The lecture covers the basic principles of the molecular origin of image contrast, how the choice of sequence type and imaging parameters influence contrast and the implementation of additional sequence components to create a specific image contrast. The theory behind the presented concepts will be discussed and illustrated with examples from relevant applications.
- Understand the basic principles of the molecular origin of image contrast
- Understand how particular sequences and imaging parameters influence contrast
- Understand how to implement additional sequence components to create a specific image contrast
This lecture explains the main principles for generating contrast in MRI. The molecular origin of these contrast mechanisms is briefly explained. The most common sequences and techniques are discussed. This list of contrast mechanisms is not comprehensive, but more sophisticated techniques can be considered to be variations of these basic methods. All presented concepts are illustrated with relevant applications.
Inherent to almost all imaging sequences is contrast resulting from variations of spin density, ρ, and relaxation of longitudinal and transverse magnetization, T1 and T2. The underlying irreversible molecular relaxation processes are illustrated briefly. Another important source for image contrast is the signal phase. In presence of microscopic susceptibility variations, the phase of the transverse magnetization varies spatially causing additional signal attenuation. In the case of static averaging, that is, spins are not moving, of the phase variations within a voxel the resulting attenuation of the transverse magnetization is referred to as T2*-relaxation. Dynamic averaging implies that spins diffuse during the echo time and, hence, the refocusing of the signal phase is incomplete. The dispersion of the residual phase within a voxel induces signal attenuation described by the so-called apparent T2-relaxation time. The signal attenuation caused by phase variations is reversible and, therefore, can at least partially be recovered. The optimization for different types of image contrast depending on sequence type and the specific choice of imaging parameters is discussed. In particular, the differences between gradient-echo (GE) and spin-echo (SE) based sequences are detailed.
Another technique producing a unique image contrast exploits mesoscopic phase variations of the MR signal. The image phase is merged with the T2*-weighted signal amplitude to form a combined image contrast. This technique referred to as susceptibility-weighted imaging (SWI) is often used for visualizing of veins and venules or mapping the iron-concentration in deep brain structures.
Additional information about tissue characteristics can be imprinted on image contrast by modifying standard imaging sequences. As an example, in diffusion-weighted imaging a magnetic field gradient pair is used to generate an image contrast based on intra-voxel incoherent dephasing of moving spins. The technique, which is often used in stroke and cancer research and diagnostic, probes incoherent motion of blood flow in randomly oriented small blood vessels often labeled as “apparent” diffusion (ADC). True diffusion weighting, which is a measure for the Brownian motion of water molecules, is applied, for instance, in diffusion tensor imaging (DTI).
Another concept is the implementation of a preparation module, which produces a specific contrast, before the imaging component of the sequence. A simple example is an inversion-recovery (IR) or a saturation-recovery (SR) pulse for T1-weighting. Another prominent technique utilizes a magnetization transfer (MT) module which probes the exchange of spins between spin populations with different relaxation properties. Important applications of the basic MT imaging approach and more advanced methods, such as, chemical exchange spectroscopy (CEST), are measuring the integrity of myelination in white matter of brain and spinal cord or the cartilage structure in joints.
Despite its long history the development of new techniques with specific image contrast remains a vibrant research field. The objective is to search for new biomarkers, especially, for early disease stages and to further improve the specificity and sensitivity for diagnostics.
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