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Magnetic Resonance Imaging:

An overview of the fundamental principles of MRI and the technology involved in the imaging process.

Kevin Stevens, ARRT RT,R,MR

Magnetic Resonance Imaging (or MRI) is an advanced medical imaging modality that is used to evaluate the internal structures of a subject. These high resolution images contain information on the characteristic differences of each type of tissue in the subject, making it an ideal modality for soft tissue, and specifically neurologic imaging. The MRI scanner contains a complex network of systems that must all work in harmony in order to produce an image. These systems include multiple magnetic fields, radio frequency transmitters and receivers, computer systems, and the inherent properties of the tissue being imaged. Also described as the MAGNETIC properties, the creation of RESONANCE, and the resulting IMAGING. Get it?

The Magnets:

There are three separate magnetic fields present during the scanning process that are manipulated in order to obtain optimum imaging information. These are known as the Static magnetic field, the Gradient magnetic field, and what is known as inherent Nuclear Magnetism.

The static magnetic field or main magnetic field is created, in this case, by a superconductive magnet. This superconductive magnet consists of a large coil of copper wire housed inside of a chamber that is filled with approximately 1000 liters of liquid Helium. When a voltage is applied to the copper wire it induces a magnetic field around it creating an electromagnet. The liquid Helium exists at a temperature of four degrees Kelvin (-269.1C)(-452F) which completely eliminates resistance in the electromagnet. Without resistance the ends of the coil can be connected and the electricity will continue to circulate through the coil without requiring a voltage to be continuously applied. This stable electromagnet will maintain its field indefinitely, as long as it remains supercooled. Magnetization is measured in Tesla (T) with conventional MRI scanners ranging from 0.2T to 7T, and research scanners going beyond 10T. To put the strength of these magnets into perspective, 1T = 10,000 Gauss, and the magnetic field strength of the entire earth is equal to 0.5 Gauss. 

Three gradient magnetic fields are also present within the scanner. These are also electromagnetic coils that are turned on and off during the scanning process. The gradient coils vary the electro-magnetic field along three axis within the MRI scanner, known as the X, Y, and Z axis. The Z gradient coil varies the intensity of the magnetic field in the “head-to-foot” direction, the X coil in the “right-to-left” direction, and the Y coil in the “anterior-to-posterior” direction. The speed, acceleration, amplitude and polarity of these three gradient coils are all adjusted to select the imaging plane, slice characteristics, and to spatially encode the MR signal that is produced.

Nuclear Magnetism is the most important fundamental building block of MRI. Without it the technology is useless, and it all exists within the patient. Nuclear magnetism refers to the magnetic characteristics of certain nuclei. There are multiple magnetically active nuclei but Hydrogen is the most prevalent in living tissue, and therefore is primarily used in MRI. Within the Hydrogen atom there is a single proton that has mass, is positively charged, and spins on its axis. The result of this spinning, positively charged particle, is the creation of a magnetic field around it. In other words, if your body is 70% water, and every water molecule has a hydrogen atom in it, and every hydrogen atom is a tiny magnet, then the collective magnetism of your whole body is rather substantial. The reason you don’t stick to the fridge though is because those magnetic fields aren’t all facing the same direction. 

 

Precession:

One way to get all of these little magnets to work together is to place them in a stronger magnetic field. Then, like a compass needle, most of these little magnets will all point “North” or in this case they will align themselves parallel to the main magnetic field. The combined magnetization of these easily manipulated Hydrogen protons is known as the Net Magnetization Vector (NMV). Now remember, these little magnets only exist because they are spinning or “precessing” around an axis, just like a spinning top. The precession of these hydrogen protons also synchronize on the axis of the static magnetic field described as longitudinal magnetization. This collective precession exerts a force that is perpendicular to the direction of its spin. This force is known as spin angular momentum and it’s the reason you are thrown away from a spinning object. These forces interact with or “bump into” each other and cause the proton to wobble on its axis. Imagine two spinning tops bumping into each other. The ratio of the spin angular momentum of a proton with its magnetic moment is referred to as the gyromagnetic ratio. The known gyromagnetic ratio for hydrogen protons in water or fat is 42.56 Megahertz per Tesla (MHz/T). Knowing this, we can determine the precise frequency at which hydrogen protons precess. This very important frequency is known as the Resonant Frequency or Larmor Frequency.

