LEC 2 - MRI + CT Scans Flashcards
What is the downside to radiography?
Not functional
Not cross sectional
Poor soft tissue appearance
What is the downside to ultrasound?
Not functional
Bad at seeing bone and lung
What are the four options for advanced imaging?
Nuclear imaging
Computed Tomography
Magnetic Resonance Imaging
Contrast Ultrasound
What is nuclear imaging used for?
Bone scans
What is the major difference between CT scan and normal radiography?
Rotating photon source
Rotating detectors
Computer constructs image
What do CT’s measure?
Attenuation
What is attenuation coefficient?
Degree to which x-ray intensity is reduced by material
How is the measurement called for CT?
Hounsefielf (HU)
What is HU’s based on?
Water = 0
What is the progression of HU units?
Bone = + Water = 0 Air = -
How do CT scanners work in regards to providing you information?
Each pixel given number that correlates to HU scale - this tells you what the substance is most likely
What color do you want the thing you are interested in to be?
Gray
What are the two things that you change on a CT scanner to get an image?
WL - Window level
– and –
WW - Window Width
how do you set a CT machine?
Set WL to the substance that you want to be gray
Set WW to the range that you would like tissues to be visable
What is the most important clinical reason to use a cone beam CT?
Thorax - Lung mass
GOLD STANDARD
What are the limitations to a cone beam CT?
only images in the transverse plane
VERYYYYYY SLOW
What is the clinical reasons for using a single slice helical CT?
Angiography
PSS + Masses + Thrombosis
What are the limitations to using a single slice helical CT?
Slow
Only imaging in the transverse plane
What are the limitations to the multislice CT?
none
What is the best reason to use MRI?
to see brain + nervous system
NO ionization or radiation
What is MRI good for in the body?
Soft tissue
– and –
Grey vs. White mater
What is MRI bad for in the body?
Bone
– and –
Air
What is the basic way in which a MRI works?
Makes a strong magnentic field
Causes protons to line up
RF pulse causes protons to act a certain way
Wobble = signal
What are the two directions that the protons can line up?
Parallel
– and –
Anti-parallel
What direction of magnetization can NOT be measured?
Longitudinal
What is another name for the wobbling of the protons?
Precession
What is RF pulse?
Radiofrequency pulse
caused by the rapid changing of magnetic and electrical fields
What are the types of RF pulses?
90 pulse
180 pulse
90 + 180 pulse
Variable flip pulse
Outcome of 90 pulse?
Rotation into transverse plane
Outcome of 180 pulse?
Rotation into longitudinal plane
Out come of 180 + 90 pulse?
Spin Echo sequence
Outcome of variable flip angles?
Gradient echo sequence
Characteristics of T1 relaxation?
Longitudinal
Characteristics of T2 relaxation?
Transverse or spin-spin
What two things does the MR signal depend on?
How long after RF pulse - TE
The time we repeat the pulse - TR
What is the “measurements” of T1?
Short TR
What is the “measurement” of T2?
Long TE
What is the “measurement” of proton?
Long TR
– and –
Short TE
What is the tissue of T1?
Fat
What is the tissue of T2?
Water (CSF)
What is T1- weighted good for seeing?
Anatomical detail
What is T2-weight food for seeing?
tissue oedema
