Magnetic Resonance Imaging 1 Flashcards
What is nuclear magnetic resonance (NMR)?
spectroscopic technique which examines the radiofrequency (RF) emissions from spinning (precessing) nuclei
What is the spectroscopic technique?
uses an interrogating RF field to ascertain the magnetic moments of nuclei. Resonance ensures that only the nuclei of a particular element respond to the RF.
How does MRI extend the spectroscopic technique?
to image the distribution of a particular nucleus (ie. hydrogen - the intensity of the proton frequency emission) throughout the body.
Why is hydrogen chosen in MRI?
it demonstrates strong nuclear magnetic resonance and because of its prevalence in the body (through H2O).
What are paramagnetic substances?
When freely suspended in the presence of an external magnetic field, diamagnetic substances align themselves with the external field and become magnetised in a direction parallel to it. The net effect is an increase in the field strength around the material. This is a temporary effect and returns to its equilibrium distribution when the field is removed (eg. aluminium)
What are diamagnetic substances?
When freely suspended in the presence of an external magnetic field, diamagnetic substances align themselves against the external field and become magnetised in a direction opposite to it. The net effect is a reduction in the field strength around the material. This is a temporary effect and returns to its equilibrium distribution when the field is removed. (eg. water, nitrogen)
What are ferromagnetic substances?
are those which demonstrate very strong paramagnetism, to the extent that the magnetic forces may be sufficient to move the material to a stronger part of the field. Such substances retain their magnetisation following removal of the field. (eg. iron)
What is electromagnetism?
Copper is not strongly magnetic itself, but it can be made to exhibit strong magnetic fields by passing a current through it. This is the principle of the electromagnet, and a ferromagnetic core by virtue of its strong magnetisation adds to the field strength. The magnetic field due to the current is not related to the copper itself, but is a property of moving charges - a fundamental effect that can be described by a relativistic argument.
What is electromagnetism?
A moving charge has an associated B-field
Field at centre of a current loop
What is the Biot-Savart Law?
look at poster
What is precession?
Precesses in response to an applied torque
Precession is in a plane orthogonal to plane of couple
Rotating direction of spinning mass affects direction of precession
The greater the applied torque, the faster the precession
What is the difference between linear and rotational mechanics?
Force = rate of change of linear momentum
Torque = rate of change of angular momentum
What is the process of precession?
Proton is a spinning, rigid body Apply B-field Proton attempts to align Proton precesses… ...swept, accelerating charge Maxwell Proton radiates (w=B) w is ‘Larmor frequency’ Bloch equations dM/dt = yMxB
What is the simplistic classical MR model of precession?
Proton: Charged body, Rigid body, Intrinsic spin
Applied B-fiel: Precession, Radiated RF
Detection: Tuned RF sense coils
Proton density signal
Why would there be no signal?
Body composed of many spins
Randomly orientated
All orientations equally represented
