Radiology and contrast agents

Question 1

how are radiographs produced

Answer 1

• X-rays transmitted through a patient
• X-rays projected through body onto a detector
• image formed based on which rays pass through ad are deteted versus those that are absorbed or scattered in patient and not detected

Question 2

digital radiography and EOS imaging

Answer 2

• X-rays strike sensors that convert the signals generated into digital information which is transmitted and converted into an image displayed on a computer
• in DR the sensors are shaped like a plate
• in EOS a linear sensor vertically scans the patient

Question 3

examples of diseases diagnosed by plain radiography

Answer 3

• various types of arthritis
• pneumonia
• bone tumours
• fractures
• congenital skeletal abnormalities

Question 4

applications of low energy projectional radiography

Answer 4

• mammography for breast cancer evaluation
• DXA for osteoporosis evaluation

Question 5

fluoroscopy and angiography

Answer 5

• fluorescent screen and image intensifier tube connected to a closed circuit television system
• allows real-time imaging of structures in motion or augmented with a radiocontrast agent

Question 6

why are radiocontrast agents used

Answer 6

• to delineate anatomy and functioning of blood vessels, genitourinary system or the gastrointestinal tract
• they strongly absorb or scatter X-rays and in conjunction with the real-time imaging allow demonstration of dynamic processes such as peristalsis in the GI tract or blood flow in arteries and veins

Question 7

barium sulphate (BaSO4) as a radiocontrast agent

Answer 7

• given orally or rectally
• evaluation of the GI tract

Question 8

iodine as a radiocontrast agent

Answer 8

• given by oral, rectal, vaginal, intra-arterial or intravenous routes
• concentrated in abnormal areas more or less than in normal tissues and make abnormalities like tumours and cysts more conspicuous

Question 9

gases as contrast agents

Answer 9

• air for the GI system
• carbon dioxide in the venous system
• the agent attenuates the X-ray radiation less than surrounding tissues

Question 10

computed tomography (CT)

Answer 10

• uses X- rays in conjunction with computing algorithms to image the body
• X-ray tube opposite an X-ray detector in a ring-shaped apparatus rotates around the patient producing cross-sectional images
• radiocontrasts often use for enhanced delineation of anatomy
• detects more subtle variations in attenuation of X-rays than radiographs

Question 11

examples of conditions diagnosed using CT

Answer 11

• cerebral haemmorhage
• pulmonary embolism
• aortic dissection
• appendicitis
• diverticulitis
• obstructing kidney stones

Question 12

nuclear medicine

Answer 12

• administration of radiopharmaceuticals labelled with radioactive tracer into the patient
• substances consist of material with an affinity for specific body tissues
• anatomical detail is limited
• useful in displaying physiological function so can measure excretory function of kidneys, iodine-concentrating ability of thyroid, blood flow to heart muscle
• gamma camera and PET scanner detect radiation emitted by tracer in body and with computer processing display it as an axial, coronal or sagittal image as SPECT or PET
• most images can be fused with CT scan so physiological information can be overlaid with anatomical structures to improve diagnostic accuracy

Question 12

commonly used radioactive tracers

Answer 12

• technetium-99m
• iodine-123
• iodine-131
• gallium-67
• indium-111
• thallium-201
• 18F-fluorodeoxyglucose (18F-FDG)

Question 13

positron emission tomography (PET)

Answer 13

• positrons annihilate to produce two opposite travelling gamma rays to be detected coincidentally
• radioactive, biologically active substance (often 18F-FDG) is injected into patient and radiation emitted is detected to produce multi-planar images
• more metabolically active tissues such as cancer cells concentrate the active substance more than normal tissues so can be imaged
• PET images can be combined with CT imaging to improve diagnostic accuracy

Question 14

interventional radiology

Answer 14

• minimally invasive procedures are performed using image guidance for diagnostic (eg. angiogram) or treatment (eg. angioplasty) purposes
• often performed with patient fully awake with little or no sedation
• primary instruments used are needles and catheters
• images provide maps that allow the clinician to guide the instruments through the body to the areas containing disease
• peripheral interventions can reduce infection rates, recovery times and hospital stays

Question 15

what is a contrast agent

Answer 15

• chemical agent administered to the patient for a scan
• improves contrast resolution (ability to differentiate between different types of tissue)
• helps characterise pathology

Question 16

what types of scans are contrast agents used for

Answer 16

• CT
• MRI
• gastrogaffin in abdominal x-rays
• angiograms
• GI fluoroscopy
• ultrasound

Question 17

how can contrast be given

Answer 17

• orally
• rectally
• intravenously
• intraarterial
• any cavity

Question 18

types of contrast

Answer 18

X-ray
- positive contrast is more radiopaque, tend to be heavy elements such as iodine or barium
- negative contrast is more radiolcent, tend to be lighter materials such as air or CO2

MRI
- gadolinium based agents are used as it interferes with how protons behave in a magnetic field

ultrasound
- microbubbles interact with sound waves

Question 19

ideal characteristics of contrast agents

Answer 19

• biologically inert
• safe and non-toxic
• stable for storage and within body
• low osmolality and viscosity
• soluble in water
• cost-effective

Question 20

idiosyncratic reactions to contrast agents

Answer 20

• acute (<1hour), can range from mild to life threatening
• higher risk if previous reaction to contrast, asthma or other allergies
• severe reactions very rare

Question 21

non-idiosyncratic reactions to contrast agents

Answer 21

• contrast extravasation, metallic taste or warm sensation
• nausea and vomiting, vasovagal syncope and other CVS effects
• can be delayed reactions (1 hour-1 week), commonly skin reactions

Question 22

what does idiosyncratic reaction mean

Answer 22

• cannot be explained by drug’s known mechanism of action
• do not occur at any dose in most patients
• mostly unpredictable

Question 23

severity of idiosyncratic reactions to contrast agents

Answer 23

• mild: flushing, itching, mild urticaria
• moderate: marked urticaria, bronchospasm, angioedema, laryngeal oedema
• severe: anaphylaxis, cardiopulmonary arrest