MRI physics ch1-4

rf excitation pulse

a short burst of Radio frequency energy applied to the area of the body being imaged

T1 weighted image

images used to show anatomy and pathology after administration of a contrast agent. The contrast depends predominantly and the differences of the T1 recovery times between fat and water

during the T1 weighing,

The TR must be short enough so that neither the vector and fat nor the vector in water has sufficient time to fully return to B0 if the TR is too long both the vectors and fat and water return to B0 and fully recover their longitude and no magnetization. when this occurs T1 recovery is complete in both tissues, and their differences in T1 recovery times are not demonstrated (for the best images!)

tissues, with a low proton density of a small transverse component of magnetization

And therefore a low signal in a relatively hypo intense

tissues with a high proton density have a large transverse component of magnetization

Which means they have a high signal and are hyperintense

the Nmv is separated into individual vectors of the tissues such as

fat, cerebrospinal fluid (CSF) and muscle

proton density PD

of a tissue is the number of mobile hydrogen protons per unit volume of that tissue

The higher the proton density of a tissue

The more signal available from that tissue

two extremes of contrast in mri

fat and water

The oxygen and water tends to steal

The electrons away from around the hydrogen nucleus

fat and water appeared differently in

MR images. due to hydrogen in fat recovering rapidly and faster than water when exposed to a magnetic field

T1 recovery in fat occurs due to

Hydrogen nuclear, giving up their energy to its surrounding molecular lattice

The NMV fat realign rapidly would be zero so the

T1 recovery time of fat is short

magnetic moments of water, hydrogen, nuclei, take longer to relax and regain their

longitudinal magnetization

The MMV of water takes longer to realign with B0 and so the T1 recovery time of water is

long

The T2 decay type of fat is

short

The T2 decay time of water

long

T1 contrast

Image contrast is derived from differences in the T1 recovery times of the tissues rather than any other mechanism

proton density contrast

differences in signal intensity between tissues that are consequence of their relative number of mobile hydrogen protons per unit volume

T2 contrast

Image contrast is derived from differences in the T2 decay times of the tissue rather than any other mechanism. Likely to occur if vectors de phase and there's a difference in coherent transverse magnetization in each tissue

In this phase fat as a low signal while water has a high signal

water has (in T1)

low signal and is hypointense

fat has (in T1)

high signal and is hyperintense

partially saturated

nmv pushed beyond 90degrees

fully saturated

nmv pushed to a full 180degrees

steady state

when achieved vectors recover to the same point and achieve the same amount of longitudinal magnetization during the TR period And they are always flipped to the same point by the 90° excitation pulse

preparatory or dummy pulses

first few RF excitation pulses are that our signals that are produced and ignored

relaxation is a

Process by which hydrogen loses its energy from RF excitation pulse

Rf excitation pulse

Produced by a transmit coil used to achieve resonance

unit of phase is

radian

x-y axis / transverse plane

causes magnetic moments of the spins to precess

larmor equation =precessional frequency which is

proportional to the field strength

Nutation

spiral motion caused by two precessional motions- happen simultaneously

excitation is…

The RF pulse that creates resonance(energy giving) Create more high energy nuclei.

during resonance

All magnetic movements moved to the same position on precessional path and in phase

flip / tip angle

MMV lies in transverse playing at 90° to B0

saturation

Caused by When spins are unable to absorb energy or to be stimulated and release more energy

lenz law

induced electric current always flows in a direction that opposes the change in the magnetic field that produced it

The hydrogen nuclei only get Resonance

if RF pulse has the frequency equal to the larmorfrequency of hydrogen

pulse sequences

form is a magnitude and timing of RF pulses. This is the basis of contrast generation in MRI.

radiofrequency

band on electromagnetic spectrum. this is where hydrogen is at

what is the relationship between angular momentum and magnetic moment of MR Active nucleus?

