Magnetic Resonance Imaging

Key Facts

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Clinical magnetic resonance imaging (MRI) measures the spatial distribution of protons in the body.
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Gradient coils are used to provide spatial information. The changing gradients are associated with noise produced during imaging.
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Relaxation times T1, T2, and T2* are important tissue characteristics for imaging.
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Low-field magnets have lower signal to noise ratio (SNR); longer scan times, making patient motion a potential problem; decreased resolution; decreased sensitivity to old blood and calcified lesions; lower gadolinium enhancement; and difficulty in spectral fat suppression.
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Gadolinium contrast medium is often used combined with fat-suppressed T1-weighted imaging to increase contrast between enhanced tissue and surrounding tissue.
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Artifacts are numerous in MRI and can lead to erroneous diagnosis if not understood or eliminated. The magic angle phenomenon produces increased signal in portions of tendons oriented at approximately 55 degrees to the main magnetic field. These areas will appear bright on short TE sequences (e.g., T1) and can lead to an erroneous diagnosis of degeneration or tear.
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Patient safety is paramount and can be maximized by thorough prescreening and other safety measures.

GENERAL PRINCIPLES

Imaging Principles

Magnetic resonance imaging (MRI) measures the spatial distribution of specific nuclear spins (usually those of protons) in the body. Electric signals from the spins are measured using precessional motion of the proton spins after they are excited by radiofrequency (RF) pulses irradiated in a static magnetic field.

Precession refers to a change in the direction of the axis of a rotating object.

The phenomenon in which the nuclear spins generate or emit electric signals of a specific frequency (Larmor frequency) in a static magnetic field is called nuclear magnetic resonance (NMR).

The electric signal (NMR signal) itself carries no spatial information. The spatial information necessary to generate an image is given by magnetic field gradients that are generated by gradient coils. Because they are driven by pulsed electric currents in a strong magnetic field, the coils receive a repetitive strong force, and a loud sound is produced during the MRI scan.

NMR Signal: Free Induction Decay and Spin Echo

Two kinds of NMR signal are generally used in MRI: free induction decay (FID) and spin echo. FID is elicited by a single RF pulse (e.g., 90 degrees) ( Figure 3-1 ). The FID decays with the time constant T2*. The decay of the NMR signal can be recovered by applying a second RF pulse, called a 180-degree pulse . At a specific time (TE/2) after the second RF pulse, the spin echo signal is observed. The intensity of the spin echo signal decays with the time constant T2.

Relaxation Times

The relaxation times of proton spins are the most important parameters in MRI. Three kinds of relaxation times are generally used: T1, T2, and T2*. T1 or longitudinal relaxation time is the time by which nuclear spins return to thermal equilibrium (initial state) after irradiation by an RF pulse(s). T1 is generally used to visualize the degree of saturation or suppression of NMR signal or image intensity because tissues with longer T1 give suppressed NMR signal in T1-weighted sequences, as described below.

Tissues with long T1 are dark on T1-weighted images.

T2, or transverse relaxation time, describes the lifetime of spin echo signal, as shown in Figure 3-1 . T2 is generally used to distinguish pathologic tissues from normal tissues, because proton spins of pathologic tissues usually have longer T2.

Tissues with a long T2 appear bright on T2-weighted images.

T2* describes the decay rate of FID signal, as shown in Figure 3-1 . Although T1 and T2 depend on NMR frequency (magnetic field strength), typical T1 and T2 values of water content in normal tissues are roughly 1000 ms and 50 ms, respectively. T1 and T2 of pathologic tissues usually become longer than those of normal tissues, making MRI very useful in diagnosis of various diseases.

PULSE SEQUENCES AND IMAGE CONTRAST

The contrast of magnetic resonance (MR) images is determined by combinations of relaxation times and pulse sequences. The pulse sequences are divided into two major categories: spin echo and gradient echo sequences.