Resonance:

“In physics, resonance describes the phenomenon of amplification that occurs when the frequency of a periodically applied force is in harmonic proportion to a natural frequency of the system on which it acts.” That’s the textbook definition but think of it this way, if you bang a drum it makes a sound. The force of you hitting the drum is translated by the drumhead as a vibration. This vibration transfers to the rest of the drum converting the energy into a sound wave that you can hear. That vibrating energy conversion is resonance. Magnetic resonance signal is produced by non-ionizing radio frequencies being transmitted via an antenna to the excited hydrogen protons. Then manipulating them in such a way that an echoing signal can be induced in a receiving antenna. In our previous analogy the radio frequency transmitter is the drumstick, hydrogen proton is the drum, the echo is the sound produced, and the receiving antenna is your ear. In this case the receiving antenna is a “coil” placed against the patient in the area of interest. This coil can also be the transmitting antenna though there are other transmitting antennas in the scanner housing. 

As an RF field at the resonant frequency is applied to magnetized tissue, the protons begin to precess in-phase with one another causing the NMV to precess. These protons begin to absorb energy from the RF pulse and move the NMV into a direction that is anti-parallel to the static field. When enough energy is applied to tip the NMV 90 degrees away (perpendicular) from the Z axis it passes through the XY plane and is termed to have a 90 degree flip angle applied to it. Essentially, the longitudinal magnetization has been converted into transverse magnetization. The flip angle can be changed to any predetermined amount by changing the amount of RF energy applied. Once the desired flip angle is reached the RF pulse is stopped so that an echoing signal can be detected from the tissue. As the NMV is precessing through the plane of the receive coil, an MR signal will be induced in that coil in accordance with Faraday’s Law of Induction. As the RF pulse is removed the NMV loses its phase coherence, and transverse magnetization, and returns to its previous state. More simply, when you stop the RF pulse from pushing the proton over the strength of the strength of the magnetic field causes the proton to line back up with the magnetic field.

Computer Systems:

Every part of the MRI system is controlled by intricately designed computer systems and software. The software signals the gradient magnetic systems to function and tells the radio frequency systems when and how to transmit a signal. The computer also needs to be able to take the information that is received from the hydrogen protons and somehow turn that information into an image that represents all of the tissue structures in a patient. This bit can get even more boring than the rest so I’ll try to keep it simple. The analog radio signal that is received goes through an analog to digital converter turning the waves into 1s and 0s that the computer can understand. The information is quickly stored in a digital “K” space which isn’t an image yet but all of the information needed to make that image. Like all of the atoms needed to make a jigsaw puzzle but not even formed into jigsaw pieces yet. Next that information gets processed by a computer function called Fourier Transform which turns that digital data into image information, now the atoms have become jigsaw pieces. That information is then processed by an image reconstruction software that puts all of the jigsaw pieces together resulting in the picture on the box. 

Encoding and Slice Orientation:

So how does the computer know what information belongs to what piece. This is what the gradient systems are for. Not only is the radio frequency giving the protons energy in order to get an echo but the specific frequencies from each gradient field tell us where that proton exits in the patient, what kind of tissue it is contained in, and where it belongs in the final image. This is known as frequency encoding and phase encoding. The number of separate spaces in the encoding matrix (phase x frequency) relates to the number of designations in the final image matrix. Therefore the higher the number of phase and frequency encodings the higher resulting image resolution. Making phase and frequency the primary elements of two dimensional pixel size. When you add a third dimension to this pixel it becomes a voxel and this depth is related to the thickness of an image “slice”. Each of the gradients (X,Y, and Z) are responsible for one of these encodings in each sequence and those assignments are determined by the slice orientation.  A slice of bread is a perfect way to think of this. It is two dimensional if you just look at the surface of the cross section but it also has thickness which makes it a three dimensional object. The same thing happens with an MR image. If you took an image slice perpendicular to the long axis of the patient you have what is called an “axial” slice. In an axial slice the slice encoding and thickness encoding is managed by the Z gradient because the Z or Head to Foot axis is the long axis through the scanner. That leaves the X and Y gradients to control the phase and frequency encoding of the slice. Slices along the X or Left to Right axis are called “sagittal” and slices along the Y or Anterior to Posterior axis are “coronal”. Each time the other two remaining gradients will manage the phase and frequency encodings. This entire process happens very fast and is repeated by the computer for every slice in every sequence contained in the study protocol. 

Imaging and Pulse Sequences:

Based on the area of interest and the symptoms of the patient, an MRI Technologist is able to plan a collection of scans that would best represent the information desired from the exam. These individual scans are referred to as “sequences” and the collection of sequences is known as the “protocol”. There are many different types of sequences at the disposal of the MRI Technologist and not all of them are necessary for every exam. There are a multitude of parameters that are manipulated to determine the type of sequence being performed. Flip angle, determines the amount of energy needed to flip the NMV through the XY plane to reach that angle. TE, or Time of Echo, is the delay between the transmission of the RF pulse and when the signal is read from the patient. Time of Repetition or TR determines the amount of time between the initial RF energy pulse and the point where that pulse is repeated.

