بسم الله الرحمن الرحيم Dr. Maged Ali Hegazy Assistant Professor Alazhar University

MRI PHYSICS

STEPS OF EXAMINATIONSTEPS OF EXAMINATION Patient is placed in a magnet. A radio wave is sent in. The radio wave is turned off. The patient emits a signal. The signal is received. Image is reconstructed from signal.

 BASIC PRINCIBLES  Atoms consist of nucleus and a shell of electrons.  Nucleus contains positively charged protons.  protons are spinning around an axis, have a spin.  Moving electrical charge is an electrical current.  Electrical current induces a magnetic field.  Thus, a spinning proton is simulating a small magnet.  Proton, as a small magnet, has its magnetic field and magnetic moment.

Spinning proton A SPINNING PROTON, HAS A CHARACERISTIC MAGNETIC MOMENT AND MAGNETIC FIELD AS A MAGNET BAR.

EFFECT OF AN EXTERNAL MAGNETIC FIELD (PUT PATIENT INTO MAGNET) Protons are aligned in EMF in one of two ways, either parallel or anti-parallel to EMF. Parallel direction requires less energy. More protons are in the lower energy state (parallel to EMF). The difference between parallel and anti-parallel states is quite small and depends on strength of EMF. This difference well result in a net magnetization vector in parallel direction, longitudinal magnetization.

PROTONS ARE ALIGNED IN EMF IN EITHER PARALLEL OR ANTI PARALLEL DIRECTIONS.

WHAT PROTONS DO ? Protons move in a certain way called precession. Precession means the axis of proton (that spin) rotates in a circle forming a cone shape. Precession frequency (how many times the protons precess per second) depends upon strength of external magnetic field.   Larmor equation   precession frequency   gyro-magnetic ratio  external magnetic field (in Tesla)

Spinning protons are precessing in EMF.

INTRODUCING COORDINATE SYSTEM Z- axis runs in the direction of external magnetic field lines. Protons are represented as a vector. Magnetic forces in the opposing directions cancel each other out in both longitudinal and transverse directions. The net result is magnetization along the direction of external magnetic field (longitudinal magnetization).

Introducing coordinate system.

AS THE NET MAGNETIZATION IS IN THE DIRECTION OF E.M.F. IT COULD NOT BE MEASURED DEVELOPMENT OF NET MAGNETIZATION IN LONGITUDINAL DIRECTION, BY SLIGHT PREFERENCE OF PROTONS TO ALIGN PARALLELTO MAGNETIC FIELD

SEND A RADIO WAVE We send a short burst of electromagnetic wave called a radio frequency pulse ( RF ). RF pulse with same frequency with protons will disturb protons by giving them energy. THIS PHENOMENON IS CALLED THIS PHENOMENON IS CALLEDRESONANCE

WHAT IS THE EFFECT OF R.F.PULSE ? Protons acquire energy and thus more protons are aligned anti parallel, and cancel more parallel ones resulting in decrease longitudinal magnetization. Protons are precessing in phase, they point in the same direction at the same time in horizontal plane, and stop canceling their magnetization along that plane, transversal magnetization will develop.

RF pulse FADING OUT OF LM AND DEVELOPMENT OF TM A NET LONGITUDINAL MAGNETIZATION Protons in phase N M V

TRANSVERSAL MAGNETIZATION T.M. moves in phase with precessing protons, then a magnetic vector by constantly moving, induces an electric current. This electric current is our MR signal, which could be received by antenna, coil. The resulting MR signal has precession frequency.

RESULTS OF RESONANCE 1. The net magnetization vector (NMV) moves out of alignment away from  2. The angle to which NMV moves out of alignment is called flip angle. 3. The magnitude of flip angle depends upon the amplitude and duration of RF pulse.

SWITCHING OFF RF PULSE The whole system goes back to its original state (it relaxes ). Longitudinal magnetization grows back to its original level ( longitudinal relaxation ). Transversal magnetization starts to disappear ( transversal relaxation ). Both processes are simultaneous but independent to each other.

LONGITUDINAL MAGNETIZATION, AGAIN! ( LONGITUDINAL RELAXATION ) 1. The protons go back to its original low energy state ( parallel to EMF ). 2. They no longer cancel out magnetization of other parallel protons adding to long. magn. 3. This is a continuous process, as if one proton after the other goes back to its parallel state. 4. The energy, they have accept, is handed over to their surroundings, ( the lattice ). 5. Thus, longitudinal relaxation is also called spin-lattice-relaxation.

