Methods - cross sectional image Flashcards
How are ultrasound images formed?
- 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
2 types of ultrasound image
- M-mode: non-anatomic images used for measurements of cardiac chambers, valves
- B-mode: cross-sectional anatomical images used for most ultrasound exams
What do ultrasound images represent?
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
What frequencies are used for ultrasound?
- 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
Which backscatter is hyperechoic?
- fat or glycogen
- collagen
- vascularity
- crystalline material
Which backscatter is hypOechoic?
- water/oedema
T/F: there is no simple correlation b/w the appearance of a lesion in an ultrasound image and its histology
TRUEE: pathological entities such as inflammation, neoplasia, hypertrophy share many gross features and do not produce any characterisstic effects on the ultrasound beam
What is a high frequency trasnducer used for?
- detailed exam of small or superficial strucutre
- 7.5MHz
Use - low frequency transducer
- less detailed exam of deeply situated organs
- 3.5MHz
Use - wide, flat (‘linear’) transducers
- flat body parts
- eg. equine metacarpus
Use - narrow, curved transducer (e.g. ‘sector transducer’)
- intercostal space
Similarities- CT and MRI
- 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
Typical image time - CT and MRI
- CT
How can the visibility of lesions that damage the BBB be increased?
- IV injection of contrast medium
- CT: aqueous organic iodides
- MRI: paramagnetic agents (gadolinum chelates)
Principle of CT
= 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
What do grey shades in CT reflect?
tissue attenuation - similar to conventional radiography
Principle - MRI
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
What do grey shades in MRI reflect?
‘relaxation times’ of tissues, which are influenced by a variety of physiochemical properties
What is radiography particularly useful for? 2
- bones and joints
* airways and lungs
What is contrast radiography particularly useful for? 2
- abdomen
- brain and spine
What is ultrasound particularly useful for? 2
- abdomen
- guiding biopsy
What is CT particularly useful for?
- bones and joints
- airways and lung
- abdomen
- brain and spine
- guiding biopsy
What is MRI particularly useful for?
- brain and spine
- bones and joints
- abdomen
What is the Hounsfield number?
a measure of attenuation of xrays relative to water
- water = 0
- 0+ = more xrays absorbed
- 0- = fewer xrays absorbed
What is meant by a narrow window?
i. e. the range of Hounsfield units
- high contrast
- to distinguish soft tissue
What is meant by a wide window?
i. e. the range of Hounsfield units
- low contrast
- more bone detail
What is ultrasound not used ofr?
brain and spine
What is contrast radiography not used for?
- bones and joints
- airways and lung
- guiding biopsy
