MR Imaging Techniques and Concepts

One of the major challenges of MRI in the abdomen has centered on the problem of acquiring data from tissue that normally moves in relation to respiration, with additional undesirable effects from cardiovascular pulsation and bowel peristalsis.
MRI of the liver initially relied upon standard spin-echo (SE) T1-weighted and T2-weighted methods. However, since these are sequences that acquire data over a long time window relative to respiratory movement [3, 8, 13] they require supplemental techniques of respiratory gating, which in turn adds to the total acquisition time. Moreover, the reliability of the examination is reduced as even minor inconsistent respiratory gating can yield non-diagnostic images. Use of these techniques can lead to total procedure times in excess of 60 min. Currently employed MR techniques focus on shorter sequences that can be completed within a breath-hold. These include T1-weighted fast spoiled gradient echo (SGE) and breath-hold half-Fourier transform single shot spin-echo (HASTE or ssfse) methods (Fig.1).Tables 1 and 2 summarize the nomenclature for the majority of the currently employed recommended sequences. The single shot spin-echo sequences are slice selective, performing all of the preparation and acquisition for an individual slice in approximately 1 sec, with the central k-space data acquired over a fraction of that time. As the image contrast is derived from the central k-space, single shot techniques are remarkably motion insensitive, and have respiratory-independent characteristics that are useful in non-compliant patients . T1-weighted 2-dimensional (2D) or 3-dimensional (3D) gradient echo sequences tend to be motion sensitive as these techniques use interleaved phase lines: the phase lines are collected from each image slice one phase line at a time moving from slice-to-slice. A consequence is that even transient motion occurring during only a fraction of the acquisition will affect all the slices. T1-weighted techniques with motion insensitive properties are also available. These use the same  basic concept applied to the T2-weighted single shot technique: 2D data with rapid filling of the central k-space are acquired by preparing and analyzing one slice at a time. In this case, however, spoiled gradient echo sequences that utilize an inversion or saturation pre-pulse are used to gener
generate the T1 contrast. These sequences have been referred to as turbo fast low-angle shot (turboFLASH) and fast inversion-recovery motion- insensitive (FIRM).Another development has been the application of 3D gradient echo sequences modified from MR angiographic techniques. These have various vendor-specific names, including the first description of this technique, known as Volumetric Interpolated Breath-hold Examination (VIBE) [33]. This approach facilitates the generation of high-resolution images of the liver, particularly out-of-phase resolution,with the ability to generate near isotropic voxel sizes in the order of 2-3 mm when combined with interpolation techniques. Such an approach allows better evaluation of hepatic vascular anatomy, and generates volumetric datasets that can be used for multiplanar reconstruction.
Another critical element in T1-weighted breathhold imaging is the use of intravenously-administered gadolinium contrast agents. These agents shorten the T1 relaxation rate of tissues resulting in marked elevation of signal on T1- weighted images. They are used to assess focal liver lesions based upon characteristic vascular enhancement patterns which can be distinguished from adjacent normal hepatic parenchyma.
The liver is unique in having a dual blood supply, receiving 70-80% of afferent blood flow from the portal vein, and the remainder from the hepatic artery. Hepatic tumors develop selective portal-venous or hepatic arterial blood supply based on their specific characteristics. Tumors that derive blood supply from hepatic arterial branches are best visualized during the hepatic arterial dominant phase of liver enhancement.Hypovascular tumors are predominantly supplied by portal-venous branches, and demonstrate enhancement-time curves that are different from normal liver, having slower and less intense enhancement, which is in part due to the lack of contribution from the hepatic arterial supply, and in part to their lower total intravascular volume per gram of tissue. Over time, measured in units of minutes, the gadolinium concentration may slowly increase in a non-uniform pattern within these tumors due to leakage of the contrast agent into the interstitial spaces. Therefore, a key to diagnostic MR liver exams is dynamic multiphase post-gadolinium SGE imaging,which provides information regarding the time intensity curves of hepatic lesions . Gadolinium contrast agents are extremely safe; the risk of serious reactions is estimated at around 1 per million while the risk for mild reactions is estimated at around 2.5%. Although contrast agents based on super-paramagnetic iron oxide (SPIO) and manganese have also been developed for MR imaging of liver lesions.
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