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when does the 180 degree rephasing pulse in a spin echo applied?
between the 90 RF pulse and the spin echo, or half TE
why isnt the FID sampled when it is formed?
- because the transmitter is still on during the early part of the FID
- because the signal decays before it can be properly spatially encoded
what happens when the slice select gradient isnt turned on when the 90 degree RF pulse is applied?
will cause every proton in the vicinity of the transmitting RF coil to be disturbed
when does the slice select turn on?
whenever the RF energy is being transmitted
when does the readout or frequency encoding gradient turn on?
during the formation of the spin echo
when is the phase encoding gradient turned on?
sometime between the 90 degree RF pulse and the 180 degree RF pulse
a positive change in frequency, followed by a 180 RF pulse, followed by another equal positive change in frequency, will have what kind of effect on the phase?
will have no net effect on phase
why is a fast spin echo faster than a regular spin echo?
because more than one echo is collected during a single repetition for a single slice
what is an echo train length?
the number of echoes collected in a single repetition
each echo represents how many phase encoded line of data?
information located at the periphery (top,bottom, left and right) of the raw data is said to contain high or low spatial frequencies? What does it correspond to based on contrast and resolution?
- high spatial frequencies because this is where the amplitudes of the phase encoding and readout gradients are the greatest.
- it corresponds to the edge definition or spatial resolution
information located toward the center of the raw data is said to contain low or high spatial frequency? What does it correspond to, contrast or resolution?
- low since this is where the amplitudes of the phase encoding and readout gradients are the lowest
- corresponds to contrast
what is a dual contrast spin echo sequence?
- type of spin echo pulse sequence which allows you to acquire an additional echo at a second TE time
- has an extra 180 degree RF pulse and generates a second spin echo
- this describes a traditional spin echo that can be turned into a dual echo spin echo. there's a fast spin echo version which has a dual contrast also
- the traditional spin echo generates a proton density and T2-weighted images
what is the purpose of a 180 degree RF pulse in a spin echo?
to rephase the dephased net magnetization in the transverse plane and to form a spin echo
what happens to the transverse net magnetization once its rephased to its maximum amplitude and the spin echo formed?
the transverse magnetization immediately dephases once again due to factors including magnetic inhomogeneities, the tissues' differing magnetic susceptibilities, and the phase effects of chemical shift
what is an inversion recovery pulse sequence?
- begins with an inversion or 180 RF pulse to disturb the net magnetization which is initially aligned along the positive longitudinal direction
- the 180 pulse causes the magnetization to spiral down and completely flip into the negative longitudinal direction
- the protons immediately begin to experience T1 relaxation and they relax back toward the positive longitudinal direction
- the protons in short T1 tissues relax back to the positive longitudinal direction quickly and the protons in long T1 tissues relax back to the postivive longitudinal direction slowly
- the inversion period therefore creates a differential spread of relaxation along the longitudinal direction, and this is what establishes T1 based contrast between the tissues
what follows the inversion pulse of 180 RF?
- 90 RF and a 180 RF pulse that essentially produce a spin echo sequence after the initial inversion
- the purpose of the inversion pulse is to prepare the T1 contrast. The spin echo part moves the magnetization into the transverse plane where rephasing of the dephasing mechanisms and sampling of the echo takes place
what is the inversion time?
the time from the first 180 RF pulse to the 90 RF pulse
what is a STIR?
- short T1 inversion recovery.
- suppresses fat to avoid obscuring pathology in certain parts of the body like the orbits, abdomen, and joints
what is one drawback to using inversion recovery method of fat suppression?
- its ineffectiveness when used with MR paramagnetic contrast agents
- since these contrast agents shorten the T1 relaxation time of any perfused tissues, and fat suppressed inversion recovery sequences reduce the signal of short T1 tissues, the pulse sequence will act to suppress the pathology that the contrast is working to enhance
what is FLAIR?
- fluid attenuated inversion recovery
- signal from CSF is suppressed
- requires a very long TI around 2000ms
- TR of 6000-9000ms is used to acquire T2 weighted images where the CSF is black
- fast spin echo is used due to the long TR time
what are the 2 main differences of gradient echo to a spin echo?
- can be run with shorter acquisition times than spin echo because it uses a shorter TR times
- it uses a RF flip angle of less than 90 degrees
- it uses no 180 RF pulse. it uses gradients with equal magnitude but opposite polarity to rephase signal and thus get a gradient echo
- since a spin echo and a gradient echo are formed by different means, the images each produces are not identical
what are the 3 dephasing mechanisms that reduce the transverse magnetization?
- magnetic field inhomogeneities- variations in the uniformity of the main magnetic field
- magnetic susceptibility- the tendency of a material to become magnetized
- chemical shift- refers to the shielding of the hydrogen proton from the full strength of the main magnetic field by the electrons of the molecule in which they exist
which pulse sequence is more sensitive to blood products thus are excellent for demonstrating hemorrhage and even determining the age of a bleed?
gradient echo pulse sequences
which pulse sequence is good for MRA studies?
gradient echo pulse sequence
which pulse sequence is good for abdominal studies?
- gradient echo pulse sequence
- they come in handy where patient motion is problematic especially respiratory motion. Gradient echo may be quick enough to complete while the patient suspends breathing
what are two types of gradient echo sequences?
spoiled and steady state gradient echo
what is a spoiled gradient echo?
the destruction of any residual transverse magnetization following the collection of the gradient echo
what is a steady state gradient echo?
- rephases and maximizes the residual transverse magnetization after the echo by adding extra gradient pulses to balance the phase once again
- is always T2*- weighted.
- not used for generating T1 weighted images
what is a fast gradient echo sequence?
- a very fast MR acquisition using a very short TR, a very short TE, and a shallow RF flip angle or low flip angle
- must prepare the contrast with RF pulses prior to the gradient echo portion of the sequence
- the T1 prepared sequence usually uses a 180 RF.
- the T2 prepared sequence may use three RF pulses (90 RF then 180 RF then a 90 RF) or may use a saturating 90 pulse
what is echo planar imaging?
- the fastest imaging technique available in MRI.
- it is to gradient echo what fast spin echo is to conventional spin echo
- a train of gradient echoes is collected for each slice following the initial RF excitation pulse
- fewer repetitions are required which reduces scan time
- capable of collecting all of the echoes needed to produce all of the images in a single TR which is called single-shot EPI
what happens in a multi-shot EPI?
several repetitions of RF excitation are are required to fill k-space
for which studies do you use EPI for?
- for studies that require extremely fast acquisition
- cardiac and coronary artery imaging
- diffusion-weighted imaging
- perfusion imaging
- functional activation studies