Methods - cross sectional image

Question 1

How are ultrasound images formed?

Answer 1

• based on pulse-echo principle
• by measuring the time taken for a pulse of sound to return to its source, the distance to the reflecting structure producing the echo can be calculated
• an ultrasound transduce emits sound 99.9% of the time
• a typical ultrasound pulse lasts a few us

Question 2

2 types of ultrasound image

Answer 2

• M-mode: non-anatomic images used for measurements of cardiac chambers, valves
• B-mode: cross-sectional anatomical images used for most ultrasound exams

Question 3

What do ultrasound images represent?

Answer 3

represent echoes (backscatter) generated in tissue)

• mainly arises from non-specific tissue elements that occur in most tissues including:
• fat or glycogen
• collagen
• vascularity
• crystalline material
• water/ oedema

Question 4

What frequencies are used for ultrasound?

Answer 4

• sound at a frequency above the audible range i.e. >20,000 Hz
• medical diagnostic ultrasound machines use frequencies in the range 2-15 million Hz or 2-15 MHz

Question 5

Which backscatter is hyperechoic?

Answer 5

• fat or glycogen
• collagen
• vascularity
• crystalline material

Question 6

Which backscatter is hypOechoic?

Answer 6

• water/oedema

Question 7

T/F: there is no simple correlation b/w the appearance of a lesion in an ultrasound image and its histology

Answer 7

TRUEE: pathological entities such as inflammation, neoplasia, hypertrophy share many gross features and do not produce any characterisstic effects on the ultrasound beam

Question 8

What is a high frequency trasnducer used for?

Answer 8

• detailed exam of small or superficial strucutre

- 7.5MHz

Question 9

Use - low frequency transducer

Answer 9

• less detailed exam of deeply situated organs

- 3.5MHz

Question 10

Use - wide, flat (‘linear’) transducers

Answer 10

• flat body parts

- eg. equine metacarpus

Question 11

Use - narrow, curved transducer (e.g. ‘sector transducer’)

Answer 11

• intercostal space

Question 12

Similarities- CT and MRI

Answer 12

• both rely on computers to process electronic signals which are reconstructed into 2D or 3D matrices using sophisticated mathematical algorithms
• expensive equipment
• high installation and running costs
• hence limited availability
• accurate positioning, still patient usually requires GA

Question 13

Typical image time - CT and MRI

Answer 13

• CT

Question 14

How can the visibility of lesions that damage the BBB be increased?

Answer 14

• IV injection of contrast medium
• CT: aqueous organic iodides
• MRI: paramagnetic agents (gadolinum chelates)

Question 15

Principle of CT

Answer 15

= computed x-ray tomography
- priduces cross-sectional images that involves irradiating the subject using high energy, fan-shaped circulating x-ray beam and capturing transmitted photons in electronic detectors positioned in a ring around the patient

Question 16

What do grey shades in CT reflect?

Answer 16

tissue attenuation - similar to conventional radiography

Question 17

Principle - MRI

Answer 17

method for producing a cross-sectional image that involves placement of the subject in a region of high magnetic flux, applying periodic magnetic pulses to change the orientation of atomic nuclei within the subject and collecting the radiowaves that are emitted when these nuclei relax

Question 18

What do grey shades in MRI reflect?

Answer 18

‘relaxation times’ of tissues, which are influenced by a variety of physiochemical properties

Question 19

What is radiography particularly useful for? 2

Answer 19

• bones and joints

* airways and lungs

Question 20

What is contrast radiography particularly useful for? 2

Answer 20

• abdomen

- brain and spine

Question 21

What is ultrasound particularly useful for? 2

Answer 21

• abdomen

- guiding biopsy

Question 22

What is CT particularly useful for?

Answer 22

• bones and joints
• airways and lung
• abdomen
• brain and spine
• guiding biopsy

Question 23

What is MRI particularly useful for?

Answer 23

• brain and spine
• bones and joints
• abdomen

Question 24

What is the Hounsfield number?

Answer 24

a measure of attenuation of xrays relative to water

• water = 0
• 0+ = more xrays absorbed
• 0- = fewer xrays absorbed

Question 25

What is meant by a narrow window?

Answer 25

i. e. the range of Hounsfield units - high contrast - to distinguish soft tissue

Question 26

What is meant by a wide window?

Answer 26

i. e. the range of Hounsfield units - low contrast - more bone detail

Question 27

What is ultrasound not used ofr?

Answer 27

brain and spine

Question 28

What is contrast radiography not used for?

Answer 28

- bones and joints - airways and lung - guiding biopsy