BIOMED 10/18b Imaging: CT and MRI

Question 1

what does CT stand for?

Answer 1

computed tomography

Question 2

what is a CT scan?

Answer 2

○ Image that is based on x-rays to acquire the image
○ Voxels (3-D pixel) and attenuation (things that are more radiodense = whiter)
○ Acquired axially (transverse), but can be reformatted
○ Degree of tissue thickness as you’re acquiring the data
○ Better contrast and are more sensitive for imaging soft tissue than plain films
-More sensitive, because have 3-D

Question 3

what are CT scans often used for?

Answer 3

-acute head trauma (quick to acquire and shorter time to complete)

Question 4

what is the significance of getting a CT scan for head stroke?

Answer 4

• you have have different kinds of strokes, two being:
  1. thromboembolytic (comes up as grey circle on CT)
  2. clot (comes up as white on CT)

the medication to treat varies and you want to make sure you treat fast enough!

Question 5

why do people directly go for CT scans when looking for bone/ligament issues?

Answer 5

tells us the type of fracture with its measure and quantification
-gets the details of the fracture

Question 6

what is windowing with CT scans?

Answer 6

1. Changes the grey of interest around -700 (range is -600 to -800 instead of a larger scale)
2. Centered around being able to see tissue that is more negative
3. See the same image as above, below to have more blood vessels
4. Look for things that show up in the lung window when they shouldn’t

Question 7

why is CT scan windowing important for bone?

Answer 7

it allows you to see the difference between spongy and cortical bone

Question 8

what are advantages of using a CT scan?

Answer 8

1. Heterotopic ossification (HO) - calcification in a place where there shouldn’t be (bone in muscle, tendon, etc)
   2. Subtle or complex fractures
   3. Degenerative changes in bone (i.e., spinal stenosis)
   4. Acute intracranial bleeding (e.g., hemorrhagic strokes or subdural hematomas)
   5. Quantitative bone mineral analysis (used for measure of OP)
2. More available, less expensive and quicker than MRI and more information than radiographs

Question 9

What are CT scan disadvantages?

Answer 9

1. More exposure to radiation than radiograph - x-ray (still very low), but cumulative
   2. Not as good for soft tissue differentiation cartilage, tendons and ligaments as MRI
2. Pt. mvmt & metal implants will produce artifacts

Question 10

what does MRI stand for?

Answer 10

Magnetic Resonance Imaging

Question 11

How are MRIs produced?

Answer 11

• produced by the interaction of hydrogen containing molecules within tissues and a magnetic field produced by the magnet
  1. hydrogen lines up with the field of the magnet
  2. a radio wave is delivered to the tissue which causes the hydrogen nuclei to absorb energy and change its alignment with the magnet (resonance)
  3. the radio pulses are stopped, the absorbed energy is released and measured by the computed
  4. info is converted to an image

Question 12

what does the radiopulse do in an MRI?

Answer 12

radio pulse disturbs alignment of the tissue which causes them to spin and resonate as a result of radiofrequency

Question 13

why are unique images produced with an MRI?

Answer 13

each tissue has a different amount of hydrogen and so they relax at different states

Question 14

what makes the image whiter in an MRI?

Answer 14

the more the water, the whiter the image

• fat is brightest on T1 weighted image
• cortical appears black because it has low signal intensity

Question 15

what factors produce different quality images of the same tissues?

Answer 15

1. repetition time (TR)
2. Echo time (TE)

varying TR and TE accentuates different tissues

Question 16

what produces a T1 weighted image?

Answer 16

a shorter TE

Question 17

what is TR

Answer 17

Repetition Time: the time that elapses between two consecutive radio wave pulses

Question 18

what is TE

Answer 18

Echo Time: time selected to wait after the start of TR to receive the signal or “echo” from the patient

Question 19

what produces a T2 fat suppression image?

Answer 19

T2 = STIR (Short Tau Inversion Recovery)
Longer TE and TR times
–suppresses fat and shows pathology - allows us to see if there is increased water uptake as a result of pathology to another location

Question 20

what are some types of images we can see with T2 fat suppressed imaged

Answer 20

• edema
• cyst
• tumor
• bone bruise
• kinked ligaments
• effusion

Question 21

what are T1 images better for?

Answer 21

better for viewing anatomy

Question 22

what are T2 images better for?

Answer 22

better for viewing pathology since these images accentuate still fluid

Question 23

high intensity to low intensity signals generate what images the best to worst in T1?

Answer 23

subQ fat 
spongy bone
still fluid
cartilage
tumor
muscle
fluid

Question 24

high intensity to low intensity signals generate what images the best to worst in T2?

Answer 24

Still fluid (inflammation)
tumor
fluid
muscle
cartilage
spongy bone
subQ fat

Question 25

advantages of MRI

Answer 25

1. heightened ability of contrast - soft tissue visualization (muscles, tendons, ligaments, nerves, articular cartilage, and menisci)
2. okay for spongy bone (high fat content)
3. no ionizing radiation, no known harm - still not used in pregnant women though!

Question 26

Disadvantages of MRI

Answer 26

iron/metal shavings contact creates horrible results because of the magnet

Question 27

CT vs MRI

Answer 27

MRI is better for:

1. spongy bone
2. soft tissue (ligaments and tendons)
3. cartilage

CT is better for:

1. cortical bone - subtle and complex fractures
2. calcifications in any tissue
3. acute intracranial bleeding