Magnetic resonance imaging of the spine

1

What is magnetic resonance imaging (MRI)?

Magnetic resonance imaging (MRI) is a noninvasive imaging technology that uses magnetic fields and radiofrequency (RF) current to generate three-dimensional anatomical images without the use of ionizing radiation. The components of an MRI system include the main magnet, gradient coils, a RF coil, a computer system, and a patient table that passes through a horizontal tube (bore) running through the main magnet. The magnet is the most important component of the MRI system; it is comprised of multiple coils through which electric current is passed to create a magnetic field. The coils are cooled to superconducting temperatures (−269 degrees C, −452 degrees F) by bathing the wires in liquid helium, to decrease resistance to flow of electricity to near zero. Electric current passed through a superconducting magnet will flow continuously and create a permanent magnetic field whose strength is quantified in units called tesla (T). MRI systems used in clinical practice range between 0.5 and 3 Tesla (T) which is equivalent to 10,000–60,000 times the strength of the Earth’s magnetic field (50 μT).

2

How are MRI images generated?

MRI works by exciting and detecting changes in the rotational axis of protons that comprise living tissue. The hydrogen atoms (protons) in the human body are single charged atoms spinning on random axes such that the body’s total magnetic field is zero. During an MRI scan, the patient is placed in a magnetic field, which causes the hydrogen nuclei to align parallel with the magnetic field. Application of RF pulses cause the hydrogen nuclei to realign and enter a higher energy state. Gradient coils within the main magnet alter the static field at a local level, which allows spatial encoding of the MRI signal. When the RF pulses are terminated, the excited hydrogen nuclei release energy as they realign in the direction of the main magnetic field at differential rates in different tissues, and return to a lower energy state in a process termed relaxation . The energy released during this transition is detected by the MRI receiver coil. Signal data are processed in terms of origin within the imaging plane and subsequently displayed on a monitor. The time between RF pulses is termed the repetition time (TR). The time between the application of RF pulses and the recording of the MRI signal is termed the echo time (TE). The process of relaxation is described in terms of two independent time constants, T1 and T2.

3

What is signal intensity?

Signal intensity describes the relative brightness of tissues on an MRI image. Tissues may be described as high (bright), intermediate (gray), or low (dark) intensity. Tissue intensity of a pathologic process relative to the intensity of surrounding normal tissue may be described as hyperintense, isointense, or hypointense. MRI signal intensity depends on the T1, T2, and proton density (number of mobile hydrogen ions) of the tissue under evaluation.

4

Explain the differences between T1-weighted, T2-weighted, and proton density–weighted MRI images.

T1 (longitudinal plane relaxation time) and T2 (transverse plane relaxation time) are intrinsic physical properties of tissues. Different tissues have different T1 and T2 properties based on how their hydrogen nuclei respond to RF pulses during the MRI scan. MRI contrast is determined by varying the scanning parameters (TE and TR) to emphasize differences in tissue-specific properties and is referred to as weighting the image.

• T1-weighted images are produced with a short TR (≤1000 msec) and a short TE (≤30 msec). T1 images are weighted toward fat. Fat typically appears bright on T1 images and less bright on T2 images. T1-weighted images are excellent for evaluating structures containing fat, hemorrhage, or proteinaceous fluid, all of which have a short T1 and demonstrate a high signal on T1-weighted images. Note that water will be dark on T1-weighted images. T1 images demonstrate anatomic structures well because of their high signal-to-noise ratio.

• T2-weighted images are produced with a long TR (≥2000 msec) and a long TE (≥60 msec). T2 images are weighted toward water. Water appears bright on T2 images and dark on T1 images (mnemonic: water [H ₂ 0] is bright on T2) . Signal intensity on T2 images is related to the state of tissue hydration. Tissues with high water content (cerebrospinal fluid, cysts, normal intervertebral discs) show an increased signal on T2 images. T2 images are most useful for contrasting normal and abnormal anatomy . In general, pathologic processes (e.g., neoplasm, infection, acute fractures) are associated with increased water content and appear hyperintense on T2 and hypointense on T1 images.

• Proton density–weighted images are produced with an intermediate TR (≥1000 msec) and a short TE (≤30 msec). Tissue contrast on proton density–weighted images is related to the number of protons within tissues. Water has intermediate signal intensity on proton density–weighted images.

5

Describe the signal intensity of common tissue types on T1- and T2-weighted and proton density–weighted MRI sequences.

On T1-weighted sequences, water has low-intermediate signal intensity, muscle has intermediate signal intensity, and fat has high signal intensity. T1-weighted sequences provide a good depiction of anatomic detail but are less sensitive to pathologic changes. T1-weighted sequences are useful to evaluate tissue enhancement after intravenous gadolinium contrast.

On T2-weighted sequences, water has high signal intensity while muscle and fat have intermediate signal intensity. T2-weighted sequences are useful in identification of pathologic processes (e.g., neoplasm, infection, acute fractures) as these entities are associated with increased water content.

On proton density–weighted sequences, fat has high signal intensity while muscle and water demonstrate intermediate signal intensity.

Note that cortical bone, tendons, and fibrous tissues demonstrate low signal intensity on both T1- and T2-weighted and proton density–weighted sequences, because these tissues contain few mobile hydrogen ions. Gas contains no mobile hydrogen ions and does not generate an MRI signal. Fat signal intensity may vary between each type of sequence depending on whether or not fat suppression is utilized. Fat signal may be suppressed using a variety of techniques for different purposes depending on the specific clinical scenario. The relative signal intensities of different tissue types on T1- and T2-weighted and proton density–weighted images are summarized in Table 10.1 .

6

How do I know whether I am looking at a T1-weighted, T2-weighted, or proton density–weighted image?

One method is to look at the TE and TR numbers on the scan. T1 images are produced with a short TR (≤1000 msec) and a short TE (≤30 msec). T2 images are produced with a long TR (≥2000 msec) and a long TE (≥60 msec). Proton density–weighted images are produced with an intermediate TR (≥1000 msec) and short TE (≤30 msec).

An alternate method is to recall the signal characteristics of water. Locate a fluid-containing structure (e.g., CSF surrounding the spinal cord). If the fluid is bright, the image is probably a T2-weighted image . If the fluid is dark, the image is probably a T1-weighted image . Water has intermediate signal intensity on proton density–weighted images .

The above criteria refer to the most basic pulse sequence, spin echo (SE). In other pulse sequences, contrast phenomenology is more complex ( Table 10.2 ).

7