CT & MRI Flashcards
What does CT mean?
computed tomography: tomos –“slice or section“, graphia- „describing“ (Greek)
A method of radiology which allows to take cross-sectional images of a certain body part.
Uses ionizing radiation.
CT history
- Allan Cormack developed in the late 50`s theoretical principles
- Godfrey Hounsfield built first late 60’s
- CT in Estonian vet med from Dec. 2015
CT machine construction
- Gantry is the doughnut-shaped part
which contains an X-ray tube and X-ray
detectors. - The source of the X-ray is high-powered X-ray tube, which rotates at a certain distance around a slice of the patients anatomy.
- The detectors measure x-rays which
penetrate the patients body on a
different angle.
CT machine physical process / what happens to the patient
- Patient moves slowly through the gantry
(doughnut) on a table. - The X-ray tube rotates around the patient.
- Radiation partially penetrates the tissues,
partially attenuates in tissues. - A lot of short images are taken and detectors transform the detected radiation into an electrical signal.
CT working principle
- CT measures radiation absorption in a thin axial (transversal) body slice
- first we get a lot of 2-dimensional x-ray images from a 3D object
- after processing the data we get images on different planes (“MPR-multiplanar reconstruction”)
- 3D reconstructions are also possible
Types of CT scanners
- Single-slice CT scanner
* only one row of detectors
* during each rotation of the tube, a single
slice of anatomy is scanned - Multi-slice CT scanner
* several rows of detectors
- during each rotation of the tube,
multiple slices of anatomy is scanned - machines with 2, 4, 6, 8, 16, 32, 40, 64,
128 and more detector rows are available - allow the measurement very thin slices
(0,5mm)
CT image acquisition
1. Describe Axial (sequential) image acquisition
- „step-and-shoot“ method
- the table is stationary, 360-degree images are taken
- X-ray tube is switched off, the table with the patient moves forward
- repeated until the body part is examined
- slower than helical method
CT image acquisition
2. Describe Spiral (helical) image acquisition
- the X-ray tube is rotating and patient moves with the table through the gantry at the same time
- data is constantly recorded
- examination is very fast
- better multi-planar and 3D
reconstructions
CT measures …?
radiation absorption in a thin axial body slice.
- based on the X-ray direction, we get different absorption profiles
- we get a „density map“- a computer-generated matrix image from the different projectional density values corresponding to the relative density of different body parts
- the pixel’s numerical value represents the voxel’s absorption
- the absorption value is represented by a certain shade of grey
- every value represents the body’s registered absorption
What is the Hounsfield scale?
- A quantitative scale for describing radio density.
HU (Hounsfield units)
o air -1000 HU
o water 0 HU
o bone +1000 HU (up to +3000 HU)
- On a greyscale, the thickest tissue is the brightest and the thinnest tissue is the darkest.
- A computer monitor can show up to 4000 different shades of grey, the human eye is able to differentiate 20-30 shades of grey.
- Therefore, it is necessary to adjust the image to adequate range and we can use „windowing“, which shows only certain area of the scale.
- Windows: bone, soft tissue, lung, brain
CT windows: bone window
CT windows: brain window
brain is also a little blurry since CT isn’t as sensitive for the brain as MRI is
lung window
soft tissue window
What’s the diff between these?
diff CT planes: sagittal, transverse, dorsal
whats this
3D reconstruction of CT images
Describe CT image evaluation
- partly similar with conventional radiography, but no superimposition and summation shadows.
- the images are usually transverse, but can be reconstructed in sagittal/dorsal planes and as a 3D image.
- possible to evaluate a scan „slice by slice“
- tissues of low density (f.ex. lungs) are dark
- tissues of high density (f.ex. bones) are bright
- know the normal anatomy and physiology!
- isodense =
- hypodense =
- hyperdense =
Specifically for CT!
- isodense – If an abnormality is the same density as the reference structure, we would describe it as isodense.
- hypodense - If an abnormality is less dense than the reference structure, we would describe it as hypodense.
- hyperdense – If an abnormality is bright (white) on CT , we describe it as hyperdense.
Describe CT contrast medium
- contrast procedures are used to increase the native contrast of organs and lesions to separate them from surrounding tissues
- generally iodine-based radiopaque compound (obstructing the passage of radiant energy, such as X-rays)
- native image + image with contrast – to compare
- timing – contrast enhancement should be maximal
- cleared primarily by kidneys, making the urinary tract visible.
