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Basic physics for magnetic resonance imaging

Magnetic resonance imaging (MRI) uses a superconducting magnet to create images of the internal structures, organs and tissues of the patient. This magnetic is very high field. In fact, a 1.5T MRI scanner has a magnetic field that is about 30,000 times stronger than that of the Earth. Radio waves are used to create data throughout the MRI scan, and equipment collects data to transmit to the computer, which is then translated into the images through the computer itself and a radiologist. In some cases, it may be necessary to know the basic physics concepts and techniques used during MRI in order to better understand the images that are being read.

Radiofrequency signals: Contained and obtained

The rooms at an imaging facility or hospital that house the magnetic resonance equipment, such as the scanner and coils, may be referred to as the MR suite. The control center for the MRI suite is next to the room that holds the MRI scanner and houses the computer that processes the data to make the acquired images. This control center is often separated from the room holding the scanner by a wall with a window in it, so the technologist can see into the room with the scanner without the computer being subject to the magnetic field of the scanner. There are many main parts of an MR suite, including the magnet, gradient coil, gradient amplifier, one set of radio-frequency (RF), RF amplifier and receiver. The distinction between the rest of the hospital and the MR suite may be due to the difference in creating a regular hospital room compared to creating a room specifically to hold an MR scanner. The room that houses the scanner is built in a way that is designed to keep the electromagnetic fields from outside the room out and those created by the scanner in.1

There are two objects in the MR suite that handle the radio-frequency waves produced by the scan: the transmit coil and the receiver coil, though there are coils that can both transmit and receive.1 The transmit coil is inside the scanner, around the bore (the hole in the middle), and the receiver coil is placed near the region of interest specific to each patient. The coils near the patient transmit the signals that are caused during the pulse sequences that the scanner emits throughout the scan. The magnetic fields alter the protons in the patient's body, which creates signals that are then intercepted by coils. In turn, the coils transmits data from the signals to the computer. Additionally, the receiver coils placed near the patient's body can help to increase the signal to noise ratio for the scan, which can help to create images from the data that potentially are less obscured by noise.

These pulse sequences cause the protons, water molecules or Hydrogen atoms in the patient's body to align and then relax in a clinical MRI scanner.2 The time between corresponding consecutive points during a repeating series of pulses and echoes is called the repetition time (TR). So, in a repetitive symmetric pulse, the TR could be the time from the beginning of one pulse to the beginning of the next pulse. The echo time (TE) is the time from the center of the center of the pulse to the center of the echo. A short TR and TE creates T1-weighted images. On the other hand, a long TR and TE creates T2-weighted images. A long TR and a short TE creates a proton density image.

T1- and T2-weighted images: What is the difference?

There are three primary types of weighting that can be produced using magnetic resonance. Among the three, T1- and T2-weighted and proton density, the first two are used most frequently. Whether the image will be T1- or T2-weighted depends on which relaxation has the most influence on the image.

T1-weighted images have more reliance on T1 relaxation than T2.1 T1 relaxation, also known as spin-lattice relaxation, involves protons that exchange energy with their surroundings to return to a lower energy state. This restores longitudinal magnetization. Different types of molecules have different tumbling rates, which is the rate of molecular motion. The tumbling of molecules generates its own magnetic field that protons in neighboring molecules are influenced by.

T2 relaxation influences T2-weighted images more than T1. T2 relaxation is also known as spin-spin relaxation because the spins of one molecule's protons is influenced by the magnetic fields from neighboring nuclei. This type of relaxation refers to the process by which protons fall out of phase in the x-y plane and transverse magnetization decreases and disappears.1

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Signals: Informing the MR scanner

Radiologists can note the differences between tissues to identify structures through the use of contrast.3 In this case, contrast is determined by signal intensities and should not be confused with the gadolinium contrast that can be used during an MRI. These signals can be obscured by noise, which is virtually unavoidable. Noise in an MRI refers to the background hiss or static that occurs with all electronic equipment. It is a random phenomenon, but it can greatly impact the scan.

The signal to noise ratio (SNR) can affect the images produced. Ideally, the technologist would want to have as high of a signal as possible with as little noise as possible. However, this can be difficult to achieve. The SNR is calculated by taking the signal intensity and dividing by the amount of noise.

Some additional factors can impact the SNR of a scan, such as blood flow or coils.1 The coils placed near the patient's body increase the signal intensity due to the proximity to the tissue emitting the signal. The closer the coil is to the patient's body, the greater the signal. Patients who are smaller than average may be a challenge for radiology technologists that are using traditional, rigid coils. Some newer coils can be wrapped around the region of interest as they are similar to blankets in structure.

Most of the aspects of magnetic resonance imaging rely on physics. The MR scanner is housed inside a room that was designed specifically so that the radio-frequency pulses and magnetic fields do not escape the room, taking the concept of a Faraday shield one step further. The scanner itself was designed to take advantage of certain principles of physics, such as the way protons, water molecules and Hydrogen atoms react to a strong magnetic field. The radio-frequency coils then pick this data up to transmit to the computer in order to create the images. Keeping these principles in mind, the technologist can then optimize the scan and obtain the desired images.

References

1. Stuart Currie, et al. "Understanding MRI: basic MR physics for physicians." Postgraduate Medical Journal. 2013; 89: 209-223. Web. 18 June 2019. doi: 10.1136/postgradmedj-2012-131342.

2. "TR and TE: What are TR and TE?" MRIQuestions.com. 2018. Web. 18 June 2019. <http://mriquestions.com/tr-and-te.html>.

3. Kathryn Mary Broadhouse. "The Physics of MRI and How We Use It to Reveal the Mysteries of the Mind." frontiersin.org. 1 March 2019. Web. 18 June 2019. <https://kids.frontiersin.org/article/10.3389/frym.2019.00023>.