seminar October, 2008 j. brnjas-kraljević

1. magnetic resonance imaging seminar October, 2008 j. brnjas-kraljević

2. Imaging (MRI) • tomography technique – the volume image is built up by images of thin slices from which data are taken • two-dimensional distribution of certain physical parameter is image of one tom • measurement of space distribution of same resonating nuclei is enabled by introduction of controlled inhomogeneity of B0 field - gradient of the field in desired direction • we measure resonance/relaxation of hydrogennuclei in water and in fat

3. in perfectly homogeneous field all protons have the same  -- only one signal is measured gradient in direction X-axis distinguishes the Larmor frequency of nuclei depending on the place in the field  =  (B0 +x Gx) Signal is measured in the presence of field gradient. The result is distribution of nuclei in desireddirection. Gradients in different direction built up space distribution of nuclei. Mathematicalalgorithm transcribes values of measured voxels signals into gray scale.

4. Imageconstruction • 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

5. How 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 q and Gx = Gf cos q • the recorded FID is treated by FT - gives the signal distribution by frequencies and phases Gy Gx

6. Imaging FT • 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 recorded tom phase differentiation frequency diff. signal

7. Successive recording of slices in big volume • frequencycontent 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

8. Determination of single volume parameters gradient u Y ax gradient u X ax gradient in Z ax chosen Larmor frequency excites only one tom changes wL 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 signal detection

9. Parameters of a single volume frequency phase • 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 phasefor each single voxel in XY plane • another FT procedure is used to calculate intensities from each voxel and to display it in gray scale

10. Detection artifacts • - 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 help: • 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

11. Contrast by saturation • IRmethod • - time TI is T1ln 2 for T1 hydrogen in fat or water • detected are only nuclei in another tissue SE 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

12. MRI 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

13. Parts 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 vacuum liquid helium liquid nitrogen housing superconducted coils

14. Three main gradients

15. Meaning of magnetic field gradient • gradient 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-axis - on during the time of detection of relaxation signal; therefore relaxation frequency is function of x coordinate • gradient 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

16. Characteristics and advantages • image – distribution of hydrogen nuclei density • contrast – enhanced by differences in T1 or in T2 • resolution – 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

17. Risk factors • alternating magnetic fields induce electric currents of ions in tissue – to weak to course the damage or local heating • static magnetic field has so far coursed no damage • method is noninvasive • method must not be applied on patients with metal implanters (pacemaker, artificial limb)

18. Spin-EchoS = kr (1-exp(-TR/T1)) exp(-TE/T2) Inversion Recovery (180-90) S = kr (1-2exp(-TI/T1)+exp(-TR/T1)) Inversion Recovery (180-90-180) S = kr (1-2exp(-TI/T1)+exp(-TR/T1)) exp(-TE/T2) Gradient Recalled EchoS = kr (1-exp(-TR/T1)) Sinq exp(-TE/T2*) / (1 -Cosq exp(-TR/T1))

20. Spin eho imaging

21. Inversion recovery

22. Gradient Recalled Echo Imaging

23. Contrast agents • Paramagnetic ions that can not diffuse through membrane • a) increase the local magnetic field • b) are inert to the biological tissues