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MRI Made Easy: A Guide for Beginners

Magnetic resonance imaging (MRI) is a type of scan that uses strong magnetic fields and radio waves to produce detailed images of the inside of the body. MRI can help diagnose and monitor various health conditions that affect organs, tissues and bones.

However, MRI is not a simple technique to master. It involves complex physics, sophisticated instrumentation and various sequences and parameters that affect the image quality and interpretation. For radiology trainees and beginners, learning MRI can be a daunting task.

That’s why this article is designed to introduce you to the basic principles, sequences and interpretation of MRI in a simple and easy way. We will use Govind B Chavhan’s PDF 33l as a reference guide throughout this article. This PDF is a revised edition of his book “MRI Made Easy”, which is one of the most popular and widely used books on MRI for beginners .

In this article, we will cover the following topics:

How do protons help in MRI?
What are T1, T2 relaxations and image weighting?
What is k-space and how does it affect scanning parameters?
What are the main components of an MRI scanner?
What are the different types of MRI sequences and when to use them?
What are the common MRI artifacts and how to avoid them?
What are the safety issues and precautions for MRI?
What are the contrast agents used for MRI and how do they work?
How to interpret MRI images of the brain and body?
What are some advanced MRI techniques and applications?

By the end of this article, you will have a basic understanding of MRI and its clinical utility. You will also be able to appreciate the advantages and limitations of MRI compared to other imaging modalities.

How do protons help in MRI?

The key to MRI is the hydrogen atom, which is the simplest and most abundant atom in the human body. Hydrogen atoms consist of a single proton in the nucleus and a single electron orbiting around it. Protons have a property called spin, which means they behave like tiny magnets with a north and south pole.

When protons are placed in a strong magnetic field, such as the one generated by an MRI scanner, they tend to align themselves with the direction of the field. However, they do not align perfectly, but rather precess or wobble around the axis of the field at a certain frequency. This frequency depends on the strength of the magnetic field and is called the Larmor frequency.

By applying a radiofrequency (RF) pulse at the same frequency as the Larmor frequency, the MRI scanner can tip the protons away from their alignment with the magnetic field. This causes them to rotate in a plane perpendicular to the field and create a net magnetization vector in that plane. This vector is called the transverse magnetization and it is the source of the MRI signal.

After the RF pulse is switched off, the protons gradually return to their original alignment with the magnetic field. This process is called relaxation and it involves two components: longitudinal relaxation and transverse relaxation. Longitudinal relaxation refers to the recovery of the net magnetization along the direction of the magnetic field, while transverse relaxation refers to the decay of the net magnetization in the plane perpendicular to the field.

As the protons relax, they emit RF signals that can be detected by a receiver coil in the MRI scanner. These signals are called free induction decay (FID) and they contain information about the location and properties of the protons. By analyzing these signals, the MRI scanner can create images of different tissues based on their proton density and relaxation times.

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What is k-space and how does it affect scanning parameters?

k-space is a term used to describe the data matrix that contains the raw MRI signal before image reconstruction. k-space is not a physical space, but rather a mathematical representation of the spatial frequencies in the MR image. Each point in k-space corresponds to a specific frequency and phase of the MR signal, and each point contributes to the final image after applying a Fourier transform.

The size and shape of k-space determine the resolution and field of view of the MR image. The size of k-space is determined by the number of data points acquired in each direction, which depends on the matrix size and the number of phase encoding steps. The larger the k-space, the higher the resolution of the image. The shape of k-space is determined by the sampling pattern of the data points, which depends on the gradient waveforms and the echo time. The shape of k-space affects the field of view and the aspect ratio of the image.

The filling of k-space determines the contrast and quality of the MR image. The filling of k-space is determined by the order and density of data points acquired, which depends on the pulse sequence and the scan time. The central region of k-space contains low spatial frequencies, which encode contrast and signal-to-noise ratio information. The peripheral region of k-space contains high spatial frequencies, which encode spatial resolution and edge information. Different filling strategies can be used to optimize different aspects of image quality.

What are the main components of an MRI scanner?

An MRI scanner is a complex device that consists of four main components: the main magnet, the shim coils, the gradient coils, and the radiofrequency (RF) system. Each component has a specific function and contributes to the generation and detection of the MRI signal.

