The Science of MRI
Welcome to Veterinary Imaging of the Chesapeake. We are excited to share our expertise and services with the community. On this page we would like to share some basic principles of MRI and maybe help you gain a better understanding of why this technology is important in the field of Veterinary Medicine.
Magnetic Resonance Imaging is an advanced imaging modality that requires multiple systems to collaborate harmoniously to achieve high resolution, diagnostic imaging of the internal structures of a subject.
The Magnets: There are three separate magnetic fields present during the scanning process that are all utilized and altered to obtain optimum information for imaging. These are known as Static, Gradient and Nuclear Magnetism.
The static magnetic field, also known as the main magnetic field, is created, in this case, by a superconductive magnet. A large electro-magnet is contained inside of the MRI scanner and it is always on. This electro magnet is inside of a chamber that is filled with approximately 1000 liters of liquid Helium. A voltage is applied to the electro-magnet to induce a magnetic field around it. The liquid Helium exists at a temperature of four degrees Kelvin, thus eliminating resistance from the electro magnet and allowing a constant static magnetic field to be present without the need for a constant applied voltage. This magnetization is measured in Tesla (T). Conventional MRI scanners that can be used on patients, range from 0.2T to 7T, with research specimen scanners going beyond 10T. To put the strength of these magnets into perspective, 1T = 10,000 Gauss, and the earth’s magnetic field strength is 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.
Precession: When a patient is placed within the static magnetic field a large majority of these Hydrogen protons will align parallel with the main magnetic field. The combined magnetization of these easily manipulated Hydrogen protons is known as the Net Magnetization Vector (NMV). With these protons aligned in the same direction, they also precess around the axis of the static magnetic field and exert a force perpendicular to the direction of its spin. This force is known as spin angular momentum. These forces interact with each other and cause the proton to wobble on its axis. 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 is 42.56 Megahertz per Tesla (MHz/T). Knowing this, we can determine the precise frequency at which hydrogen protons precess. This is known as the Resonant frequency or Larmor Frequency.
Resonance: Magnetic resonance signal is produced by non-ionizing radio frequencies being transmitted, via an antenna, to the excited protons then manipulating them in such a way that an echoing signal can be induced in a receiving antenna. In this case the receiving antenna is a coil placed against the patient in the area of interest. This is also sometimes the transmitting antenna, however there are other transmitting antennae 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 an anti-parallel direction to the static field. When enough energy is applied to tip the NMV 90 degrees away 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.
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 and when the signal is read from the patient. Time of Repetition or TR determines the amount of time between the initial pulse of 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.
T1 (left) and T2 (right) images of a canine brain.
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. 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.
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 TI Inversion Recovery sequence is a T1 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.
A Flair Image of a canine brain. Notice the clear deliniation between grey and white matter and the signal nulling of the CSF.
In neurological imaging, a FLAIR or Fluid Attenuated Inversion Recovery sequences 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.
Thus far we’ve been discussing, with minor exception, Spin Echo Pulse sequences. Where RF pulses are used to excite, refocus, and manipulate the hydrogen protons. Gradient Echo pulse sequences are also utilized in MRI, where and 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 use 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, very susceptible to artifacts caused by inhomogeneities and metallic items in or around the area being imaged.
We greatly appreciate you taking the time to view this page and learn more about our services here at Veterinary Imaging of the Chesapeake. Thank you.