There are three major properties of tissue that are exploited in the image formation process. They are Proton Density, T2 Relaxation and T1 Relaxation. Proton Density is quite self explanatory, it simply refers to the number of hydrogen protons present in a selected volume of tissue. Proton density is usually measured as a percentage where air is approximately O% and CSF is 100%. T2 Relaxation is the measurement of time it takes a tissue to lose 63% of its transverse magnetization. As T2 relaxation occurs by the dephasing protons, the net magnetization begins to recover along the Z axis and is referred to as T1 Relaxation. T1 Relaxation is defined by the time, in milliseconds, for this recovery to reach 63% of its initial longitudinal magnetization. 

By exploiting these characteristics the technologist can define sequences that are more or less, T1, T2, and Proton Density Weighted. There are many other parameters that can be manipulated to “fine tune” a sequence based on different variables in the anatomy, physiology, morphology, or pathology of an area of interest, but this short list is the basis of what you need to perform most routine ​MRI scans.

Spin Echo Sequences:

A spin echo pulse sequence is one in which only RF pulses are used to excite, refocus, and manipulate the hydrogen protons.

A T1 weighted sequence will have a Short TR and TE. The short TE will make the resulting image less T2 weighted by not allowing enough time to lapse for significant T2 dephasing to occur. The Short TR will increase the T1 contrast of the image where tissues with a short TR will be able to recover and tissue with a long TR will not have enough time and will therefore be more muted on the image. An image that is T1 weighted will have hyperintensities from fat and hypo-intensities from fluid. A Gadolinium contrast agent, when injected, actually changes the T1 time of the tissue it interacts with causing it to appear bright in contrast to its normal T1 weighted state.

​When a sequence is designed to result in a T2 weighted image, the sequence will have a Long TR and a Long TE. The long TR makes it so the T1 decay has finished. This leaves only the signal from the large amount of dephasing that is provided by the long TE. T2 sequences are generally the most important for clinical diagnosis. These images show fat and fluid as hyperintense which is helpful determining pathologies. Most pathological processes include either intra or intercellular fluid. Gadolinium contrast agents will also reduce the T2 times of affected tissue causing them to appear darker on T2 weighted images. This is post contrast imaging always carries a T1 weighting. 

If an image with a Proton Density weighting is desired then the sequence is constructed with a Long TR and Short TE so that none of the T1 or T2 information is present. At this point the only information that remains is the number of mobile water protons. Proton density is widely used in orthopedic imaging to delineate between the complex tendon structures, cartilage, muscle, bone, and fluid that can be present within an area of interest. 

Inversion Recovery:

With the addition of another 180 degree pulse prior to the standard 90 degree sequence pulse, the resultant sequence is known as an inversion recovery. By starting with a 180 degree pulse the 90 degree pulse then is applied when the recovery has reached zero thereby nulling a desired tissue. The TI or time of inversion is selected based on what tissue is desired to be nulled. 

A STIR or Short Tau Inversion Recovery sequence is a T2 weighted sequence in which the signal from fat is nulled. This way normal tissue appears hypointense compared to diseases that have a high fluid content. 

 

In neurological imaging, a FLAIR or Fluid Attenuated Inversion Recovery sequence is employed to null the signal from CSF that can often mask the appearance of plaques or other pathologies. The FLAIR sequence also gives excellent contrast between gray matter and white matter structures. 

Gradient Sequences:

Gradient Echo pulse sequences are also utilized in MRI, where an initial RF pulse is put in place but then reversing gradient magnetic fields are used to form the echo. Without the 180 degree refocusing pulse, the transverse magnetization decay is formed not only by T2 dephasing but field inhomogeneities, chemical shifts within the patient, and the effects of the main magnetic field. These sequences are often referred to as T2*. Gradient imaging is used for a wide array of purposes including evaluating susceptibility artifacts formed by pooling blood in the brain from strokes. The iron in the blood appears as a metallic artifact on the image revealing the tissue breakdown. Gradient imaging is also used to create extremely fast T1 and T2 weighted images without significantly decreasing the resolution of the image. This comes in very useful with reducing motion and performing three plane postcontrast imaging. Gradient imaging techniques are, however, extremely susceptible to artifacts caused by inhomogeneities and metallic items in or around the area being imaged. Metallic items such as orthopedic implants cause the magnetic field to bend around them resulting in miss encoding the protons and miss registering the information back to the system. The result is a black void on the image with a warped appearance to the structures immediately adjacent to it. This artefact will occur on an MR image though gradient images are more susceptible due to the fluctuation of the gradient magnetic field. 

I hope this “brief” look at the science of MRI can not only give you the information you need to get started, but also provide you with a foundation to explore more about what else is possible with this incredible diagnostic imaging modality.

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