ARRT PRINCIPLES OF MRI FINAL EXAM REVIEW-WCUI 3D gradient echo axial views can be used in cervical spine imaging to provide -Answer- Thin, contiguous sections of the spine, the ability to reformat into any other imaging plane, retrospectively, the ability to get either T1, spin density or T2* information by changing parameters. In complete spine imaging, to rule out metastatic lesions of the spinal cord, contrast enhancement can be used with T1 weighted images because -Answer- metastatic lesions enhance and normal cord does not Due to the differences in frequencies across the magnetic field we -Answer- use gradients, use sat bands, . use different coils To avoid the "magic angle" (an artifactual brightness seen in the medial segment of the posterior horn of the lateral meniscus when short TEs are used) artifact when imaging the knee, -Answer- use a longer TE, reposition the knee, orient the affected anatomy to another angle To best visualize the anterior cruciate ligament on knee MR -Answer- externally rotate the leg about 15 When using MRA to evaluate intracranial vascularity, flow within smaller vessels can best be demonstrated by -Answer- 3D time of flight MRA or 3D phase contrast MRA To produce the echo, a gradient echo pulse sequence uses -Answer- a gradient magnetic field only The rate of dephasing due to inhomogeneities is called -Answer- T2 inhomogeneity If the TR of a gradient echo pulse sequence is considerably less than the T2 (and T2*), the condition that will exist is known as -Answer- steady state The gradient that is on during the production of the echo is the __________ encoding gradient -Answer- frequency The gradient that is on longest is the __________ encoding gradient -Answer- frequency The four phases of a spin echo pulse sequence, in correct order, are -Answer- Excitation, encoding, refocusing, readout. Using an NSA of 3, if a phase resolution of 192 is desired, then the TR must be repeated -Answer- 576 times In a fast spin echo sequence, the effective TE is the echo that is performed with the -Answer- Low amplitude phase encoding gradient. In a 2D multiecho pulse sequence, ETL = 8, scan time is given by the equation TR x -Answer- Number of signals averaged x number of phase encodings / the number of echoes. When using MRA to evaluate extracranial vascular flow, such as that within common carotid arteries, a recommended technique is -Answer- 2D time of flight MRA When using bright blood MRA to evaluate peripheral vascular flow, such as that within the arteries of the legs, saturation pulses are -Answer- placed inferior to the acquired slices In a 3D acquisition, the slices are produced by -Answer- A phase encoding gradient applied in the slice direction. Acquired by volume imaging. To create a projection image in MRA, the technique MOST commonly employed is -Answer- 19. maximum intensity pixel If a given conventional spin echo pulse sequence takes 12 minutes to acquire, a multiecho sequence, using a 6 echo train, with all factors that affect scan time remaining the same, will take ________________ to acquire the same images -Answer- 2 minutes Gadolinium chelates are -Answer- Paramagnetic. Acquiring half the phase views of k space and then calculating the data for the other half is a technique known as -Answer- partial Fourier The MRA sequence that is more sensitive to slow flow is -Answer- 2D time of flight A major advantage of MRA over conventional angiography is that -Answer- 2D time of flight The removal of signal from vessels in an MRA sequence is achieved by -Answer- spatial presaturation In a spin echo sequence, flowing blood is normally seen as a signal void because -Answer- the 90o pulse and the 180o pulse are both slice selective In a time of flight sequence, the tissue is hypointense relative to flowing blood because of -Answer- saturation effects Phase contrast techniques produce image in which the signal intensity within the vessel is dependent on (among other parameters) the -Answer- velocity of the flowing blood A major advantage of 3D time of flight techniques over 2D time of flight is the ability to -Answer- visualize smaller vessels