- side effects: haemodynamic alterations, allergic reactions/anaphylaxis, nefro- and cardiotoxity
- angiography increases the opacity of
blood, making vascular structures visible
- allows to evaluate tissue/organ perfusion
- allows to evaluate contrast media passing
CT advantages and disadvantages
Describe why CT safety is necessary.
- higher radiation dose than conventional radiography
- The absorbed radiation can break chemical bonds in tissues, generating free radicals, which are chemically very reactive.
They can attack complex compounds that make up chromosome. After physical, chemical and biological processes, may evolve cell death, obstruction of fission or
permanent change inside the cell which is genetically transmitted. Cells changed due to radiation can be the start of a neoplastic process.
+ anesthesia/contrast media risk factors and side effects
What is ALARA
As low as reasonably achievable (ALARA) is a principle of radioprotection stating that whenever ionizing radiation has to be applied to humans, animals or materials -exposure should be as low as
reasonably achievable.
MRI uses what to image?
MRI (magnetic resonance imaging) uses electromagnetic radiation
MRI history
- first human MRI scan in 1977
- MRI in Veterinary Medicine: since 1980
- low field MRI in veterinary private practice: last 15 years
- first MRI scan in our animal clinic: January 2011
Why are water and hydrogen so important
when we are talking about MRI?
Because MRI’s working principle is to measure hydrogen proton magnetic fields after they are lined up.
If we place a patient into the MRI machine then we place all its protons in the body in a strong external magnetic field (B0)!
Protons „line up“ in the direction of the
main magnetic field (parallel and
anti-parallel states). Protons precess („wobble“) by magnetic field (B0).
Image:
1. intracellular
2. extracellular
3. in between myelin sheath
What is Radiofrequency (RF) pulse
The MRI machine applies a RF pulse to protons and protons absorb the energy.
This causes the protons orientation to change 90 degrees from
longitudinal to transverse planes – evolves
transverse magnetization.
- precession (wobble) is synchronized
When RF (Radiofrequency) pulse is turned off…
T1 RELAXATION
* protons transverse magnetization stops and longitudinal magnetization recovers
T2 RELAXATION
* precession is not synchronized
any more
- protons return to their natural random alignment
T1 and T2 weighted images:
T1: fluid is dark
T2: fluid is bright
Tissue colors dependent on T1 & T2 relaxation.
3 MRI planes (SAG, TRA, DOR)
Contrast agent used in MRI.
- dimeglumine gadopentetat (gadolinium)
- given i.v.
- passes blood-brain barrier (BBB), when it’s damaged
- paramagnetic influence to protons
- selectively shortens protons
T1 relaxation time - signal is more intensive on T1 images
MRI with contrast agent
Name these.
The MRI machine in our clinic:
* low magnetic field - 0,25 T
* designed for animals (dog, cat, horse)
- special coils for different body parts
- patient always under general anesthesia
- long examination time
Key differences between CT and MRI.
With suspected CNS neoplasia what imaging modality should you choose?
MRI is the best choice
Definitive dx requires histology. Sometimes cells can be retrieved from the CFS but not all tumors “shed” cells in to CSF.
brain macroadenoma on both CT and MRI
With suspected CNS vascular ddx what imaging modality should you choose?
MRI preferably
MRI
With suspected CNS inflammation ddx what imaging modality should you choose?
choose MRI
frontal lobe inflammatory lesion in a chihuahua
With suspected CNS trauma what imaging modality should you choose?
CT is better
With suspected CNS spinal diseases what imaging modality should you choose?
MRI better, except if you have classic IVD prolapse
CT Thorax vs MRI thorax
CT better for thorax according to Savvi
far left: pneumothorax
middle: lesion at bottom left
CT abdomen vs MRI abdomen
CT better
Left: hydronephrosis with dilated ureter secondary to accidental ligation during OVH
Right: adrenal neoplasia (yellow arrow)
CT skull vs MRI skull
CT mostly good but in some cases, like potentially spread inner ear issues, MRI is better
OI = otitis interna
CT extremities vs MRI extremities
CT good usually
CT Whole body imaging vs MRI Whole body imaging
Summary of Indications for CT and MRI.
