MRI ch 21 Flashcards
Most scanners today have a magnetic field strength between ___ and ___ Tesla.
0.5 and 3T
The coils withing the magnet are responsible for____
either transmitting radiofrequency (RF ) pulses = trasnmitter coils
or recieving the signa / or echo given off by these excited protons = reciever coils
What is T1?
T1 = relaxation or recovery is the time it takes for the tissue to recover to its longitudinal state (parallel to the magnetic field); i.e. the time before the radiofrequency pulser was administered
What is T2?
T2 relaxation (or decay) is the time it takes for the tissue to regain its transverse orientation (perpendicular to the magnetic field) before the radiofrequency pulse was administered.
The two main pulse sequences are ___ and ___.
spin echo and gradient recalled echo
What is pulse sequence?
Pulse sequence consist of a set of imaging parameters determined in advance by protocols for specific diseases and body parts,
Pulse sequence can last between
20s to 15 min
What is TR?
TRis the repetition time between two RF pulses, and it indlueces the amount of T1 weighting.
What is TE?
TE is th echo time between pulse and its resultant echo and it influences the amount of T2 weighting.
Tissues that have a short T1 will be ____
bright
Tissues with a long T2 will be ___
bright
Water will be ____ on T1 images and ____ on T2 images.
dark on T1 and bright on T2
Certain tissues and structures are typically bright on T1-weighted images. Those are:
fat,
hemorrhage (depends on the age of hemorrhage), proteinaceous fluid (in renal/hepatic cysts) – but water cyst will be dark,
melanin
gadolinium and other paramagnetic substances
Certain tissues and structures are typicallybright on T2-weighted images. Those are:
fat,
water, edema, inglammation, infection, cysts
hemorrhage (depends on age of hemorrhage)
What 2 structures are bright on both T1 and T2 imaging
fat and hemorrhage
____ is the most common IV agent used in clinical MRI imaging
Gadolinium
What is gadolinium’s effect?
its effect is to shorten the T1 relaxation times of hydrogen nuceli (and to a lesser extent also shorten T2)
T1 shortening will cause a brighter signal on T1-weighted images than the same images without gadolinium and it is for this reason that images obtained after gadolinium administration are usually T1 weighted to take advantage of this eeftc.
Structures that become BRIGHT on postgadolinium images are typically ___ or are ____ tissues, and are described as ____.
Structures that become BRIGHT on postgadolinium images are typically vascular (e.g. tumor) or are inflamed tissues, and are described as enhancing.
What is an absolute contraindication for MRI exam
ferromagnetic objescts inserted ina location where motion of the object may be harmful to the patient
e.g. cerebral aneurysm repair clips, vascular clips, surgical staples
In general what are contraindications and considerations for MRI exam?
- claustriphobic patients
- those who have ferromagnetic objects (staples, foreign bodies - bullet, metal in the eyes)
- mechanical or electrical devices - pacemakers, pain stimulator implants, insulin pumps, implantable drug infusion pumps, cochlear implants
- pregnant patients (we cannot administer gadolinium here)
- nephrogenic systemic fibrosis
