T1 & T2 Weighted Images PDF

T1 recovery/ weighted images - Fatty structures are seen brighter - White matter has myelin in it which is very fatty, will look paler than the grey matter - Fluid is seen dark - TR time around 400-650ms to allow recovery of structures excluding fluid - 300ms for pure fat -...

T1 recovery/ weighted images - Fatty structures are seen brighter - White matter has myelin in it which is very fatty, will look paler than the grey matter - Fluid is seen dark - TR time around 400-650ms to allow recovery of structures excluding fluid - 300ms for pure fat - 400ms for fatty structure - CSF, grey and white matter , we need to give TR long enough to allow contrast differentiation - This send another RF pulse, knowing these structures into the transverse plane and sends the fluid which has barely decayed or recovered another 90 degrees, out of transverse plane - We can take the image on echo time or TE of 15 ms which is right before fat decays again - TE time must be short because fat dephases fast - T1 recovery/ weighted= Short TE and short TR - Fat has a larger longitudinal magnetization resulting is faster T1 recovery T2 decay/ Weighted image - Fluid seen as bright - Golden standard - Cancers have fluid in them - Edema and inflammation also associated with cancers - Fluid needs to be in the transverse plane and in phase to see it as bright - Need to wait for fluid to recover and decay to flip but all structures will also be in the axis if we do this - This isn't what we want, TR cant control T2 - TE controls T2 weigthed images - Wait for fatty structures to decay, and we choose the TE time right before fluid dephasing - Fluid dephases at a 90TE and + - If we just want black and white, and ONLY see fluid: we increase TE time super high and a long TR because of high fluid recovery times (around 3000ms) - Called a "long drink" long TE - Depends on decay process Choose best sequence for pathology and then try to continue with the other sequences if possible with complicated patients - Ask to give patient something if they are agitated - Call radiologist to ask for prescription for something TR controls the image weighting - The TR controls how much T1 is weighed - Longer TR= more structures are brighter - Determines amount of T1 relaxation - The time between RF pulses TE - Time we wait to listen to the signal - Controls dephasing - Will never be more than TR - Time signal is taken Myographic effect - In spine: CSF surrounds chord - Do very bright T2 weighted image to only see the fluid bright, increase TE by a lot - For biliary system increase TE to almost 800ms Water has a higher energy than fat and doesn't absorb the RF pulses energy as easily - Causes slow T1 recovery - Water doesn't diphase or decay as easily also because the protons wont smack into each other as much as fat - Molecular tumbling is faster than the frequency which is why the recovery is slow Matrix of 256 X 192 - The second number in the matrix is what determines how many RF pulses (or TR's) that happen - In this example, 192 TR's - Gives you an idea of the scan time - If T1 takes 300ms to recover for each TR x 192 times, a T1 weighted image takes less time than the T2 which has 3000ms for each TR X 192 times - Scan time is ms per TR X matrix - Decreasing matrix will decrease time however will decrease spatial resolution - Matrix is dependant on resolution of images - High resolution = high matrix = small pixel - Used for looking at smaller structures - i.e in the IAC (external auditory cranial nerves), pituitary - = longer scan time Pulse sequences - Operator controlled - Series of RF PULSES signals and recovery periods - TR and TE are measured in ms Proton density images - Based on concentration of hydrogen in total - Some structures have more or less hydrogen - Bone minus marrow has hardly any hydrogen - Bone marrow gives signal (on a T1 particularly) because of the fat inside - Bones show up as bright on a T1 because of this - Tightly bound hydrogen nuclei will create higher signals - Shows lots of signal because the many structures are all made of H, just different amounts - CSF is very rich in H , the other structures will look like different shades of gray - Good for white and grey matter in the brain, differentiate structures easily in the brain - Good for multiple sclerosis - We use a plane that shows us the white matter best - Always include a sagittal proton density for MS to see corpus collosum beautifully - The axial will show the brain best, there will also probably be an axial T2 T1 short TR and short TE (\ - flip angle for spin echo: 90 - Gradients - wire coils that are magnets themselves - T2 and PD will take a lot of time especially with the 180 echo time added - By applying gradient to the z magnet, we create an increase in magnet of main magnetic field and decreasing it in another direction - Can change processional frequency in some area of the body - Changes main magnetic field strength - Can activate z gradient to increase dephasing of slow dephasing protons and decrease dephasing of fast dephasing protons to create homogeneity - Additional magnetic field and can add or subtract from the main magnetic field **linearly** and **predictably** (we control this) Image weighting and contrast in gradient echo is determined by TR, TE and flip angle - gradient is added (T2\*) to dephase so that we can predict how to apply the gradient in the opposite direction to properly align the protons in phase - Predictably diphase it more and then slap down to put everything in phase Magnetic susceptibility artifacts - Metal will cause artifacts - T2\*(gradient echo) will show a larger artifact - for a brain bleed we want to use this magnetic susceptibility artifact shown - blood has iron Smaller flip angles creates smaller TR times, this speeds up the exam Even a 20 degree - make sure metal isn't present because it will show horribly in gradient echo pulsing - never use gradient echo for those with metal implant - instead call radiologist and ask to replace with a spin echo short TR: 300ms for a gradient echo flip angle for T2 weighted images is shorter \- fluid will recover faster with smaller flip angles Higher gradient used results in thicker slices The slices will precess at different frequencies with a gradient applied as the magnetization is altered which allows us to predictably excite a slice at a time \- resonance won't occur in the other slices at a specific frequency, only the slice at that frequency Gradients (vs spin echo) - scan time is shorter - T2\* or gradient echo sequence - doesn't have a 180 pulse, has bipolar gradients that spoil and reverse dephasing and rephasing process - because it is a gradient, it is a magnet which is inhomogeneous - spin echo T1 has better signal inhomogeneities but takes long - spin echo is in the 1000's ms, the gradient is in the 100's ms - smaller flip angles allow shorter scan time in T2\* - susceptible to metal artifact inhomogeneities but useful for a bleed - metal implants will not allow a good image - the z gradient will excite slices independently - x and y help us encode things Scan time: TR X phase encoding (second number of matrix) X \# of excitations For a SE (spin echo) T2: 256 X 192 Pe: 192 TR: 4000ms Scan time: 4000x192x1 = 768000ms or 768 seconds = 12 mins and 8 seconds For a GE (gradient echo) T2\*: Same parameters with... TR 300: 57.6s with is about 1 min Compensates for premature dephasing of nuclei in trasnverse plane \- from the inhomogeneous MF \- the rephasing is at different times until the 180 flip \- best signal to noise is at 90 degree pulse in transverse plane the gradients cause the loud noises, not the RF pulses for gradient echo pulse sequencing: \- short flip angle for T1 and PD to be able to see the fluid and allows for quicker recovery thus faster scan time , no more 180 degree pulse \- TR in the 100's instead of 1000's \- short TR allows fluid to be shown For gradient.... (incorporate flip control if necessary) T1 weighted \- larger flip angle (90-110) allows for fat to be seen -don't want water to recover fast \- to delay fluid, short TR still but a bit longer with gradients \- short TE \- flip angle and the TR controls for T1 T2 weighted \- long TR and small flip angle (10-15) -- want water to recover fast \- large TE \- just the TE controls T2 PD \- long TR, short flip and short TE \- anything offsetting the T1 and T2