Ultrasound Flashcards
How does ultrasound work?
- uses sound waves with frequencies 1-20 MHz
- images created by interpreting sound reflections
- images composed of a mosaic of white and grey dots which represent an echo of a structure
What are the benefits of ultrasound? (8)
- non ionising
- mobile equipment
- quick and cost effective
- well tolerated by patients
- good soft tissue information
- real time
- accurate measurement of structures
- can carry out blood studies (ensure needle is in correct spot)
What are the cons of ultrasound? (4)
- many areas not suitable to image (bone, lung, abdominal structures covered by bowel)
- hand held transducer results in scan plane variablilty and hard to reproduce same scan plane
- high operator dependent
- diffifult image interpretation
What is the transverse plane?
- divides the body sup and inf
- transducer at ant of body
What is the sagittal plane?
- divides body left and right
- midsagittal on midline
- para-sagittal slightly of midline
- image always viewed with patient head to left of screen
What is the coronal plane?
- divides the body ant and post
- transducer on the lateral aspect of patient in lonitudinal plane
- ultraosund beam only go through half of patient so the inf of the image is medial
What is the viewing of a transverse image?
- ant and post
- left and right
What is the viewing of a sagittal image?
- ant and post
- sup and inf (sup on left, inf on right)
What is the viewing of a coronal image?
- lat and medial (lat on ant, medial on post)
- sup and inf (sup on left, inf on right)
What is echogenicity?
- described how bright the tissue is (e.g. how intense the echoes are)
Define anechoic
- area that has no echoes (black)
- e.g. fluid (blood and urine)
Define hypoechnoic
- area where the echo intensity is low (dark)
- e.g.
Deinfe Echogenic
- area where the echo is more intense (bright)
- e.g. bone, gas, calcification
What is echotexture?
- describes the pattern of echoes
What are the 4 main types of echotexture?
- fine
- coarse
- homogeneous
- heterogeneous
What is an acoustic window?
- structure or anatomical configuration that allows deeper anatomy ro be visualed
- causes little beam attenuation
What are examples of acoustic window?
- bladder to see prostate and uterus/ovaries
- liquid filled stomach to see pancreas
- liver and spleen to see kidneys
- amniotic fluid to see fetus
- eye
- anatomical configuration such as intercostal space
What is the ultrasound equipment?
- transducer
- monitor to display
- housing for electronics and controls
- recording device
What is the transducer frequencey?
- 1-20MHz
What is lower frequency for?
- lower frequency allows deeper penetration but slighly decrease image quality of superficial structures but better deeper structures
What is higher frequency for?
- more superficial and improved image quality of superficial
What is linear array?
- used for superficial
- 8-17 MHz
What is convex array?
- used for abdomen and pelvis
- 3-8 MHz
What is the pros of linear array?
- no moving parts
- wide near FOV
- adjustable foccussing
- multiple focal zones can be used
- colour doppler flow capability
What are the cons of linear array?
- large probe footprint
- narrow far FOV
What are the pros of convex array?
- smaller footprint
- wide far FOV
- wide near FOV
What are the cons of convex array?
- footprint too large for very small acoutsic window
- popular for general sonography
What is dynamic range?
- express a range of values (max and min signal values)
- range of echoes make up grey scale on image
What are the principle elements of attenutation?
- Material itself
- frequency of beam
- depth of tissue
What are the factors of the material itself?
- Viscositiy: more viscous material, more energy is expended moving the molecules and thus more attenuation
- relaxation time: speed molecules return to their rest postition. If slow, molecule still moving when next wave hits and thus more energy to stop movement and reverse
What are the factors of frequency of beam?
- higher frequency, faster molecules are moved and more energy is expended as heat
- if waves close together, molecule has less time to return to rest potition and thus same long relaxation time
What are the factors of depth of tissue?
- further beam travels, more attenuation
How are beams attenuated in ultrasound?
- attenauted at a rate of 1dB/cm/MH in tissue
- echoes returning from deeper structures become progressively weaker
- divergence is spreading of beam as increased distance from source (increased divergence = increased attenuation)
What is the biological effects of ultrasound?
- thermal caused by transfer of energy from the wave to the tissue causing molecules to vibrate
- mechanical
What are the thermal effects?
- beam characeristic
- frequency: ^ freq, ^ absoprtion = ^ heat
- intensity: ^ intensity = ^ heat
- mode of ultrasound: M-mode and doppler use pulsing on single path ^ heating
- pulse length: ^ pulse length = ^ energy in each pulse
- exposure time: heating continuous until equilibrium occurs - Tissue characteristics
- absoprtion characteristics: ^ absorption = ^ heat
- vasculatity: ^ blood = ^ heat removal
- harmful thermal effects are not considered possible with current imaging
What are the mechanical effects?
- cavitation
- under the influence of repeated compression, tiny mucrobubbles form
- dissolved gas comes out of solution under low pressure condition
- bubble size grows to size determined by wave length
- resonance occurs within the bubble, wih large vibrational amplitudes - transient
- at higher intensities, the microbubble may grow rapidly then collapse suddenly under a compression wave
- the collapse causes shock waves and higher local temperatures which can disrupt cellls
- high intensity U/S is needed to produce cavitation
- it is considered possible to produce cavitation with currect equipment if there is pre-existing cavitation nuclei in the tissue
What are the general rules for safety?
- diagnostic U/S should only be used when medically indicated or when there is an expected benefit
- should be completed in the shorest possible time
- output power should be kept to a minimum
- active transducer should not be rested on skin while not scanning
What are the general considerations?
- obtin full patient hisory and clinical presentation
- prior surgery or imaging
- only representative images of each oragn are recorded
- each image should be annotated
- sonographer doesnt discuss findings
What is the method for pelvis trans-abdominal?
- patient prep of empty bladder 2hrs prior then drink 1-1.5L of water
- all areas of pelvis scanned in multiple planes
- patient supine using 3-5 MHz transducer
- initial scout should be performed
What are some difficulties with pelvis U/S?
- innappropriate bladder filling
- patient obesity and abdominal scarring
- failure to define normal anatomy
- lack of specificity of pelvis pathology (benign or malignant)
