6) Intro to MRIs

proton signals

MRIs are based on ______

• interaction of hydrogen-containing molecules within tissues and a magnetic field produced by the magnet.

• The hydrogen lines up with the field of the magnet.

• A radio wave is delivered to the tissue, which causes the hydrogen nuclei to absorb energy and change their alignment within the magnet (resonance).

How are MRIs produced?

• The radio pulses are stopped, and the absorbed energy is released and measured by the computer detector.

• This information is converted to an image.

How are the MR images made?

each tissue (and even the same tissues over time) have a different amount of H+ and so relaxes at a different rate.

unique images are produced bc

signal intensities of the same tissues

the length and sequence of the pulse produces different

repetition time (TR)

the time that elapses b/w two consecutive radio-wave pulses (how frequent a knock is)

echo time (TE)

time selected to wait after the start of the TR to receive the signal or “____” from the pt (pauses btw the knocks)

T1 weighted image

short echo time… pauses between echoes are short

T2 weighted image

long echo time… pauses between echoes is longer

T1-weighted image

using a short TE and TR will produce a

T2-weighted image

using a long TE and TR will produce a

T2-weighted or STIR (only bright white if there is pathology present bc of abnormal proton presence)

what imaging makes dark signaling?

T1

which images are better for viewing anatomy?

T2 bc these images accentuate still fluid

which images are better for pathology

routine MRI

both T1 and T2 and all 3 planes

• 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

what are some advantages/uses of MRI

Thought to be very safe; however, cannot use with patients who work with metal shavings, metal pacemakers, certain types of fixative devices (ferrous metals)

what are some disadvantages of MRI

• Spongy bone

• Soft tissue (ligaments & tendons)

• Cartilage

what is MRI better for?

• Cortical bone—subtle and complex fractures

• Calcifications in any tissue

• Acute intracranial bleeding

what is CT better for?