seminarseminar

October, 2008October, 2008 j. brnjas-kraljevićj. brnjas-kraljević

Imaging (MRI)Imaging (MRI)

tomography techniquetomography technique – the volume image is built up by images of thin slices from which data are taken

two-dimensional distributiontwo-dimensional distribution of certain physical parameter is image of one tom

measurement of space distributionspace distribution of same resonating nuclei is enabled by introduction of controlled inhomogeneity of B0 field - gradient of the gradient of the fieldfield in desired direction

we measure resonance/relaxationresonance/relaxation of hydrogenhydrogen nucleinuclei in water and in fat

dz

dBG

dy

dBG

dx

dBG zyx

in perfectly homogeneous field all protons have the same -- only one signal is measured

Signal is measured in the presence of field gradient. The result is distribution of nuclei in desired direction. Gradients in different direction built up space distribution of nuclei. Mathematical algorithm transcribes values of measured voxels signals into gray scale.

gradient in direction X-axis distinguishes the Larmor frequency of nuclei depending on the place in the field = (B0 +x Gx)

ImageImage constructionconstruction

by projection of reordered spectra each volume part, voxel, is give the value of measured parameters parameters are displayed in gray scale specters have to be measured in thin slices - the 3D-image is built up from many slices

How is it recordedHow is it recorded ? ? 90-FID method recording pulls simultaneously with gradient in the field

direction – selects the desired tom changing of the angle of gradient, Gf, for

frequency differentiation is realized by combination of two linear gradients in Y i X direction:

Gy = Gf sin and Gx = Gf cos the recorded FID is treated by FT - gives the

signal distribution by frequencies and phases

G

yGx

ImagingImaging change of gradient angle is realized by

combination of two linear gradients and mathematical processing of signal – analyses by Fourier transform

the time of applying and the with of gradients pulses in Y- and X- axes the voxels are differentiated by frequency and by phase

third gradient in Z- axis defines tom

FTFT

signal

recorded tom

phase differentiation

frequency diff.

Successive recording of Successive recording of slices in big volumeslices in big volume

frequency content of excitation RF- pulls is changed – to successively excite single tom along Z- axes gradient pulses in X- and Y-direction follow the frequencies after TR interval the first slice is excited again it is very important not to overlap the frequencies – toms are not exactly defined

Determination of single Determination of single volume parametersvolume parameters

chosen Larmor frequency excites only one tom

changes L in Y- ax; after that gradient pulls all moments have again the same frequency but differ in phase

distinguishes frequencies along X-ax

gradient is on during gradient is on during signal detectionsignal detection

gradient gradient inin Z Z axax gradient u Y gradient u Y axax gradient u X gradient u X axax

Parameters of a single volumeParameters of a single volume

ph

phaasese

frefreququencencyy

FID detected with X- gradient on contains frequencies and

phases of

precession of protons depending on the space distribution two-dimensional FT method determines the value of frequency

and phase for each single voxel in XY plane another FT procedure is used to calculate intensities from each

voxel and to display it in gray scale

DetectionDetection

artifactsartifacts - because of spin mobility between different voxels during

detection - because of diffusion - because of covering the small signals by higher ones from

undesired structures - because of to weak signal or undistinguishable signal in the

whole volume of interest

helphelp:: suppression of signals from structures not desired (water or fat) addition of paramagnetic ions signal detection in intervals of periodic flow or by special pulls

sequences

Contrast by saturationContrast by saturation

IRIR methodmethod - time TI is T1ln 2 for T1 hydrogen in fat or water detected are only nuclei in another tissue

SE methodSE method selective saturation pulls has frequency spectra in resonance with longitudinal magnetization of fat applied before standard pulls sequence courses the disappearance of fat magnetization phase gradient rules out fat transversal magnetization imaging sequence does not see fat

MRI angiographyMRI angiography

angiography – imaging of blood flow MRI detects flow - intensity proportional to flow speed 1. excitation pulls and detection pulls have different frequencies – two different slices along Z-ax – with correct TE sees the same blood volume 2. bipolar gradients – do not detect static protons – enhances signal from the ones that flow in direction of gradient 3. contrast agents – decreases T1

in blood – the signal from surrounding tissue, can be saturated

Parts of imaging systemParts of imaging system

B0 field is oriented along the patients bed – main axis

B1 field is in transversal plane RF field coil for excitation is

also the detection coil it emits and detects certain

white interval of frequencies detector coils have different

shapes – field shape three systems of coils build

up the gradients of magnetic field B0 in direction X,Y and Z axis

vacuumliquid heliumliquid nitrogenhousingsuperconducted coils

Three main gradientsThree main gradients

Meaning of magnetic field Meaning of magnetic field gradientgradient

gradient in Z-axisgradient in Z-axis - on while the initial RF- pulls is applied; determines tom in which spins are excited

toms width is determined by steepness of gradient and by frequency content of RF-pulls

gradient in X-axisgradient in X-axis - on during the time of detection of relaxation signal; therefore relaxation frequency is function of x coordinate

gradient in Y-axisgradient in Y-axis - regularly on and off between two RF-pulses; it determines phase distribution and resolution in XY-plane; 128, 256, 512; meaning 360/256 = 1,4o phase shift

typical voxel is 2 mm thick, and by matrices of 5122 has the area of 1mm2

for B0 of 1 T and Y- gradient of 0,15 mT/cm frequency resolution is 190 Hz

Characteristics and advantagesCharacteristics and advantages

imageimage – distribution of hydrogen nuclei density contrastcontrast – enhanced by differences in T1 or in T2 resolutionresolution – determined by magnetic field gradient

bones are “transparent” – the structures inside are easily

seen dynamics of processes can be investigated fMRI – follow the activation of certain centers in the brain

during different activities

Risk factorsRisk factors

alternating magnetic fieldsalternating magnetic fields induce electric currents of ions in tissue – to weak to course the damage or local heating

static magnetic fieldstatic magnetic field has so far coursed no damage

method is noninvasive method must not be applied on patients with

metal implanters (pacemaker, artificial limb)

Spin-Echo S = k (1-exp(-TR/T1)) exp(-TE/T2)

Inversion Recovery (180-90) S = k (1-2exp(-TI/T1)+exp(-TR/T1))

Inversion Recovery (180-90-180) S = k (1-2exp(-TI/T1)+exp(-TR/T1)) exp(-TE/T2)

Gradient Recalled Echo S = k (1-exp(-TR/T1)) Sin exp(-TE/T2*) / (1 -Cos exp(-TR/T1))

Spin eho imagingSpin eho imaging

Inversion recoveryInversion recovery

Gradient Recalled Echo ImagingGradient Recalled Echo Imaging

Contrast agents

Paramagnetic ions that can not diffuse through membrane

a) increase the local magnetic field

b) are inert to the biological tissues