Intro to Imaging - CT/MRI (10/18b) [Biomedical] Flashcards
Computerized Tomography (CT) - Overview
CT also uses x-rays to acquire the image
Displayed in voxels (3D pixels)
Attenuations — in Hounsfield units (HU) → higher number is more radiodense, appears whiter
Acquired axially but can be reformatted
Have better contrast and are more sensitive for imaging soft tissue than plain films
CT - Hounsfield Unit Trend
Most radiodense (white) → Most radiopaque (black)
Dense metals Cortical bone Spongy bone Muscle Blood Water Fat Soft tissues Lungs Air
Some uses of CT
Routinely used for trauma, generally more available than MRIs in rural areas
Conventional Axial Tomography (CAT) scan — can be used for bone issues/fractures if more contrast is needed than an x-ray
- Especially where geometry of the bone might be tricky (EX: knee, spine)
How would a clot vs a hemorrhagic stroke appear on CT?
Clot will appear darker, hemorrhagic stroke appears bright white
CT - Windowing
set the level of the shades of gray of interest around the tissue they’re interested in
Must be done when the imaging occurs
Bone windowing can enhance ability to distinguish between cortical and spongy bone
___ tissue is the most negative substance in the body
Lung
Uses/Advantages of CT
Used for bone issues or head trauma
Subtle or complex fractures
Degenerative changes in bone (EX: spinal stenosis)
Acute intracranial bleeding (EX: hemorrhagic strokes or subdural hematomas)
Quantitative bone mineral analysis (used for measure of OP)
Heterotopic Ossification (HO)
More available, less expensive and quicker than MRI and more information than radiographs
Disadvantages of CT
More exposure to radiation than radiograph (still very low), but cumulative
Not as good for soft tissue differentiation — cartilage, tendons and ligaments better with MRI
Patient movement and metal implants will produce artifacts
Magnetic Resonance Imaging (MRI) - Overview
MR Images are produced by the interaction of hydrogen containing molecules within tissues and a magnetic field produced by the magnet
MRI Steps
1) Hydrogen lines up with the field of the magnet
2) A radio wave is delivered to the tissue which causes the hydrogen nuclei to absorb energy and change its alignment within the magnet (resonance)
3) The radio pulses are stopped, the absorbed energy is released and measured by the computer detector
4) This info is converted to an image.
5) Unique images are produced because each tissue has a different
amount of hydrogen and so relax at different rates
What is MRI based on?
Not based on density, but on water content
Increased sensitivity and ability to differentiate soft tissues
Fat vs cortical bone in T1 MRI
Fat has highest amount of water in our body, appears bright white in T1
Cortical bone will appear black in T1
In an MRI, the ___ and ___ of the pulses produces different quality images of the same tissues
length and sequence
MRI - Repetition time (TR)
time that elapses between 2 consecutive radio wave pulses
MRI - Echo Time (TE)
time selected to wait after the start of the TR to receive the signal or “echo” from the patient
MRI - Varying TE and TR
Varying the TR and TE will accentuate different tissues
T1 and T2 images are different for each tissue and produce different intensities (degrees of whiteness) of the same tissue
MRI - T1 Weighted Image
Using a shorter TE
Better for viewing anatomy
MRI - T2 Weighted Image
Using a longer TE
Better for pathology since the images accentuate still fluid
Suppresses fat and shows pathology – increase in water brightness (abnormal water uptake) and fat suppressed
Uses/Advantages of MRI
Superior soft tissue visualization, especially muscles, tendons, ligaments, nerves, articular cartilage and menisci
Okay for spongy bone (high fat content)
No ionizing radiation, no known harm
Disadvantages of MRI
Can’t use with patients who work with metal shavings, metal pacemakers, certain types of fixative devices (ferrous metals)
CT vs MRI
CT IS BETTER FOR
- cortical bone: subtle and complex fractures
- calcifications in any tissue
- acute intracranial bleeding
MRI IS BETTER FOR
- spongy bone
- soft tissue (ligaments, tendons)
- cartilage
MRI - T1 Weighted Appearance
High intensity (white) → Low intensity (black)
subcutaneous fat spongy bone still fluid cartilage tumor muscle fluid
MRI - T2 Weighted Appearance
High intensity (white) → Low intensity (black)
still fluid (inflammation) tumor fluid muscle cartilage spongy bone subcutaneous fat