The main magnet is the largest and most expensive component of the MRI scanner. It produces a strong and uniform magnetic field that aligns the protons in the body along its direction. The strength of the main magnet is measured in tesla (T) and typically ranges from 0.5 T to 3 T for clinical scanners. The main magnet can be either superconducting or permanent. Superconducting magnets use coils of wire that are cooled by liquid helium to achieve very high magnetic fields with low power consumption. Permanent magnets use blocks of ferromagnetic material that are magnetized by an external field and do not require cooling or power.

The shim coils are smaller coils that are placed around the main magnet to correct any inhomogeneities or variations in the main magnetic field. These variations can be caused by imperfections in the magnet design, environmental factors, or the presence of the patient. Shim coils can be either passive or active. Passive shim coils are fixed metal plates that are placed inside the magnet bore to adjust the field locally. Active shim coils are electrically powered coils that can be controlled by a computer to adjust the field dynamically.

The gradient coils are another set of coils that are placed inside or outside the main magnet to create spatially varying magnetic fields that are superimposed on the main magnetic field. These fields are used to encode the spatial location of the protons in the body by changing their resonance frequency according to their position. There are three gradient coils that correspond to the three spatial directions: x, y, and z. The gradient coils are switched on and off rapidly during the scan to create different pulse sequences and image contrasts.

The radiofrequency (RF) system consists of two parts: the transmitter and the receiver. The transmitter is a device that generates RF pulses at a specific frequency and power level and delivers them to an RF coil that is placed near or around the body part to be imaged. The RF pulses tip the protons away from their equilibrium alignment with the main magnetic field and create transverse magnetization. The receiver is a device that detects the RF signals emitted by the protons as they relax back to their equilibrium state and converts them into electrical signals. The receiver also uses an RF coil that can be either separate from or combined with the transmitter coil.

What are the common MRI artifacts and how to avoid them?

An MRI artifact is a visual anomaly that appears in an MRI image that is not present in the original object. Artifacts can degrade the image quality and interfere with the diagnosis. Artifacts can be caused by various factors, such as patient motion, magnetic field inhomogeneity, hardware malfunction, or software errors. Some of the common MRI artifacts and their causes and solutions are:

Motion artifact: This artifact occurs when the patient or a part of the patient moves during the scan. Motion can cause blurring, ghosting, or misregistration of the image. Motion artifact can be reduced by using faster scan techniques, such as echo planar imaging (EPI) or parallel imaging, applying motion correction algorithms, or using external devices to immobilize or monitor the patient.
Aliasing artifact: This artifact occurs when the field of view (FOV) is smaller than the object being imaged. Aliasing causes a part of the object to appear wrapped around or superimposed on another part of the image. Aliasing artifact can be avoided by increasing the FOV, applying anti-aliasing filters, or using oversampling techniques.
Chemical shift artifact: This artifact occurs when different chemical species, such as fat and water, have different resonance frequencies due to their different magnetic susceptibilities. Chemical shift causes a misregistration or a signal loss at the interface between fat and water tissues. Chemical shift artifact can be minimized by using a higher magnetic field strength, a smaller voxel size, a lower bandwidth, or a fat suppression technique.
Susceptibility artifact: This artifact occurs when there is a local distortion of the magnetic field due to the presence of magnetic materials, such as metal implants, air bubbles, or blood products. Susceptibility causes a signal loss or a geometric distortion in the image. Susceptibility artifact can be reduced by using a higher magnetic field strength, a shorter echo time (TE), a smaller voxel size, a gradient echo sequence, or a susceptibility-weighted imaging (SWI) technique.
Gibbs artifact: This artifact occurs when there is a truncation of the k-space data due to finite sampling or zero filling. Gibbs causes a ringing or an overshooting effect at the edges of sharp transitions in the image. Gibbs artifact can be avoided by using a higher matrix size, a higher number of averages, or an apodization filter.

Conclusion

MRI is a powerful and versatile imaging modality that can provide detailed information about the structure and function of various tissues and organs in the body. However, MRI is also a complex technique that requires a thorough understanding of the basic principles, sequences, and artifacts that affect the image quality and interpretation. In this article, we have introduced some of the fundamental concepts and common applications of MRI using Govind B Chavhan’s PDF 33l as a reference guide. We hope that this article has helped you to gain a basic knowledge of MRI and its clinical utility.