Many physicians feel apprehensive about not being able to interpret this terminology since they are not trained as radiologists. To help prepare you for when you encounter these terms, we will show you how to gain a better understanding of this useful tool in the medical field so you can provide better healthcare.

What Is Magnetic Resonance Imaging and How Does It Work?

Magnetic resonance imaging (MRI) is a diagnostic method used to produce detailed images of organs and tissues by means of a magnetic field and radio waves that change rapidly. The resulting images are then displayed on a computer to determine whether a lesion is present and to what extent.

Magnetic resonance imaging is an interaction between an external magnetic field, radiofrequency waves, and atomic nuclei. When a body is subjected to a magnetic field and subsequently stimulated by electromagnetic waves (radiofrequency waves), the result is resonance of its atomic nuclei. 

The basis for obtaining images is measuring the energy released and the time it takes to return to a relaxed state once stimulation ceases. Through this radiological study, high-quality images of the human body are obtained, enabling accurate diagnoses. Thanks to magnetic resonance imaging, we have been able to reveal the different cross-sections of tissues, organs, and vascular structures.

Basic Radiology Concepts in Magnetic Resonance Imaging

Setting aside purely technical aspects, what is important to understand is the sequential process:

1. The hydrogen nuclei are the key players in forming the image by MRI. When the patient is placed in the MR scanner, the hydrogen atoms, previously oriented at random, align with the static magnetic field.

2. To detect the signal, a radiofrequency pulse is briefly applied, producing a net change in the alignment of these atomic nuclei.

3. When the radiofrequency pulse ceases, the spins return to their equilibrium state, releasing energy to the surrounding molecules.

4. The rate of energy release is determined by the intrinsic relaxation properties of each tissue, characterized by the longitudinal relaxation time (T1) and the transverse relaxation time (T2).

5. T1 represents the recovery of longitudinal magnetization in the direction of the main magnetic field.

6. T2 represents the loss of magnetization in the transverse plane, perpendicular to the axis of the field.

7. Substances with a long T1 (for example, fluids) will appear dark on T1-weighted images, while those with a short T1 (fatty tissues) will show a high-intensity signal.

8. On T2-weighted images, a substance with a long T2 (fluid) will appear bright.

9. The main advantages of MRI are its excellent contrast resolution, high spatial resolution, and the absence of ionizing radiation.

10. Among the agents approved for clinical use in MRI,  the most commonly used contrast agent is a drug that shortens T1, called gadopentetate dimeglumine (or Gd-DTPA), because it contains gadolinium, a paramagnetic agent (atoms with unpaired electrons in their outer shells).

11. Tissue relaxation is altered by the interaction between gadolinium's unpaired electron and tissue hydrogen protons, which significantly shortens the T1 of blood relative to the surrounding tissues.

12.  MR images can be obtained using different “sequences.” The most commonly used are known as spin-echo (or spin echo) sequences and can be weighted in either T1 or T2.

T1-, T2-, TR-, and TE-Weighted Images

MRI allows healthcare professionals to detect abnormalities and lesions that help confirm a diagnostic hypothesis and improve treatment guidelines. This is why we must deepen our understanding of T1-, T2-, TR-, and TE-weighted imaging.

Many physicians fear they will be unable to interpret this terminology since they are not trained as radiologists. Below we show you how to gain a better understanding of this useful tool in the medical field and its most common terms.

To more precisely understand what relates to a weighted image, we must recall the basic planes of every magnetic resonance study, understanding that it works with three planes: the axial, coronal, and sagittal planes.

When it comes to gaining a deeper understanding of the qualities of this imaging study, in this case magnetic resonance imaging, it is important to be familiar with certain concepts described previously in order to understand what is meant by a weighted image. 

The pixel value and contrast of an image are determined by intrinsic and extrinsic factors:

• Intrinsic factors: proton density, T1, T2, flow, and diffusion.
• Extrinsic factors: TE, TR, flip angle, TI (inversion time), turbo factor, echo train length, b-value, among others.

Definition of Key Terms in Radiology

•  T1-weighted images. This is the measure of relaxation time as it manifests in the longitudinal plane. These images are created primarily using data from the relaxation rates that differentiate protons in the longitudinal plane of a main magnetic field.
  
• T2-weighted images. This refers to the measure of relaxation time as it manifests in the transverse plane. These images are created using the same principle as T1-weighted imaging, except that it occurs in the transverse plane.  
  
• TR-weighted images. Describes the measure of repetition time. In other words, it is the time needed to complete a full excitation/relaxation cycle for a pulse sequence, expressed in milliseconds (ms).
  
