Mri basics

Editor's Notes

• #3: Magnetic resonance imaging (MRI) is an imaging technique used primarily in medical settings to produce high quality images of the inside of the human body. MRI is based on the principles of nuclear magnetic resonance (NMR), a spectroscopic technique used by scientists to obtain microscopic chemical and physical information about molecules. The technique was called magnetic resonance imaging rather than nuclear magnetic resonance imaging (NMRI) because of the negative connotations associated with the word nuclear in the late 1970's. MRI started out as a tomographic imaging technique, that is it produced an image of the NMR signal in a thin slice through the human body. MRI has advanced beyond a tomographic imaging technique to a volume imaging technique. This package presents a comprehensive picture of the basic principles of MRI. Before beginning a study of the science of MRI, it will be helpful to reflect on the brief history of MRI. Felix Bloch and Edward Purcell, both of whom were awarded the Nobel Prize in 1952, discovered the magnetic resonance phenomenon independently in 1946. In the period between 1950 and 1970, NMR was developed and used for chemical and physical molecular analysis. In 1971 Raymond Damadian showed that the nuclear magnetic relaxation times of tissues and tumors differed, thus motivating scientists to consider magnetic resonance for the detection of disease. In 1973 the x-ray-based computerized tomography (CT) was introduced by Hounsfield. This date is important to the MRI timeline because it showed hospitals were willing to spend large amounts of money for medical imaging hardware. Magnetic resonance imaging was first demonstrated on small test tube samples that same year by Paul Lauterbur. He used a back projection technique similar to that used in CT. In 1975 Richard Ernst proposed magnetic resonance imaging using phase and frequency encoding, and the Fourier Transform. This technique is the basis of current MRI techniques. A few years later, in 1977, Raymond Damadian demonstrated MRI of the whole body. In this same year, Peter Mansfield developed the echo-planar imaging (EPI) technique. This technique will be developed in later years to produce images at video rates (30 ms / image). Edelstein and coworkers demonstrated imaging of the body using Ernst's technique in 1980. A single image could be acquired in approximately five minutes by this technique. By 1986, the imaging time was reduced to about five seconds, without sacrificing too much image quality. The same year people were developing the NMR microscope, which allowed approximately 10 mm resolution on approximately one cm samples. In 1987 echo-planar imaging was used to perform real-time movie imaging of a single cardiac cycle. In this same year Charles Dumoulin was perfecting magnetic resonance angiography (MRA), which allowed imaging of flowing blood without the use of contrast agents. In 1991, Richard Ernst was rewarded for his achievements in pulsed Fourier Transform NMR and MRI with the Nobel Prize in Chemistry. In 1993 functional MRI (fMRI) was developed. This technique allows the mapping of the function of the various regions of the human brain. Six years earlier many clinicians thought echo-planar imaging's primary applications was to be in real-time cardiac imaging. The development of fMRI opened up a new application for EPI in mapping the regions of the brain responsible for thought and motor control. In 1994, researchers at the State University of New York at Stony Brook and Princeton University demonstrated the imaging of hyperpolarized 129Xe gas for respiration studies. MRI is clearly a young, but growing science.
• #15: A radiofrequency pulse that has the same frequency as the precessing protons, can cause resonance, transfer energy to protons. This result in more protons being anti-parallel and thus neutralizing more protons in the opposite direction. Consequence the longitudinal magnetization decreases.
• #16: The RF pulse also causes the protons to precess in synch, in phase this result in new magnetic vector the transverse magnetization.
• #20: When the RF pulse is switched off 1. Longitudinal magnetization increases again; this longitudinal relaxation is described by a time constant T1, the longitudinal relaxation time
• #21: The time that it takes for the longitudinal magnetization to recover, to go back to its original value , is described by the longitudinal relaxation time also called T1 . This actually is not the exact time it takes but a time constant describing how fast this process goes. This is like taking time for one round at a car race. The time gives you an idea of how long the race may take, but not the exact time. Or T1 is a time constant.
