked/concave; spine visible with tented soft tissues; vertebrae clearly seen
Young animal (less than 2-3 months)	Normal finding in juveniles who lack intra-abdominal fat; open vertebral physes visible
Peritonitis/carcinomatosis	May see focal or generalized loss; gas bubbles outside bowel suggest perforation
Technical factors	Underexposure creates apparent poor detail; check spine penetration over scapulae on lateral view

Section 6: Contrast Radiography

Contrast radiography uses radiopaque or radiolucent substances to enhance visualization of structures that are normally poorly defined on survey radiographs. Although many contrast procedures have been supplanted by ultrasound and advanced imaging, several remain clinically important.

Types of Contrast Media

High-YieldNever use IONIC iodinated contrast for myelography - the high osmolarity causes severe damage to neural tissue and can be fatal. Always use NON-IONIC agents for spinal studies.

MEMORY TIP - Barium vs Iodine Decision: "BARIUM = BETTER for GI (unless perforation). IODINE = INJECTABLE (except ionic in spine)." If GI perforation is suspected, use water-soluble iodinated contrast because it will be absorbed if it leaks.

Common Contrast Procedures

Upper GI Study (Barium Series)

Indication: Suspected partial obstruction, masses, motility disorders when survey films inconclusive
Dose: 6 ml/lb (12 ml/kg) of 20-30% w/v barium sulfate via orogastric tube
Timing: Radiographs at 0, 15, 30 min, 1, 2, 3 hours in dogs; more frequent in cats (faster transit)
Normal transit: Barium reaches colon in 180 plus/minus 90 minutes in dogs, 30-60 minutes in cats

Excretory Urography (IVU/IVP)

Indication: Evaluation of kidneys, ureters, bladder (ectopic ureters, ureteral obstruction, renal mass)
Dose: 400-800 mg iodine/kg IV (typically 2 ml/kg of 300-400 mgI/ml solution)
Timing: Nephrogram phase at 0-1 min; pyelogram phase at 5-15 min; delayed films if poor renal function
Preparation: Empty bladder (pneumocystogram recommended), fast patient, obtain survey films first

Myelography

Indication: Spinal cord compression when MRI/CT not available; localization of disc disease
Agent: NON-IONIC iodinated contrast ONLY (iohexol, iopamidol) - 0.3 ml/kg
Injection site: Cerebellomedullary cistern (C-M tap) or lumbar subarachnoid space (L5-L6)
Findings: Extradural, intradural-extramedullary, or intramedullary compression patterns

High-YieldFor ectopic ureter evaluation: Perform pneumocystography BEFORE IV contrast injection and obtain 20-degree oblique views of the trigone region to visualize ureteral insertion points.

MEMORY TIP - IVU Timing Phases: "Neph-NOW, Pyelo-FIVE" - Nephrogram (kidney parenchyma opacification) is immediate (0-1 min); Pyelogram (collecting system and ureters) is at 5 minutes.

Radiographic Physics

kVp controls penetration and contrast (quality); mAs controls density (quantity)
15% rule: 15% increase in kVp = doubling of density (same as doubling mAs)
ALARA principle guides radiation safety: Time, Distance, Shielding

Positioning

Orthogonal views (minimum 2 projections) are standard for all studies
Left lateral = gas in pylorus; Right lateral = gas in fundus (critical for GDV evaluation)
Three-view abdominal series is the current standard of care

Thoracic Interpretation

Normal VHS: Dogs 8.4-10.5; Cats less than 8.1 (breed variation exists)
Pulmonary patterns: Alveolar (air bronchograms), Bronchial (rings/lines), Interstitial (structured/unstructured), Vascular
Air bronchograms = alveolar pattern = pneumonia/edema

Abdominal Interpretation

Poor serosal detail: effusion (rounded abdomen) vs emaciation (tucked abdomen) vs juvenile (normal)
SI dilation: Dogs greater than 1.6x L5 height; Cats greater than 2x L2 height
GDV: compartmentalized stomach, dorsally displaced pylorus, shelf sign on right lateral