• TE-weighted images. This is the measure of echo time. That is, it is the time elapsed between the start of a pulse sequence and the acquisition of data from excited protons, expressed in milliseconds (ms).

What Is a Pulse Sequence? 

The echo time (TE) and the repetition time (TR) refer to a concept known as the pulse sequence, a series of instructions repeated numerous times, which allows data to accumulate in order to ultimately create an MRI image.

The pulse sequence has two essential variants:

• Spin echo pulse sequence. This event begins with a 90-degree pulse and continues with a 180-degree pulse, generating protons that create the MRI signal. This type of sequence is used to weight images in T1 and T2.
  
• Fast spin echo (FSE) pulse sequence. The advantage of this type of technique is that, after the 90-degree and 180-degree pulses, multiple echoes can be obtained. In other words, multiple slices of the scan are obtained.  This modality is much faster than conventional spin echo, especially for T2, since it requires a shorter repetition time (TR) to create the image.

Difference Between a T1-Weighted and T2-Weighted Image Based on TE and TR

Generally speaking, a T1-weighted image has a short TR and TE, the former being less than 1,000 ms and the latter less than 20 ms. On the other hand, substances with a long T1 will be hypodense, while a short T1 will result in a substance that is hyperdense in terms of intensity.

A T2-weighted image has a long TR and TE, the former being more than 2,000 ms and the latter more than 40 ms. In this case, substances with a long T2 will be hyperdense, while hypodense substances will remain hypodense.

How to Distinguish a T1-Weighted from a T2-Weighted Image?

• On  T2-weighted images, fluid is hyperintense, whereas on T1 it has an intermediate signal.
• Fat is brighter on T1, although it is not the ideal predictor. On T2 it is hypointense.
• On T1, hyperintense structures include blood, proteinaceous substances, melanin, and paramagnetic agents (gadolinium).
• On T1/T2, structures such as air, cortical bone, ligaments, tendons, fibrous tissue, and blood flow appear hypointense.

On a T1-weighted image, the bone marrow of a normal adult (fatty or yellow marrow) has high signal, that is, it is hyperintense or hyperdense. This means it appears white in tone, while cerebrospinal fluid has low signal,  that is, it is hypointense or hypodense, giving it a dark tone.

Nervous tissue, such as the spinal cord or nerve roots, has an intermediate signal intensity. Cortical bone, which lacks mobile protons to produce a signal, is hypointense on all pulse sequences.

On T2-weighted images, bone marrow has lower signal intensity, cerebrospinal fluid becomes hyperintense, and nervous tissue maintains an intermediate signal intensity. However, the spinal cord has relatively lower signal intensity, since it is surrounded by cerebrospinal fluid, which has a much higher signal intensity.

The intervertebral discs of normal individuals typically have an intermediate signal intensity on T1-weighted images and, due to their water content, appear hyperintense on T2-weighted images.

Diagnostic Value of T1- and T2-Weighted Images 

One of the main uses of a T1-weighted image lies in its application in neurology, since the image quality and the precision of anatomical detail allow for a more accurate diagnosis. It also helps determine the best therapeutic course for the patient.

The advantage of a T2-weighted image is that it shows fat as a low-intensity signal (hypo) and fluid as a high-intensity signal (hyper).  This, for example, is very useful in lesions and pathologies characterized by an increase in fluid content. There is also a reversal with respect to white and gray matter, with white matter showing lower intensity.

The mechanism used by radiology and imaging equipment may seem complex, but understanding how an image is obtained allows us to improve patient diagnosis. Let us remember that the type of contrast used is key when performing a paraclinical study.

Today, this type of procedure is enhanced thanks to the technology used by PACS systems (Picture Archiving and Communication System), which give specialists the opportunity to improve image contrast and quality using a variety of techniques and tools.

References

• E. Scott Pretorius and Jeffrey A. Solomon. 2006. Radiology Secrets. Elsevier.
• Joaquín Costa Subias and Juan Alfonso Soria Jerez. 2015. Resonancia magnética dirigida a técnicos superiores en imagen para el diagnóstico (Magnetic Resonance Imaging for Senior Diagnostic Imaging Technicians). Elsevier.
• Michael Y. M. Chen, MD, Thomas L. Pope Jr., MD, and David J. Ott, MD. 2006. Radiología básica (Basic Radiology). Mcgraw-hill - Interamericana de España.
• “Resonancia magnética” .Temas especiales. Manual MSD versión para profesionales.
• “Cómo interpretar las Imágenes de Resonancia Magnética”. Radiología 2.0.