• #22: Transverse magnetization decreases and disappears . This transversal relaxation is described by a time constants T2 the transversal relaxation time.
• #23: Transverse relaxation time is also a time constant describing disappearance of transverse magnetization.
• #25: Previously it was believed that measuring the relaxation times would give tissue characteristic result, and thus enable exact tissue typing. But there is quite some overlap of time range, and also T1 is dependent on the magnetic field strength used for examination. What is
• #28: Actually, T1 depends on tissue composition, structure and surroundings. T1 relaxation is due to exchange of thermal energy to the surrounding, the lattice. The precessing protons have a magnetic field, that constantly changes directions, that constantly fluctuate with the Larmor frequency. The lattice also has its own magnetic fields. The protons now want to hand over energy over the lattice to relax. This can be done very effectively when the fluctuation of the magnetic fields in the lattice occur with a frequency. When the lattice consists of pure liquid/water, it is difficult for the protons to get rid of their energy, as a small water molecules move too rapidly. And the protons cannot hand over their energy over to the lattice quickly, they will only slowly go back to their lower energy level, their longitudinal alignment. Thus it takes a long time for the longitudinal magnetization to show up again, and this means that liquid /water have long T1s. When the lattice consists of medium size molecules that move and have fluctuating magnetic fields near the Larmor frequency of the precessing protons, energy can be transferred much faster, thus T1 is short. The carbon bonds at the ends of the fatty acids have frequencies near the Larmor frequency, thus resulting in effective energy transfer. It is easy to imagine that in a stronger magnetic field it takes more energy for the proton to align against it. Thus these protons have more energy to hand down to the lattice, and this takes longer than handing down just a small amount of energy. Though it is logical it is wrong. The precessing frequency depends on magnetic field strength. If we have a stronger magnetic field then the protons precess faster. And when they precess faster , they have more problem handling down the energy to a lattice with more slowly fluctuating magnetic fields. T2 relaxation comes about when protons get out of phase, which has two causes; inhomogeneities of the external magnetic field, and the local magnetic fields within the tissues. As water molecules move around very fast, their local magnetic fields fluctuate fast, thus kind of average each other out, so there are no big net differences in internal magnetic fields from place to place. And if there are no big difference in magnetic field strength inside of a tissue the protons stay in step for a long time, and so T2 is longer. With impure liquids, e.g. those containing some larger molecules, there are bigger variation in the local magnetic fields. The larger molecules do not move around as fast so their local magnetic fields do not cancel each other out as much. These larger difference s in local magnetic fields consequently cause larger differences in precessing frequencies, thus protons get out of phase faster T2 is shorter.
• #30: We have 6protons pointing up; we send in a RF pulse, which lifts up 3 of them to higher energy level. The result we no longer nave a longitudinal, but a transversal magnetization When RF pulse is switched off protons go back to their lower state of energy and they lose phase coherence. Both process occur simultaneously and independently. First one proton goes back to the lower energy state resulting in 4 protons up and two pointing down. In net effect we now have a longitudinal magnetization of 2. Then the next proton goes back up, now 5 protons point up and one down resulting in a net longitudinal magnetization of 4. After the next proton goes up, longitudinal magnetization equals 6. At the same time, transversal magnetization decreases.
• #32: A changing magnetic force can induce an electrical current, which was the signal that we receive and use in MR. If we put up an antenna we get a signal as illustrated If you think of the antenna as a microphone, and the sum magnetic vector as having a bell at its tip. The further the vector goes away from the microphone m the less loud the sound. The frequency of the sound however, remains the same because the sum vector spins with the precessing frequency. This type of signal is called FID signal from free induction decay. 90degree pulse gives a very good strong signal. The magnetic vector directly determine the MRI signal and signal intensity by inducing electrical currents in the antenna. Instead of term longitudinal and transversal magnetization we can also use the term “signal or signal intensity “ at the axis of our T1 and T2 curves.