Spin Echo Sequence

Spin echo (SE) sequences utilize spin echo signal and produce spin echo images, in which image intensity, I(x,y) , is expressed as:

<SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='I(x,y)=kρ(x,y){1-exp(-TR/T1(x,y))}exp(-TE/T2(x,y)),’>I(x,y)=kρ(x,y){1−exp(−TR/T1(x,y))}exp(−TE/T2(x,y)),I(x,y)=kρ(x,y){1-exp(-TR/T1(x,y))}exp(-TE/T2(x,y)),
I ( x , y ) = k ρ ( x , y ) { 1 – exp ( – TR / T1 ( x , y ) ) } exp ( – TE / T 2 ( x , y ) ) ,

where k, ρ (x,y), TR , and TE are a constant, proton density, repetition time of the pulse sequence, and spin echo time, respectively (see Figure 3-1 ). This equation shows that spin echo images are proton density images modified by the ratios of TR/T1 and TE/T2. Although ρ (x,y), T1(x,y) , and T2(x,y) can be computed by combinations of several spin echo images, three practically useful images are widely used: T1-weighted images (T1WI), T2-weighted images (T2WI), and proton density weighted images (PDWI). Because T1 and T2 of water-content normal tissues are roughly 1000 ms and 50 ms, respectively, the pulse sequences shown in Table 3-1 are used for T1WI, T2WI, and PDWI acquisition. Typical and instructive T1WI, T2WI, and PDWI of a chicken egg are shown in Figure 3-2 . The yolk and white of the egg are visualized with various image contrasts because they have different T1, T2, and proton densities.

TABLE 3-1

Parameter-Weighted Spin Echo Sequences

	TR << 1000 ms	1000 ms << TR
TE << 50 ms	T1WI	PDWI
50 ms << TE	Not used	T2WI

Because imaging appearances vary depending on the imaging parameters used, attention should be directed to the parameters listed on the image itself. The TR and TE, among other parameters, are indicated adjacent to the MR image ( Figure 3-3 ).

Table 3-2 explains the appearance of common tissues on SE imaging. The signal may change depending on the combination of the TR, TE, and inversion time (TI) used for obtaining the sequence.

TABLE 3-2

Examples of Tissue Appearance on Common Imaging Sequences

	T1 weighted	T2 weighted	STIR
Bone cortex, calcification	Very low signal	Very low signal	Very low signal
Bone marrow	High signal	High signal	Low signal
Cartilage	Iso-signal	Slightly low signal	Iso-signal
Joint effusion	Iso-signal	High signal	High signal
Acute hemorrhage	Low to iso-signal	Low to iso-signal	Low to iso-signal
Subacute hemorrhage	High signal	High signal	Various signal
Hemosiderin	Very low signal	Very low signal	Very low signal
Fat	High signal	High signal if FSE	Low signal

Comparison is made to the signal of muscle. FSE, Fast spin echo.

In actual clinical settings, SE sequences are usually performed as fast spin echo (FSE) sequences to shorten the imaging time by using multiple spin echoes. Basic image contrasts are similar to those obtained by the traditional or conventional spin echo sequences described above. However, fat tissue is of higher signal on FSE T2-weighted imaging than on spin echo T2-weighted imaging.

Gradient Echo Sequence

Gradient echo (GRE) sequences utilize FID signal and are characterized by sequence parameters TR, TE, and FA (flip angle). The FA is the angle by which nuclear spins are rotated from the direction of the static magnetic field. However, the image contrasts of GRE images are not determined solely by the sequence parameters but are strongly affected by the pulse sequence design.

Regarding the pulse sequence design, GRE sequences are categorized into three groups: incoherent acquisition sequence (e.g., FLASH, SPGR), partially coherent acquisition sequence (e.g., GRASS, FISP), and coherent acquisition sequence (e.g., TrueFISP, SSFP). MRI manufacturers use different names for their own GRE sequences. However, for simplicity, we use the terms FLASH, GRASS, and TrueFISP to represent the three acquisition methods described above. FLASH is mainly used as a T1-weighted sequence. GRASS is used as a T2*-weighted or T1-weighted sequence. FLASH and GRASS are faster than the spin echo T1-weighted sequence, but the image contrasts are slightly different ( Figure 3-4 ). TrueFISP is a very fast sequence and is mainly used for visualization of fluids such as blood.