IF WE PLOT THE LONGITUDINAL MAGNETIZATION VERSUS TIME AFTER RF PULSE WAS SWITCHED OFF WE GET WHAT SO CALLED T 1 curve TIME LONG.MAGN. LONGITUDINAL RELAXATION TIME : T 1

TRANSVERASL MAGNETIZATION WHY IT DISAPPEARS ! 1. After RF pulse is switched off, the protons get out of phase. 2. They cancel their magnetization in transversal plane, transversal magnetization fades out. 3. This due to differences in precession frequency of protons. 4. Differences in precession frequency is due to external and local magnetic inhomogenity. 5. As local magnetic field inhomogenity is influenced by surrounding protons, the process is called spin-spin-relaxation.

IF WE PLOT THE TRANSVERSAL MAGNETIZATION VERSUS TIME AFTER RF PULSE WAS SWITCHED OFF WE GET WHAT SO CALLED T 2 curve TIME TRANS.MAGN. TRANSVERSAL RELAXATION TIME : T 2 Protons are progressively dephasing

SUMMARY: The NMV is created by two components at 90 degree to each other, LM & TM.The NMV is created by two components at 90 degree to each other, LM & TM. Before resonance, RF pulse, there is full LM and no TM.Before resonance, RF pulse, there is full LM and no TM. After RF pulse, NMV is flipped fully into transverse plane, provided sufficient energy is applied.After RF pulse, NMV is flipped fully into transverse plane, provided sufficient energy is applied. There is now full TM and zero LMThere is now full TM and zero LM The created TM is our MR signal.The created TM is our MR signal. Once RF pulse is switched off, LM grows again and TM decreases.Once RF pulse is switched off, LM grows again and TM decreases. As a result, the signal decays as relaxation takes place.As a result, the signal decays as relaxation takes place.

WHAT AFFECTS T 1 TIME OF LONGITUDINAL RELAXATION T 1 depends on tissue composition, structure and surroundings, lattice. T 1 depends on tissue composition, structure and surroundings, lattice. Energy handling from precessing protons to surrounding lattice occurs effectively when magnetic field of lattice occurs with frequency near that of protons. Energy handling from precessing protons to surrounding lattice occurs effectively when magnetic field of lattice occurs with frequency near that of protons. Liquid and water molecules move too rapidly, and protons can not hand their energy rapidly. Liquid and water molecules move too rapidly, and protons can not hand their energy rapidly. This means liquid and water have a long T 1 s. This means liquid and water have a long T 1 s.

WHAT AFFECTS T 2 TIME OF TRANSVERSAL RELAXATION Inhomogeneities of the EMF and local magnetic field. Inhomogeneities of the EMF and local magnetic field. As water molecules moves very fast, there are no big difference in local MF from place to place. As water molecules moves very fast, there are no big difference in local MF from place to place. So, proton stay in phase for a long time. So, proton stay in phase for a long time. So, water has a long T 2. So, water has a long T 2. Impure liquids have a big variations in local MF. Impure liquids have a big variations in local MF. Protons get out of phase rapidly Protons get out of phase rapidly So, they have a short T 2. So, they have a short T 2.

AN EXPERIMENT 1. We have tissues A & B. tissue A has a shorter longitudinal and transversal relaxation times. 2. We send a 90 RF pulse, and wait a certain time (TR long ), then we send a second 90 RF pulse. WHAT WILL HAPPEN? 3. After TR long tissue A and B have regained their longitudinal magnetization (frame 5). 4. The transversal magnetization, (our MR signal), after the second RF pulse will be the same for both tissues, (no tissue contrast).

TR long RF pulse TM is the same for both tissues after second RF pulse when TR is long

ANOTHER EXPERIMENT 1. What if we don't wait so long from RF pulse to another. 2. We send a 90 RF pulse, and wait a certain time (TR short ), then we send a second 90 RF pulse. WHAT WILL HAPPEN? 3. Tissue A have regained more of its longitudinal magnetization than tissue B. 4. When LM is tilted by 90 pulse, the transversal magnetic vector ( MR signal) of tissue A is larger than that of tissue B.[A brighter than B].

TR short RF pulse TM vector is larger in A than B after short TR

THE CONCLUSION The difference in signal intensity in this experiment depends on the difference in LR time. This means the difference in T 1 between tissues. When we use more than one RF pulse, we call it a pulse sequence, with TR, time to repeat 90 pulse. With a long TR, tissues will not further differentiate based on their LR time (T 1 ). With a short TR, there is difference in signal intensity based on T 1. The resulting picture is called T 1 – weighted image (Partial saturation recovery).