• #33: I figure a we have two tissue, A and B, which have different relaxation times (tissue A has a shorter transversal as well as longitudinal relaxation time). We send in a 90d RF pulse, and wait a certain time TR long. After the time TR long tissue A and tissue B have regained all of their longitudinal magnetization (frame 5), the transversal magnetization after the second pulse will be the same for both tissues, as tit was in frame 1. If we give RF pulse early, as in fig b, after frame 4 TR short. At this time tissue A has regained more of its longitudinal magnetization than tissue B. When the second 90d pulse now “tilts” the longitudinal magnetization 90d. The transversal magnetic vector of tissue A is larger than that of tissue B. And when this vector of A is larger , it will reach closer to our antenna; thus the imaginary bell at the tip of vector A will cause a louder, stronger signal in our “microphone”, the antenna, than vector B. The difference in signal intensity in this experiment depends on the difference in longitudinal magnetization, and this means on the difference in T1 between the tissue. Using these two pulses, we can now differentiate tissue A from tissue B, which might have been impossible choosing only one 90d pulse or two 90d pulses that are a long time apart. When you use more than one RF pulse –a succession of RF pulses – you use a so called pulse sequence . As you can use different pulses, e.g. 90d or 180d pulse, and the time intervals between successive pulses can be different, there can be many different pulse sequence. As we saw the choice of a pulse sequence will determine what kind of signal you get out of a tissue. So it is necessary to carefully chose and also describe the pulse sequence for a specific study. The pulse sequence that we used was made up of one type of pulse only , the 90d pulse . This was repeated after a certain time , which is called TR = time to repeat. With long TR we got similar signal from both tissues, both would appear the same on a MR picture. Using a shorter TR there was a difference in signal intensity between the tissues, determined by their difference in T1. The resulting picture is called T1 weighted picture. This means that the difference of signal intensity between tissues in that picture , the tissue contrast is mainly due to their difference in T1. A TR is less than 500mse is considered to be short, a TR greater than 1500 msec to be long.
• #34: Why are the signals after a very long time TR between pulses not identical? When we wait a long time , T1 does not influence the tissue contrast any more, however, there may still be a possible difference in the proton density of the tissues in question. And when we wait a very long time TR in our experiment from above figure, the difference in signal is mainly due to different proton densities, we have a so called proton density, weighed image.
• #35: After 90degree pulse, the longitudinal magnetization is tilted we get a transversal magnetization. After the pulse is switched off, longitudinal magnetization starts to reappear, the transversal magnetization however starts to disappear due to loss of proton phase coherence. Protons are in phase a, but increasingly spread out, as they have different precession frequencies b and c . The loss of phase coherence results in decreasing transversal magnetization and thus loss of signal. After a certain time (TE/2, half of TE) we send a 180degree pulse. The protons turn around and precess in exactly opposite direction. The result is that the faster precessing protons are now behind the slower ones. If we wait another time TE/2, the faster ones will have caught up with the slower ones. At that time the protons are nearly in phase again, which results in a stronger transversal magnetization, and thus in a stronger signal again. A little later however, the faster precessing protons will be ahead again, with the signal decreasing again.
• #36: A race between a rabbit and a turtle starting at the same line. After a certain time (TE/2), the rabbit is ahead of the turtle. When you make the competitors run in the opposite direction for the same length of time , they will be both back at the starting line exactly the same time. The 180degree pulse in our experiment act like a wall, from which the protons bounce back , like a mountain reflecting sound wave as echoes. This is why the resulting strong signal is also called a echo or spin echo.
• #37: After we have our signal , our spin echo, the protons lose phase coherence again, the faster ones getting ahead as we have heard. We naturally can perform the experiment again with another 180degree pulse , and another and another… If we plot time vs. signal intensity, we get a curve like above fig. From this curve we can see that the spin echo, the resulting signal, decreases with time. Responsible for this is the fact that out 180 0 pulse only “neutralizes” effects that influences the protons in a constant manner, and these are the constant inhomogeneities form local magnetic fields inside of the tissue cannot be “ evened out” , as they may influence some protons before the 180 0 pulse differently than after the 180 0 pulse. So some of the protons may still be behind or in front of the majority of the protons, that will reach the starting line at the same time. So from echo to echo the intensity of the signal goes down due to the so-called T 2 effects.