Skeletal Interpretation

Aggressive lesions: moth-eaten/permeative lysis, interrupted periosteal reaction, long zone of transition
Salter-Harris Type II is most common physeal fracture (best prognosis)
4 As of healing assessment: Alignment, Apposition, Apparatus, Activity

Contrast Radiography

Barium: GI studies; contraindicated if perforation suspected
Ionic iodine: urography, angiography; NEVER for myelography
Non-ionic iodine: myelography, all other contrast studies; safest option

Species	Abnormal Small Intestine Diameter
Dog	Greater than 1.6x the height of the L5 vertebral body; maximum diameter should not exceed 12 mm in small dogs
Cat	Greater than 2x the height of the L2 vertebral body; maximum diameter should not exceed 12 mm

Classification	Definition	Examples/Notes
By Location	Anatomic position within the bone	Diaphyseal, metaphyseal, epiphyseal, physeal (growth plate), articular
Transverse	Fracture line perpendicular to long axis	Usually result of direct trauma; inherently stable
Short Oblique	Fracture line less than 2x bone diameter	Common; result of combined bending and compression
Long Oblique	Fracture line greater than 2x bone diameter	More prone to shortening and displacement
Spiral	Fracture wraps around bone axis	Result of rotational forces; often extends further than appears on single view
Comminuted	More than two fragments present	Butterfly fragment = 2 oblique lines with large cortical fragment
Greenstick	Incomplete fracture; cortex broken on one side only	Common in juveniles; immature bone bends before breaking completely
Pathologic	Fracture through diseased/weakened bone	Suspect with minimal trauma history; look for underlying lysis or proliferation

Type	Description	Prognosis
I	Fracture through physis only (Separation)	Good - growth plate intact
II	Fracture through physis and metaphysis (Above)	Good - most common type
III	Fracture through physis and epiphysis (Lower)	Fair - articular surface involved
IV	Fracture through metaphysis, physis, and epiphysis (Through/Total)	Guarded - crosses germinal layer
V	Crush injury to physis (Ruined/Rammed)	Poor - growth arrest likely

Feature	Non-Aggressive	Aggressive
Zone of Transition	Short, well-defined margin between normal and abnormal bone	Long, indistinct, blurred margin between normal and abnormal
Bone Lysis Pattern	Geographic - single, well-marginated lucency	Moth-eaten (multiple small holes) or Permeative (tiny, confluent holes)
Periosteal Reaction	Smooth, continuous, well-organized (can trace with pencil without lifting)	Interrupted, irregular, spiculated ("sunburst"), lamellated ("onion skin")
Cortical Integrity	Intact or uniformly thinned	Destroyed, irregular destruction, cortical breakthrough
Rate of Change	Slow progression over weeks to months	Rapid progression over days to weeks
Common Causes	Fracture callus, bone cyst, osteoma, panosteitis, OCD, osteoarthritis	Osteosarcoma, other neoplasia, bacterial osteomyelitis, fungal osteomyelitis

Type	Characteristics	Uses/Contraindications
Barium Sulfate	Positive contrast; colloidal suspension; not water soluble; not absorbed from GI tract; excellent mucosal coating; inexpensive	GI studies (esophagography, upper GI, barium enema). CONTRAINDICATED if perforation suspected - causes severe peritonitis. Never inject IV.
Ionic Iodinated (High osmolar)	Positive contrast; water soluble; high osmolarity causes fluid shifts; irritating; can be absorbed from peritoneum	Excretory urography, angiography, GI if perforation suspected. CONTRAINDICATED for myelography (causes seizures/death). Caution with dehydration.
Non-ionic Iodinated (Low osmolar)	Positive contrast; water soluble; low osmolarity; fewer side effects; more expensive; not diluted as quickly	Myelography, urography, CT contrast, safe for GI studies. Preferred for high-risk patients and when aspiration is possible.
Negative Contrast (Air, CO2)	Radiolucent; creates dark areas on radiograph; air is readily available	Pneumocystography, pneumocolon, double contrast studies. Risk of air embolism with bladder studies - use CO2 or left lateral recumbency.

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Diagnostic Imaging: Radiography &ndash; BCSE Study Guide