The overall purpose of this educational lecture is to discuss conventional and state-of-the-art acquisition methods for DCE-MRI. Topics will include protocol design informed by contrast mechanisms, sensitivity and spatiotemporal demands and advances in DCE-MRI acquisition methods based on technological advances (e.g. novel sampling schemes) and designed to enhance sensitivity to a greater range of physiologic properties.
At the end of this lecture participants should be able to:
Introduction
Dynamic Contrast Enhanced (DCE)-MRI methods rely upon the acquisition of dynamic MRI signals in order to track the dynamic passage of exogenously administered contrast agents (CA). DCE-MRI methods track changes in CA induced T1 changes in order to assess kinetic features such as the volume transfer coefficient, Ktrans, and the volume fraction of the extravascular space (ve). As one may expect, the reliability of the extracted kinetic features from DCE-MRI is heavily influenced by the pulse sequence type and input parameters. Acquisition details should account for the underlying biophysical basis of the contrast mechanisms, spatial, and temporal resolution and SNR requirements, and site-specific optimizations. The goal of this lecture will be to describe the motivation and requirements for robust DCE-MRI data acquisition, current best practices and opportunities for development.DCE-MRI Acquisition Methods
In order to estimate Ktrans and ve (the parameters most commonly assessed using DCE-MRI), the CA-induced MRI signal changes need to be assessed. The signal evolution in tissues of interest is slower than the first-pass kinetics measured with DSC-MRI, and enables the use of acquisitions with lower temporal and higher spatial resolution. With lower temporal resolution requirements, DCE-MRI protocols do not require EPI and are typically acquired using a 3D fast spoiled gradient recalled echo sequence. Organizations (e.g. RSNA-QIBA) have released standardized, best practice protocols for DCE-MRI to improve multi-site consistency and facilitate comparisons (1). Since DCE-MRI can be used in most human tissues, organ specific pulse sequence optimizations are frequently developed (2-4).
Kinetic analysis of DCE-MRI data requires pre-contrast T1 mapping in order to relate signal changes to CA concentration. While inversion recovery techniques are known to provide the most robust estimates of T1 their scan times are prohibitive, which has led to the use of multi-flip angle (which is the recommended approach) and, to a lesser extent, look locker techniques. For accurate kinetic analysis the arterial input function also needs to be quantified, thereby necessitating the use of higher temporal resolution (typically <10 sec/image) and rapid bolus injection (3 – 4 mL/sec). To enable higher spatial resolution multi-dose strategies may be used, where a low dose injection and high-temporal resolution imaging is used initially for AIF quantification followed by a standard dose of CA and higher spatial resolution dynamic imaging (5,6). AIF quantification may also be improved though the use of multi-echo acquisitions, as they enable the removal of unwanted, competing T2* contributions.
Technological improvements continue to shape DCE-MRI acquisition options, offering improved spatiotemporal resolution. Numerous studies have leveraged time-resolved MR angiography sequences, which are typically keyhole imaging methods, in order to achieve higher spatiotemporal DCE-MRI data in prostate and breast cancer (7-9). Further improvements have been gained through the application of compressed sensing or multi-band acquisition strategies (2,3,10-12).