MRI pulse sequence
An MRI pulse sequence in magnetic resonance imaging is a particular setting of pulse sequences and pulsed field gradients, resulting in a particular image appearance.
A multiparametric MRI is a combination of two or more sequences, and/or including other specialized MRI configurations such as spectroscopy.
Spin echo
T1 and T2
Each tissue returns to its equilibrium state after excitation by the independent relaxation processes of T1 and T2.To create a T1-weighted image, magnetization is allowed to recover before measuring the MR signal by changing the repetition time. This image weighting is useful for assessing the cerebral cortex, identifying fatty tissue, characterizing focal liver lesions, and in general, obtaining morphological information, as well as for post-contrast imaging.
To create a T2-weighted image, magnetization is allowed to decay before measuring the MR signal by changing the echo time. This image weighting is useful for detecting edema and inflammation, revealing white matter lesions, and assessing zonal anatomy in the prostate and uterus.
The standard display of MRI images is to represent fluid characteristics in black and white images, where different tissues turn out as follows:
| Signal | T1-weighted | T2-weighted |
| High |
| |
| Inter- mediate | Grey matter darker than white matter | White matter darker than grey matter |
| Low |
Proton density
Proton density - weighted images are created by having a long repetition time and a short echo time. On images of the brain, this sequence has a more pronounced distinction between grey matter and white matter, but with little contrast between brain and CSF. It is very useful for the detection of arthropathy and injury.Gradient echo
A gradient echo sequence does not use a 180 degrees RF pulse to make the spins of particles coherent. Instead, it uses magnetic gradients to manipulate the spins, allowing the spins to dephase and rephase when required. After an excitation pulse, the spins are dephased, no signal is produced because the spins are not coherent. When the spins are rephased, they become coherent, and thus signal is generated to form images. Unlike spin echo, gradient echo does not need to wait for transverse magnetisation to decay completely before initiating another sequence, thus it requires very short repetition times, and therefore to acquire images in a short time. After echo is formed, some transverse magnetisations remains. Manipulating gradients during this time will produce images with different contrast. There are three main methods of manipulating contrast at this stage, namely steady-state free-precession that does not spoil the remaining transverse magnetisation, but attempts to recover them ; the sequence with spoiler gradient that averages the transverse magnetisations, and RF spoiler that vary the phases of RF pulse to eliminates the transverse magnetisation, thus producing pure T1-weighted images.For comparison purposes, the repetition time of a gradient echo sequence is of the order of 3 milliseconds, versus about 30 ms of a spin echo sequence.
Inversion recovery
Inversion recovery is an MRI sequence that provides high contrast between tissue and lesion. It can be used to provide high T1 weighted image, high T2 weighted image, and to suppress the signals from fat, blood, or cerebrospinal fluid.Diffusion weighted
measures the diffusion of water molecules in biological tissues. Clinically, diffusion MRI is useful for the diagnoses of conditions or neurological disorders, and helps better understand the connectivity of white matter axons in the central nervous system. In an isotropic medium, water molecules naturally move randomly according to turbulence and Brownian motion. In biological tissues however, where the Reynolds number is low enough for laminar flow, the diffusion may be anisotropic. For example, a molecule inside the axon of a neuron has a low probability of crossing the myelin membrane. Therefore, the molecule moves principally along the axis of the neural fiber. If it is known that molecules in a particular voxel diffuse principally in one direction, the assumption can be made that the majority of the fibers in this area are parallel to that direction.The recent development of diffusion tensor imaging enables diffusion to be measured in multiple directions, and the fractional anisotropy in each direction to be calculated for each voxel. This enables researchers to make brain maps of fiber directions to examine the connectivity of different regions in the brain or to examine areas of neural degeneration and demyelination in diseases like multiple sclerosis.
Another application of diffusion MRI is diffusion-weighted imaging. Following an ischemic stroke, DWI is highly sensitive to the changes occurring in the lesion. It is speculated that increases in restriction to water diffusion, as a result of cytotoxic edema, is responsible for the increase in signal on a DWI scan. The DWI enhancement appears within 5–10 minutes of the onset of stroke symptoms and remains for up to two weeks. Coupled with imaging of cerebral perfusion, researchers can highlight regions of "perfusion/diffusion mismatch" that may indicate regions capable of salvage by reperfusion therapy.
Like many other specialized applications, this technique is usually coupled with a fast image acquisition sequence, such as echo planar imaging sequence.
Perfusion weighted
Perfusion-weighted imaging is performed by 3 main techniques:- Dynamic susceptibility contrast : Gadolinium contrast is injected, and rapid repeated imaging quantifies susceptibility-induced signal loss.
- Dynamic contrast enhanced : Measuring shortening of the spin–lattice relaxation induced by a gadolinium contrast bolus.
- Arterial spin labelling : Magnetic labeling of arterial blood below the imaging slab, without the need of gadolinium contrast.
In cerebral infarction, the penumbra has decreased perfusion. Another MRI sequence, diffusion-weighted MRI, estimates the amount of tissue that is already necrotic, and the combination of those sequences can therefore be used to estimate the amount of brain tissue that is salvageable by thrombolysis and/or thrombectomy.
Functional MRI
measures signal changes in the brain that are due to changing neural activity. It is used to understand how different parts of the brain respond to external stimuli or passive activity in a resting state, and has applications in behavioral and cognitive research, and in planning neurosurgery of eloquent brain areas. Researchers use statistical methods to construct a 3-D parametric map of the brain indicating the regions of the cortex that demonstrate a significant change in activity in response to the task. Compared to anatomical T1W imaging, the brain is scanned at lower spatial resolution but at a higher temporal resolution. Increases in neural activity cause changes in the MR signal via T changes; this mechanism is referred to as the BOLD effect. Increased neural activity causes an increased demand for oxygen, and the vascular system actually overcompensates for this, increasing the amount of oxygenated hemoglobin relative to deoxygenated hemoglobin. Because deoxygenated hemoglobin attenuates the MR signal, the vascular response leads to a signal increase that is related to the neural activity. The precise nature of the relationship between neural activity and the BOLD signal is a subject of current research. The BOLD effect also allows for the generation of high resolution 3D maps of the venous vasculature within neural tissue.While BOLD signal analysis is the most common method employed for neuroscience studies in human subjects, the flexible nature of MR imaging provides means to sensitize the signal to other aspects of the blood supply. Alternative techniques employ arterial spin labeling or weighting the MRI signal by cerebral blood flow and cerebral blood volume. The CBV method requires injection of a class of MRI contrast agents that are now in human clinical trials. Because this method has been shown to be far more sensitive than the BOLD technique in preclinical studies, it may potentially expand the role of fMRI in clinical applications. The CBF method provides more quantitative information than the BOLD signal, albeit at a significant loss of detection sensitivity.