• #38: Imagine two buses full of people ,e.g. after a soccer game. They are standing at a starting line. With two microphone , you record the signal (e.g. the signal from the crowd) that is coming from each bus . Then the bus leaves in the same direction . Recording the signal, you may recognize, that one signal disappears faster , than the other. This has two different causes : may be one bus drives faster than the other (loss of signal would thus be due to external influences, the external magnetic field inhomogeneities.). Or the difference in signal intensities, the difference in signal, may be due to difference of inherent properties of the two groups (internal inhomogeneities); may be in one bus, there are only the “party animals”, who do not become tired fast, as the people in the other crowd.. To figure out what actually is the reason for the signal disappearing , you can make the buses turn around after a certain time TE/2, and have them drive back with the same speed also for the time TE/2. After time 2 x TE/2 = TE the buses will be back at the starting line. The signal intensity, that you record with your microphone then depends only on inherent properties, i.e. how tired the crowd have become. If you do not use a 180 0 pulse to neutralize constant external inhomogeneities , the protons will experience larger difference in magnetic field strength when the RF pulse is switched off. Due to this they will be out of phase faster, the transversal relaxation time will be shorter. To distinguish this shorter transversal relaxation time from the T 2 After the 180 0 pulse, it is called T* 2 (T2 star). The corresponding effects are named T* 2 -effects are important with the so called fast imaging sequences. The type of pulse sequence, that we used in our experiment, is called a spin echo sequence, consisting of 90 0 pulse and a 180 0 pulse (causing the echo). This pulse sequence is very important in MR imaging, as it is the workhorse among the pulse sequences, which can be used for many things. It is important to realize that with a spin echo sequence, we cannot only produce T2-, but also T1- and proton density- weighted pictures.
• #39: First we sent in a 90 0 pulse we have a maximum transversal magnetization. However , this transversal magnetization disappears, due to T 2 -effect. How fast transversal magnetization disappears can be seen from a T2-curve. Tissue A having a short T2(Brain), tissue B having a long T2 (Water). Both curve start at 0, which is the time immediately after the 90 0 pulse is switched off. When we wait for a certain time TE/2 to send the 180 0 pulse, transversal magnetization will have become smaller. After waiting another time TE/2 (that is TE after the 90 0 pulse is switched off ), we will receive a signal , the spin echo . The intensity of this echo is given by the T2 curve at the time TE. This time TE between the 90 0 pulse and spin echo is called TE = time to echo. The time TE can be chosen by the operator. And as we can see from the T 2 curve , TE influences the resulting signal and thus also the image. The shorter the TE , the stronger the signal that we get from a tissue. To get the best, strong signal, it may seem reasonable to use a short TE, because with longer TEs signal intensity decreases. With a short TE, however, there will be a problem. If we wait a very short TE, the difference in signal intensity between tissue A and tissue B is very small, both tissue may hardly be distinguished as there is hardly any contrast (which is the difference in signal intensity of tissue). Consequence; with a short TE, difference in T2 do not influence tissue contrast very much. As both T2 curves diverge, with a longer TE the difference in T2- curves and thus the difference in signal intensity = contrast, is more pronounced in our example. So it might be reasonable to wait a very long TE; the resulting picture should be very heavily T2-weighed. But if wait longer, the total signal intensity becomes smaller and smaller. The signal to noise ratio becomes smaller, the picture appears grainy
• #40: A short TR is one that is about as short as the smallest/shortest T1 that we are interested in. a long TR is 3 times as long as a short TR. A TR of less then 500msec is considered to be short, a TR of more than 1500msec to be long. A short TE is one that is as short as possible, a long TE is about 3 times as long. A TE of less than 30 mesc is considered to be short, a TE greater than 80 msec to be long.