similarities between conventional radiograph & CT
both based on same principles, both use X-rays and get attenuated by the body's tissues and produce images based on tissue radiodensity
differences between conventional radiograph & CT
CT uses many projections from different angles that get captured by up to 1,000 detectors encircling the patient and creates images based on these slices
what are the three planes viewed as slices in a CT?
images as collected are not limited to axial plane (volumetric scanning) but can be constructed in any plane and rotated in 3 dimensions. Viewed in axial (transverse), saggital, and coronal (frontal).
Windowing
operator selects radiodensities displayed in an image to determine what tissues are emphasized (ie. bone v soft tissue)
What does CT image best?
Bone: subtle and or complex fractures, degenerative changes, spinal stenosis (especially when combined with CT myelography), and measurement of osseus allignment in any plane.
BEST MODALITY FOR evaluation of loose bodies in a joint, and can be combined with diskogram to evaluate intervertebral discs in the spine.
Also, imaging choice in serious trauma because it can detect injuries to muscle tissue types, less time-consuming than MRI or ultrasound, less expensive than MRI. 3D imaging.
limitations of CT
identifies tissues based on radiodensity and some tissues are similar in that regard making them difficult to distinguish. HIGH RADIATION EXPOSURE.
T1
return to longitudinal magnetization when protons lose energy to surrounding molecules, faster recovery=stronger signal, shorter T1
SIGNAL CAUGHT EARLY AND CAN NOTE DIFFERENCE IN FAT & WATER
TR= short time to repitition,
TE= time to echo
highest signal intensity on T1 (brightest)
yellow bone marrow and fat, ACUTE HEMMORAGHE
lowest signal intensity on T1 (darkest)
cortical bone, ligament, tendon, fluid
T2
relaxation of transverse magnetization toward equilibrium
slower the decay=stronger the signal
long TR: long time to repition
long TE: long time to echo
highest signal intensity on T2 (brightest)
water and inflammation (reluctant to release energy)
SOFT TISSUE TUMORS AND CHRONIC HEMMORAGHE
what does MRI image best?
soft tissue details (sport injury), changes in bone marrow (stress fracture), disk herniation, staging of neoplasm, alternative to invasive procedures
limitations of MRI
low signal intensity of corticol, increased time to produce image (high cost)
contradictions and health concerns related to MRI
metals can become projectile, surgical clips can become displaced & can be fatal, claustrophobia
advantages of MRI > CT
greater contrast of soft tissues and ability to image organs surrounded by bone, no ionizing radiation, various visibility using different sequences decrease the risk of missing a pathology
what are the disadvantages of MRI as compared to CT?
MRI more expensive & less available/takes longer. Thicker slices and less resolution of corticol bone and image quality can be lost due to motion. Contraindications with metal or metal implants.