Brain has a shorter LR time (T1) than CSF. With a short TR, the signal intensities of brain and CSF differ more than after a long TR. TIME Signal intensity brain CSF TR short TR long

HOW TO OBTAIN A PROTON DENSITY WEIGHTED IMAGE ? WE HAVE ALREADY JUST ANSWERED THIS QUESTION. When we use a very long TR, the resulting signal (tissue contrast) will no further depends upon T 1. However, the resulting signals are not identical. What makes the difference in signal is difference in proton density of the tissue in question. So, the resulting image is called proton density or spin density weighted image (Saturation recovery).

HOW TO OBTAIN A T 2 WEIGHTED IMAGE ? LET US PERFORM ANOTHER EXPERIMENT  We send 90 pulse. What will happen.  After switching off RF pulse TM starts to disappear, why, because the protons became out of phase.  WE DO SOMETHING NEW  After a certain time called (TE/2), we send in a 180 pulse.  180 pulse makes the protons precess in the opposite direction, thus the faster protons became behind the slower ones.  We wait for another (TE/2)time, at that time the protons come in phase again, regain TM (MR signal).  A little latter the protons will dephase again, loosing signal.

a bcdef 180 pulse Time TE/2 Switched off 90 pulse After 90 EF pulse switched off the protons dephase (a-c). The 180 pulse causes them to precess in the opposite direction, so they rephase again (d-f) after time TE. Time TE

 The 180 pulse acts like a mountain reflecting sound waves as echoes.  The resulting signal is called an echo or spin echo.  When the protons start to dephase again, we can perform the experiment with another 180 pulse and another.  But, from echo to echo, after 180 pulse, the intensity of the signal goes down due to the so called T 2 effect.  If we don't use 180 pulse, the protons will get out of phase faster, in time T 2 *, as transversal relaxation time is shorter.  Corresponding effects named T 2 * effects, which are valuable in the so called fast imaging sequences.

T 2 * FID T 2 decay FID refocused to give Spin-Echo Time 0 TE/2 TE RF 90° 180°

WHAT IS TE TE = time to echo TE is time between 90 pulse and the spin echo. TE is time between 90 pulse and the spin echo. The shorter the TE the stronger the signal, with long TE the signal decreases. The shorter the TE the stronger the signal, with long TE the signal decreases. But with a very short TE, the difference in signal between tissues (A&B) in our example, is very small, less contrast. But with a very short TE, the difference in signal between tissues (A&B) in our example, is very small, less contrast. With longer TE the difference in signal intensity (contrast) is more pronounced. With longer TE the difference in signal intensity (contrast) is more pronounced. So, it is reasonable to wait a long TE to get a T 2 weighted image. So, it is reasonable to wait a long TE to get a T 2 weighted image. But, if we wait longer the signal becomes smaller with small signal to noise ratio. But, if we wait longer the signal becomes smaller with small signal to noise ratio.

Spin-Echo 1. Spins dephase: fast and slow 2. Apply 180° at t = TE/2 3. Echo at t = TE

Signal Time A B TE short TE long T 2 –curve for tissue A with shorter T 2 than tissue B, thus loses transversal magnetization faster. With a short TE the difference in signal intensity is less pronounced than after a long TE.

REVIEW 1. The spin echo sequence consists of a 90 and a 180 pulse. 2. After 90 pulse protons are dephasing due to external and internal magnetic field inhomogeneties. 3. The 180 pulse rephases the dephasing protons (spins) and a strong signal, the spin echo, results. 4. The 180 pulse neutralize the EMF inhomogeneties. 5. When using multiple 180 pulses, signal decreases from one echo to the next due to internal T 2 effect. 6. By choosing different TEs, signals can be T 2 weighted in varying degrees. 7. With short TE, T 2 effect has no time to affect signal. 8. With long TE, T 2 will affect signal of different tissues. 9. With a very long TE, signal would be small as transversal magnetization decreases markedly.

PDT2 T1 NOT APPLICABLE TR TR TE TE

HOW DOES FLOW INFLUENCE THE SIGNAL Flow void phenomenon: We have a body section through which a vessel is crossing. We send 90 RF pulse, so all the protons in the cross section are influenced. We then record the signal from the section. At this time, all the blood in the vessel have left the examined slice. No signals come from the vessel which appears dark.

Flow related enhancement: Situation before 90 RF pulse Immediately after the pulse Before second 90 RF pulse

MR CONTRAST MEDIA Called paramagnetic substances. Cause shortening of the relaxation times of the surrounding protons. This effect is called proton relaxation enhancement. Examples, degradation products of hemoglobin, molecular oxygen and Gadolinium

TIME Signal intensity A TR B The T1 curves for tissue A and B are very close to each other, with small difference in signal intensity between them at specific TR.

TIME Signal intensity A TR B The T1 curves for tissue A is shifted to the left as contrast agent entered it. At the same time TR there is much greater difference in signal intensity (tissue contrast).