MR Physics

General MR Physics References

http://www.imaios.com/en/e-Courses/e-MRI

Q: What is T1?

A: T1 is the time constant for the recovery of the longitudinal magnetization.

S (t)=So[1-e(-t/T1) ][e(-t/T2)]

At t = T1, S=So[1 - e-1] = 0.63 So This is the basis for the typical Core exam physics question.

T1 is the time that it takes for the longitudinal magnetization to recover to 63% of thermal equilibrium (So )

Q: What is T2?

A: T2 is the time constant for the decay of the transverse magnetization.

S=So[1-e(t/T1) ][e(-t/T2)]

At t = T2, S= So[e-1} or 0.37 So This is the basis for the typical Core exam physics question.

Q: What is a sat pulse? (see Q 52, p 169)

A: It is a 90 degree pulse applied prior to the imaging sequence. The tissue that experiences the 90 degree pulse is out of sync with the rest of the tissue and therefore gives no signal. Sat pulses can be used in the field of view (e.g. to suppress heart signal on a T-spine image or artifact from the orophayrnx/swallowing on a C-spine MRI).

Sat pulses can be used out of the field of view (e.g., a sat pulse parallel to and just above the imaging plane during 2D-TOF neck MRA will suppress inferiorly directed (usually venous) flow)

Sat pulses can be used in the field of view (e.g. on a sagittal spine images to reduce bowel or heart motion artifact from degrading the spine)

A: There is a superior saturation pulse.Q: When does the sat pulse cause confusing neck MRA images?

A: When there is inferiorly directed flow in arteries.

Q: What are examples

A: In subclavian steal, there is reversal of flow in the vertebral artery.

In some cases of common carotid occlusion, the ECA supplies the ICA. In these case the ICA appears apparently out of nowhere on the MRA.

Subclavian steal is not listed in the ABR Core Exam Study Guide

Examples for the interested reader include HSS 2278938 (thanks to MaryAnn Ro '17)

Q: How would you generate an MRV of the neck?

A: Position the sat pulse just below the imaging plane during 2D-TOF neck MRA

Q: How do differnet parts of K-space control spatial resolution v contrast resolution

A: The central portion of k-space influences contrast while the peripheral portion infuluence spatial resolution

http://images.slideplayer.com/11/3280813/slides/slide_31.jpg

Thanks to John (Big Red) Choi '18 for the link.

This is why the "keyhole" technique works.

References

Gory Detail on GRE/Steady State MR

spoiled vs various varieties of refocussed residual transverse magnetization

http://radiographics.rsna.org/content/28/4/1147.abstract

some reviews of popular books:

http://www.revisemri.com/other/books.php

Dr. Joffe recommends:

All You Really Need to Know About MRI Physcs

http://simplyphysics.com/PRODUCTS.HTM#Products_textbook