Lecture 3 - Introduction to Medical Imaging

Question 1

What are the 3 anatomical sections an image can be taken in?

Answer 1

• Trasnverse (top and bottom)
• Coronal (front and back)
• Saggital (left and right)

Question 2

How do X-rays work? What determines whether a structure appears as light or dark on the X-ray?

Answer 2

• Focus beam of high energy electrons passed through body and onto detector
• Some electrons absorbed, some scattered (known as attenuation)
• More dense/higher atomic number = more attenuation = Lighter appearance on X-ray

Question 3

Order these from lightest to darkest on X-ray:

• Soft tissue
• Fat
• Bone
• Air
• Metal

Answer 3

Lightest

• Metal
• Bone
• Soft tissue
• Fat
• Air

Darkest

Question 4

What is the difference between an AP and PA X-ray?

Answer 4

• AP = anterior to posterior (front facing)

- PA = posterior to anterior (back facing)

Question 5

Name 2/3 advantages + disadvantages of X-rays

Answer 5

Adv:

• Cheap
• Portable
• Quick & Simple

Disadv:

• Can’t see all pathology
• Uses radiation (although small)
• Only 1 plane per image so always in 2D
• Poor soft tissue imaging

Question 6

Name 3 uses of X-rays

Answer 6

• Chest X-rays for lung abnormalities
• Abdominal X-rays for bowels and organs
• Orthopaedic X-rays for fractures

Question 7

Explain how fluoroscopy works and what it allows examination for.

Answer 7

• Constant stream of X-rays creates a moving image, enhanced by contrast (i.e.: barium) by strongly absorbing X-rays.
• Allows for examination of both anatomy and motion

Question 8

What are the uses of fluoroscopy?

Answer 8

• Angiography
• Contrast GI studies
• Therapeutic joint infections
• Screening in theatre

Question 9

Give an advantage + disadvantage of fluoroscopy

Answer 9

Adv = Dynamic studies, cheap, interventional
Disadv = Uses radiation so clinical exposure must be minimised

Question 10

How do CT scans work? How is density described in CT scans?

Answer 10

• Rotating gantry with X-ray tube one side and detector on the other.
• Images put together by computer using same principal of attenuation
• Transverse imagine produced where everything is flipped to other side.
• Density measured in HU (water = 0HU, less dense = negative, more dense = positive)

Question 11

Describe advantages, disadvantages and uses of CT scans.

Answer 11

adv:

• Quick
• Can scan most areas

Disadv:

• Radiation
• Affected by artefacts
• Requires breath holding
• Incidental findings

Uses = diagnosis, e.g.: head scans, or monitoring conditions like cancer.

Question 12

How does MRI work?

Answer 12

• Magnetic field aligns hydrogen atoms
• Some point towards head, some toward feet, some are unmatched
• They emit energy when radiofrequency pulse is applied
• this is turned into an image by a computer

Question 13

What are uses of MRI scans?

Answer 13

• Scan brain + SC for tumours
• Scan bones + joints
• Scan gallbladder for stones
• Scan internal organs

Question 14

Name a couple adv + disadv for MRI scans

Answer 14

Adv:

• Uses magnets so NO ionising radiation
• Good contrast resolution

Disadv:

• Expensive + Time consuming
• Claustrophobic for patients/some wont fit
• Need to lie very still

Question 15

How do ultrasound scans work? What are the terms that are used?

Answer 15

• High frequency sound waves from a transducer probe
• Sound wave reflected back from tissues where density (impedance) differs
• Probe detects sound waves and creates electrical signal.
• Hyperechoic = more reflective (shows white)
• Hypoechoic = vice versa (shows dark grey)
• Anechoic = no reflection (shows black)

Question 16

What are the uses of ultrasound scans?

Answer 16

• Look at solid organs
• Look at urinary tract for stones, dilation, volume
• Look at body cavities, e.g.: transvaginal
• in Obs and gynae for pregnancy + uterus

Question 17

Supply some adv + disadv of ultrasound scans

Answer 17

Adv:

• No Ionising radiation
• Low cost + portable
• Provides dynamic studies

Disadv:

• Operator dependent (especially for image)
• No bone or gas penetration (gas can obscure deep structures)

Question 18

What colour does water + fat show on a T1 weighted MRI and a T2 weighted MRI?

Answer 18

T1 = water is dark (grey), fat is white
T2 = water is white (lighter), fat is black

Question 19

What are the dimensions of the image for CT, MRI, X-ray and ultrasound?
Which ones used radiation and which ones don’t?

Answer 19

CT = 3D, high radiation
MRI = 3D, no radiation
X-ray = 2D, less radiation 
Ultrasound = 2D, no radiation