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Medical Imaging > 0.1 - X-ray Basics > Flashcards

0.1 - X-ray Basics Flashcards

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USMLE® Step 1 flashcards
1
Q

What is an X-ray?

A
  • X-rays are a form of electromagnetic radiation
  • It is an ionizing radiation
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2
Q

What is the wavelength and intensity of X-rays?

A
  • wavelength = 0.01-10nm
  • photon energy = 100 eV - 100 keV

wavelength is inversely proportional to the intensity of the X-ray (rate of penetration and beam hardness)

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3
Q

How are X-rays produced?

A

A direct current, hot cathode releases electrons that are accelerated in a vacuum tube by high-voltage. The accelerated high velocity electrons collide with a metal anode to create X-rays.

The current and voltage of the X-ray machine determines the properties of the X-ray beam

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4
Q

What are the main properties of X-rays?

A
  • electromagnetic radiation
  • traverse thick objects without much absorption or scattering
  • ionizes atoms and disrupts molecular bonds
  • higher atomic number materials attenuate a greater percentage of the beam than lower atomic number materials
  • induces variable chemical changes in film emulsion
  • certain materials can convert X-ray into visible light (fluoroscopy)
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5
Q

Magnification of X-rays

A
  • X-ray beams diverge in straight lines from the source causing magnification and distortion
  • structures closer to the X-ray source and magnified
  • structures closer to the X-ray detector appear more realistic in size
  • magnification/distortion is not an issue multislice modalities (PET/MR/CT)
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6
Q

What are common examples of static X-ray images?

A
  • chest X-ray
  • bone X-ray
  • abdominal X-ray
  • mammography
  • bowel follow-through study
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7
Q

What are examples of common fluoroscopy studies?

A
  • cholangiography
  • catheterisation of hepatic artery
  • urogram of transplanted kidney
  • GI studies with barium contrast agent
  • placement of medical devices
  • angiograms
  • orthopedic surgeries
  • placement of medical devices
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8
Q

What is fluoroscopy?

A
  • dynamic/functional imaging (X-ray movie)
  • continuous X-ray radiation is used to examine movement of structures and contrast agents
  • higher radiation dose than in X-ray images
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9
Q

What does the attenuation of X-ray depend on?

A

atomic number, density and thickness of the material

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10
Q

What appears bright on X-rays?

A
  • described as radiolucent or increased transparency
  • denser structures
  • structures that attenuate large amounts of X-rays
  • higher atomic number materials
  • ie. bone, calcification, contrast agents
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11
Q

What appears dark on X-rays?

A
  • described as radio-opaque or decreased transparency
  • more transparent structures
  • structures that are less attenuating
  • ie. air, fat
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12
Q

Brightness in fluoroscopy imaging

A
  • reversed compared to X-ray
  • dense structures appear dark
  • more transparent structures appear brighter
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13
Q

What are the major X-ray densities?

A
  • air
  • fat
  • soft tissue (water)
  • bone
  • metal
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14
Q

How is the resolution of X-ray?

A

X-ray has excellent spatial resolution but poor soft tissue resolution

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15
Q

What is the benefit of X-ray in lateral decubitus position?

A

patients in poor condition benefit from taking abdominal X-rays in lateral decubitus position to detect air-fluid levels and free peritoneal air

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16
Q

Detecting breast malignancy in mammography

A

it is easier to detect microcalcifications in adipose breast tissue than dense breasts, thus malignancy can easily go unnoticed in denser breasts

17
Q

What should be considered in case of posterior fat pad in elbow joint?

A
  • appearance of lucent crescent of fat in the olecranon fossa (true lateral view)
  • indicates trauma
  • consider supracondylar fracture or intra-articular fracture (ie. lateral condyle fracture)
18
Q

How to diagnose pleural effusion on X-ray?

A
  • homogenous decreased transparency
  • costophrenic angle is not visible
19
Q

What is the anatomy of bowels in abdominal X-ray?

A
  • colon: more meteoristic than small bowels
  • small bowels: minimal gas content, located more centrally
20
Q

What are the advantages and disadvantages of X-ray imaging for bone?

A
  • the higher attenuation of calcification in bones makes it easier to be visualised in contrast to surrounding tissue
  • sclerotic bone tissue can be visualised but there isn’t much information on the bone marrow
  • X-ray has great linear resolution so it is great for fractures, at least two projections are always required
  • X-ray does not detect soft-tissue injuries well
21
Q

The use of barium as a contrast agent in radiography

A
  • barium coats the gastro-intestinal mucosa in a thin layer so it is useful for GI studies
  • if contraindicated iodine-based contrast agent is used
  • barium causes severe peritonitis and mediastinitis if it gets into contact with the peritoneum or pleura
  • contraindications of barium: GI postoperative setting, perforation, anastomosis insufficiency, aspiration
22
Q

What is a small bowel follow-through exam?

A
  1. non-contrast X-ray image is taken
  2. patient drinks iodine-based contrast agent
  3. image is taken every 30-60 minutes to follow the advancement of the contrast agent through the bowels
  4. contrast agent appears in the colon usually 30-120 minutes later (normal small bowel transit time)
23
Q

When is X-ray not indicated?

A
  • nephrolithiasis: gold standard is low-dose non-enhanced abdominal CT
  • head trauma: non-enhanced head CT is recommended
  • acute sinusitis: clinical diagnosis is sufficient
  • chronic/recurrent sinusitis, polyposis, fungal infections, surgical planning: non-enhanced CT
  • back pain with neurological symptoms: non-enhanced MRI
  • lung cancer screening: low-dose non-enhanced CT
24
Q

What are the advantages of radiography?

A
  • excellent spatial resolution
  • easily accessible
  • quick
  • available as portable devices
  • contrast agents
  • fluoroscopy is a dynamic, functional imaging modality
  • specific signs are seen for a few diseases
25
Q

What are the disadvantages of radiography?

A
  • ionising radiation
  • 2D imaging
  • summation
  • magnification and distortion
  • poor soft tissue resolution
  • contraindicated in pregnancy and childhood
  • non-specific signs for many diseases
  • many diseases do not have detectable signs on X-ray

Medical Imaging (10 decks)

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