What are the TR and TE values for T1 imaging?
- ** T1 weighted images = short TR and short TE
- Short TR (100-1000 ms) (<1000)***
- Short TE (20-30 ms) (<30)
What are the TR and TE values for T2 imaging?
- **T2 weighted images = long TR and long TE
- Long TR (1600-3000 ms) (>2000)
- Long TE (70-100 ms) (>60)***
T1
Fat image
- Fat = white, water = black
- Operator manipulates TR and TE to produce images in which contrast between tissues reflects differences in T1 of those tissues
- T1 images produced by a short TR
- Those tissues that return to Bo the fastest (fat) give off the most energy and appear bright (white) on T1 images
- The water (H2 nuclei) in fat is closest to the resonant frequency of the RF pulse, so it gives off the most energy and appears brightest.
- That is why the T1-weighted image is often referred to as the fat image
- Fat has a shorter T1 than does free water
- Free water gives off the least signal on a T1 image and appears black.
T2
Water image (T2 or “H20” image)
- Fat = black, water = white
- Operator manipulates TR and TE to produce images in which contrast between tissues reflects differences in T2 of those tissues
- T2 images produced by long TE
- Those tissues that hold their transverse magnetization longest give of the strongest signal and appear bright
- Free water gives off the strongest T2 signal and appears bright (white) on a T2 image
- That is why the T2-weighted image is often referred to as the water image
- Free water has a longer T2 than does fat
- Bound water (protein) moves more slowly as a molecule, and has a less intense signal
What is Gadolinium?
IV contrast material that can be used to suppress fat on a T2 image
o This has the same type of effect as STIR imaging (super T2), but IV
Gadolinium does NOT suppress fat uniformly
Identifying fluid filled lesions
o PRIMARY USE OF GADOLINIUM IS FLUID-FILLED LESIONS
o You will see a bright “rim” sign surrounding lesion
o MOST COMMON use of IV gadolinium is for identifying soft tissue structures that have fluid in them like a ganglion
o When using gadolinium on a mass, you are checking to see if you are looking at a fluid filled image or not, so the image will be a T1
o Generally speaking, cystic lesions (water-filled lesions) tend to be non-malignant
What disease can gadolinum cause?
o An association between gadolinium and the development of nephrogenic systemic fibrosis (NSF) has been recognized in patients with severe renal impairment, but it doesn’t have to be a patient with prior renal failure
Magic angle effect
- Common 4th year question at residency sites
- This is an artifact that produces false pathology on a T1
- Occurs whenever the body part is orientated 55 degrees from the plane of the magnetic field (Bo)
- In podiatry, we typically see this in images around the ankle
o It will occur where ever structures change direction, so it happens to tendons and ligaments around the ankle
o It occurs in tissues with highly structured collagen fibers (tendon/ligament) - Normal tendons and ligaments appear dark on T1, so if you see brightness in a tendon or ligament, you would typically think there is some injury/pathology
- What you have to do in order to determine if this is the magic angle artifact or actual pathology is look at the T2 image
- ***If the tendon/ligament is truly torn, there will be bright signal on T2 as well
- ***If the structures look normal on T2, there is no damage and this is the magic angle effect
