magnetic resonance imaging

what is MRI?

a cross-sectional imaging technique that uses a magnetic field and radiofrequency signals to cause hydrogen nuclei in water and fat molecules to emit their own signal, which is then converted into an image by a computer

hydrogen, fat, and water**

what is a sequence in MRI?

the timing of the radiofrequency pulse and the capturing of the energy signal

most sequences are generally ______

T1 weighted or T2 weighted

how is pathology diagnosed?

comparison of tissue on T1 and T2 sequences

what is the basic tenet of MRI?

define the anatomy

identify the pathology (which usually has a component of edema/inflammation)

what is a T1 weighted sequence?

used to define anatomy

what is T2 weighted sequence?

identify fluid and therefore pathology (T2 = H2O)

what is a fat-suppressed sequence?

can be added to T1 or T2 - this quiets the signal from fat making water/inflammation more evident

what are characteristics of T1?

short TR (time to repetition) (500ms) and short TE (time to echo) (20ms)

tissues that recovery quickly with T1 will produce ________

a bright or high intensity signal (fat)

tissues that recovery slowly with T1 will produce ________

a dark or low intensity signal (water)

what are characteristics of T2?

long TR (>2000ms) and long TE (60-100ms)

tissues slow to recover with T2 will image ________

bright with high intensity signal (water)

tissues fast to recover with T2 _________________

do not get measured and produce dark or low intensity signals (fat)

most anatomy slides will be in ________

T1 because it demonstrates anatomy well

when should T1 vs T2 be looked at?

look at T1 to identify the anatomy and the T2 image to identify where the fluid from inflammation is located

why is MRI the most sensitive for detecting stress fractures?

detects inflammation around the fracture

what is a proton density MRI?

refers to a specific type of MRI sequence run by an MRI machine

it looks for the small ligament structures in the neck and back, and can tell whether such tissues are fully intact or have been torn by a traumatic event.

what are proton density image characteristics?

when an MRI sequence is set to produce a PD-weighted image, it is the tissues with the higher concentration or density of protons (hydrogen atoms) which produce the strongest signals and appear the brightest on the image

proton density weighted sequence produces contrast mainly by minimizing the impact of T1 and T2 differences with long TR (2000-5000ms) and short TE (10-20)

how does inflammation appear on T1 vs T2?

T1 = low signal intensity (dark)

T2= high signal intensity (white)

how does acute hemorrhage appear on T1 vs T2?

T1= high

T2= low

what is the difference between coronal, sagittal, and axial MR image?

coronal images are viewed from the front as if facing the patient, just like CT

sagittal images on either side of the body are viewed from left to right, just like CT

axial images are viewed from below and move up the body, just like CT

what are intrinsic factors of image quality in MRI?

quantity of water present or proton density

what are extrinsic factors of image quality in MRI?

choice of imaging sequence

strength of magnet-size matters (standard was 1.5 Tesla, now 3-7 Tesla Some even up to 20 T)

what can enhance MRI?

paramagnetic agents like gadolinium, intravenously or intra-articularly injected (MRA - arthrogram)

what is an MRI with contrast? what is it used for?

a gadolinium based medium is injected intravenously to improve resolution in tissues when evaluating blood vessels, tumor, infection, inflammation, or if prior surgery altered the tissue

used to further investigate a finding on an initial pre-contrast part of the scan. (MRI with and without contrast)

what sequence is typically used for MRIs with contrast?

T1 fat-suppressed FS sequence

what is an MR arthrography?

injection of a diluted gadolinium contrast medium

what is MR arthrography used for?

distends the joint capsule, allowing smaller structures that are normally difficult to see in small spaces to be seen

contrast is bright and highlights tears or defects in the capsule or tissues within the capsule, most often used to evaluate labral and ligament tears in hip and shoulder and ligament tears in wrist and ankle

what is MR myelography?

noninvasive and does not require contrast injection into the subarachnoid space

special sequences are use to increase the signal from CSF and suppress surrounding background signal

what is MR myelography used for?

assesses spinal stenosis

what are clinical indications for MRI?

soft tissue injuries of any kind

bone tumors, stress fractures, osteomyelitis, avascular necrosis, acute bone injuries

intervertebral disc pathology

what are limitations of MRI?

imaging cortical bone because of low signal intensity (low water)

claustrophobia → need for sedation

orthopedic hardware is not ferromagnetic so is not a hazard, but can distort images

high cost

what are contraindications of MRI?

magnetic field can lift heavy metal objects in the proximity

surgical clips, brain aneurysm clips, can displace any magnetic metal

pacemaker malfunctions

what are differences between radiographs, CT, and MR imaging?

structures that appear bright on radiographs and CT typically appear dark on MR

when would you use a CT over MRI?

Less expensive

Greater availability

Thinner slices

Less loss of image quality owing to motion

Better cortical bone

Ferrous implants Okay

when would you use MRI over CT?

Better for soft tissues

No ionizing radiation

Greater image quality in non-axial planes

No artifacts due to intervening bone

Less risk of missing disease because it will show up on another sequence