Gyromagnetic ratio!

graded/ sloped magnetic fields =

gradients

oscillating field

B1, magnitude lower than B0

the speed the atoms precess around B0 is called…

Precessional Frequency!! or larmor frequency.

signal to noise

increases at higher field strengths

fringe field

mag field outside of scanner

magnetic resonance

phenomenon that affects a magnetic dipole when placed in uniform static magnetic field

Measures strength of mri machines

T (tesla)

raymond damadian

built first patent for MRI - discovered hydrogen is a signal of cancerous tissue

paul lauterbur

produced first NMR image of test tube. won prize and took credit.

richard ernst

proposes using phase and frequency encoding. initial physics proposed to mri.

1977

first scan of healthy human body in mri. followed by cancer body the year later!

block and purcell were

pioneers of mri. learned the matnetic diapole static energy infield.

fMRI

functional MRI that shows speech and brain, finger, tapping and listening in real time in the brain

growth of MRI happened in

1980s. continued to 2000s for cardiac and fetal imagine

who produced functional mris?

filler and colleagues

most abundant atom in human body

hydrogen

classical theory

mechanical view of how the universe and MRI works. uses concepts of mass, spin angular momentum on a a larger or bulk scale

quantum theory

Subatomic level operates in a much smaller scale that refers to energy level levels of protons, neutrons, and electrons

isotopes

atoms are the same number of protons, but a different number of neutrons

ionization

knocking out of electrons

even atomic and mass number

= nucleus will have no spin

angular momentum

A nuclei with odd number of protons, neutrons or odd number of both spin directions are not equal in opposite so the nuclear is itself has a net spin

MR active nuclei

only nuclear with an odd mass number or atomic weight characterized by their tendency to align Their access of rotation to an applied magnetic field

law of electromagnetic induction

connection between electrical and magnetic fields and motion

classical theory is where the magnetic field

is denoted by a magnetic moment

MRI can see inside bones produce images of blood vessels, cartilage, bone marrow, and ligaments and tool to map the brain

what can MRI see?

Protium

isotope of hydrogen (most commonly used MR active nuclei in MRI)

nucleus Of Prodium has a magnetic field induced around it

it acts as a small magnet.

nuclei available for MRI have a net spin and contain at least

one (positive)proton

magnetic moment

an arrow that denotes the magnetic field of nucleus

allignment

can be a normal straight alignment or random alignment

classical theory uses direction of magnetic moments of spins to create

allignment!

parallel alignment

Magnetic moments same direction as main B0 field spin up

anti-parallel allignment

magnetic moments in opposite direction to main B0 field (spin down)

NMV reflects balance of

spin up vs spin down nuclei!

the number of low energy nuclei vs high energy nuclei is

directly proportional to the strength of B0

difference between low n high energy nuclei determines

overall size of net magnetization vector

strong mag field?

many more low energy nuclei, because it would take much more energy to allign in opposition to the magnetic field

the NMV and magnetic field strength is also directly

proportional

stronger the magnetic field…

larger the NMV

how bit the NMV is controls how strong the what is?

MRI Signal!

stronger mag field→ large NMV

stronger MRI signal!

Rf Excitation is tuned to what?

larmor frequency of hydrogen in the magnetic field.

B1

Rf excitation pulse

B0

magnetic field

when only exposed to subzero,

The net magnetic vector or MMV is in alignment with the longitudinal plane

what happens when rf excitation pulse excites the NMV at a 90 degree angle?

flipped into transverse plane

once flipped into the transverse plane, the nmv loses

longitudinal magnetization, while GAINING transverse magnetization

transverse magnetization is…

what we can measure in an MRI Scanner!

flip angle

The amount that the MMV is flipped Away from the longitude and a plane is called

short rf pulse flip angle may be

shorter than 90 degrees

long rf excitation may have flip angle may be

exactly 90 degrees

Flip angle play a role in

Determining the contrast and brightness of an MRI image based on the different properties of different tissues

when the RF excitation pulse is stopped the hydrogen nuclei release the energy that they absorbed, and begin to phase with one another…

phase called relaxation!

relaxation is a result of..

excitation

no resonance=

No excitation