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4. Medical Imaging > 0. Basics of XRAY, Nuclear Medicine, US, CT > Flashcards

0. Basics of XRAY, Nuclear Medicine, US, CT Flashcards

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USMLE® Step 1 flashcards
1
Q

What type of radiation is XRAY?

A

Electromagnetic, IONIZING radiation

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2
Q

How are X rays generated?

A

Through a vacuum tube that uses high voltage to accelerate electrons released by a cathode to a high velocity - those electrons collide with a metal target, the anode, creating the XRAY

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3
Q

Examples of anode metals

A
  • Molybdenum (used in mammography)
  • Wolfram
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4
Q

Types of X-Ray images

A
  • static snapshot (photography)
  • dynamic / functional imaging (“movie”)
  • continuous XRAY radiation
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5
Q

What does the attenuation of XRAY depend on?

A

It depends on atomic number, density and thickness - dense structures appear brighter.
(it’s the opposite in fluoroscopy)

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6
Q

How is XRAY resolution?

A
  • Excellent spatial resolution
  • Poor soft tissue resolution
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7
Q

How can we visualize pneumoperitoneum in XRAY?

A

By layin the patients in lateral decubitus position for their XRAY, we can detect air-fluid levels and free peritoneal air

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8
Q

What is dense breast on mammography?

A

Dense fibroglandular tissue with less adipose tissue - makes it harder to see cancer & microcalcification

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9
Q

What is the fat-pad sign on XRAY?

A

Luscent crescent of fat that indicates join effusion after trauma (aka fracture)

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10
Q

What is the significance of posterior fat pad on XRAY?

A

Supracondylar fracture or intra-articular fracture

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11
Q

What is the GI contrast agent for XRAY?

A

Barium, but if contraindicated it can be switched out for iodine-based contrast agentsW

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12
Q

When and why can barium be contraindicated?

A
  • It is contraindicated in cases of GI postoperative bleeding, perforation…
  • Because it causes severe peritonitis or mediastinitis if it gets in contact with the peritoneum or pleura
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13
Q

What is a small bowel follow through exam?

A
  • First a non contrast XRAY is taken
  • Then patient drinks iodine-based contrast agent
  • Images are taken after every 30-60 minutes to follow advancement of contrast agent
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14
Q

Advantages of XRAY

A
  • Accessible
  • Quick
  • Can use contrast agents
  • Specific signs in certain diseases
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15
Q

Disadvantages of XRAY

A
  • Ionising radiation
  • 2D
  • Summation image
  • Not good for soft tissues
  • Can’t detect many diseases
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16
Q

Can we use XRAY to detect kidney stones?

A

No, it’s not sensitive enough, the gold standard is low dose enhanced abdominal CT

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17
Q

What is nuclear medicine ?

A

medical activities using unsealed radioactive isotopes in diagnosis, treatment, and research of diseases

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18
Q

Types of radiations used in diagnostics and therapy

A

Diagnostic : 99mTc (gamma - SPECT), 18F (positron - PET)
Therapy : alpha or beta emitting isotopes

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19
Q

Half life of Technetium-99m

A

6 hours

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20
Q

What does SPECT detect?

A

Gamma emitting radioisotopes

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21
Q

Which has the best resolution and sensitivity between PET and SPECT?

A

PET

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22
Q

Advantages of PET/SPECT

A
  • Sensitive
  • More specific than CT
  • Gives metabolic information
23
Q

Disadvantages of PET/SPECT

A
  • Lower resolution
  • Lack of precise localization
  • Longer acquisition time
24
Q

What radioisotope is used in bone scintigraphy and why?

A

Tc99m-diphosphonate - adsorbs to bones in proportion to osteoblast activity

25
Q

Advantages and disadvantages of bone scintigraphy

A
  • Very sensitive : can detect lesions before XRAY
  • Not specific : fractures, inflammation, primary tumors…
26
Q

What radioisotope is used in thyroid scintigraphy and why?

A

Tc99m-pertechnetate - taken up by thyroid cells through Na-I symporter (so same as iodine)

27
Q

What are some tumors that cannot take up FDG?

A

Prostate, hepatocellular, some neuroendocrine tumors…

28
Q

What 3 components does the creation of sonographic images depend on?

A
  • production of high frequency sound waves
  • reception of reflected waves
  • conversion of that echo into the actual image
29
Q

Frequency of probes in US

A
  • Curvilinear : 3-5MHz
  • Linear : 7-10MHz
30
Q

How can you get better resolution and penetrability in US?

A

Higher frequency results in better resolution, while lower frequency results in better penetrability

31
Q

What is the most used type of ultrasound in diagnostics?

A

B-mode

32
Q

What is an echodense tissue?

A

A tissue that reflects all echo. It will be bright with a posterior acoustic shadowing

33
Q

What is M-mode used for?

A

To visualize moving structures, like heart valves in echocardiography

34
Q

What is power doppler used for?

A

More sensitive than color doppler, so it’ sused for flow with low velocity

35
Q

What is the ultrasound contrast agent?

A

Microbubbles : bubbles with a shell made up from albumins, galactose, lipids and polymers

36
Q

What is US used for in kidneys?

A

To detect hydronephrosis. The ureters are difficult to visualize on US, so we would use CT instead

37
Q

Why is a full bladder necessary for US?

A

To provide an acoustic window, to avoid the air filled bowels when assessing prostate or pelvic structures

38
Q

What is ureteric jet?

A

Detecting normal periodic flux of urine from ureters into the bladder - to exclude renal tract obstruction

39
Q

What is US used to detect in the thyroid gland?

A
  • Vascularization
  • Nodules
40
Q

Normal lymph node on US

A
  • Fatty hyperechoic hilum
  • Hypoechoic cortex
41
Q

What is the modality used to screen stenosis?

A

Doppler ultrasound

42
Q

What affects the risk of rupture of an aneurysm?

A

Its size (4-5cm have 25% risk of rupture)

43
Q

What is the screening of choice to check for aortic aneurysm?

A

US - blood will be anechoic, thrombus will be echogenic

44
Q

HOw do we check for deep vein thrombosis on US?

A

Veins containing thrombus will not completely collapse after compression, and the thrombus is echogenic

45
Q

What does FAST US stand for?

A

Focused Assessment With Sonography in Trauma

46
Q

What are the 5 main points of focus of FAST?

A
  • Heart
  • IVC
  • Morrison’s pouch (x3)
  • Aorta
  • Lungs
47
Q

Classical sign of pneumothorax on M mode FAST US?

A

Normal lung sign would be “seashore” but in PTX, there is just beach : only static

48
Q

Benefit of helical / spiral CT

A

Multiple regions can be scanned during a single breath-hold, in just a couple of seconds

49
Q

What is bright vs dark in CT?

A

Dark : hypodense - low density regions like air, fat
Light : hyperdense - dense structures like metal, bone

50
Q

GI contrast agents in CT

A
  • Iodine based, like gastrografin (bowel leaks, fistulae)
  • Water or macrogol -hypodense, useful in suspected bowel ischemia)
51
Q

Phases of contrast enhancement

A
  1. No contrast phase
  2. Arterial phase / angiography
  3. Late arterial / renal cortical phase
  4. Hepatic / late portal phase
  5. Nephrogenic / venous phase
  6. Excretory / delayed phase
52
Q

Do we always measure the GFR before contrast CT?

A

No, not in emergency situations - kidney function is measured after the examination

53
Q

Advantages of CT

A
  • 3D image
  • Good spatial resolution
  • Good for soft tissues
  • Fast
  • Can visualize calcifications and lung tissue
54
Q

Disadvantages of CT

A
  • High ionizing dose
  • Less good than MRI for soft tissue
  • Contraindications (pregnancy, kidney failure)

4. Medical Imaging (7 decks)

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