14 High-Field Clinical MR Imaging
14 High-Field Clinical MR Imaging
Dramatic advances have been made in medical MR imaging in recent years.
The magnetic field strengths typically used for routine clinical imaging range from 0.2 to 1.5 T. Over the past several years, systems operating at higher field strengths have become more prevalent, particularly at research centers. At the same time, interest in clinical imaging at 3 T is increasing as well. Available data suggests that magnetic field strengths above 2 T involve no increased risk for patients. The maximum field strength approved by the US Food and Drug Administration (FDA) for routine clinical applications is 4 T. Current clinical interest focuses on 3-T scanners although it is already evident that even higher field strengths will be used to examine patients in the future. In the framework of scientific studies, MR scanners operating at 7 tesla have already been used in humans.
The 3-T MR imagers that are commercially available today do not dif- fer from 1.5-T machines in terms of scanner architecture. These are whole- body scanners just like the MR systems operating at 1.5 T or lower field strengths.
The strongest argument in favor of switching to a higher field strength is the expected boost in signal-to-noise ratio (SNR) as the MR signal increases roughly in proportion to the field strength. In theory, the SNR would thus be doubled at 3 T compared with 1.5 T. The better SNR achieved with a high-field scanner can be used to improve spatial resolution or reduce im- aging time. An improved spatial resolution might permit a better evaluation of anatomy so far only inadequately visualized by MRI. Alternatively, with shorter scan times, MR systems can be operated more economically because more patients can be examined. Finally, imaging at 3 T or even higher field strengths has the potential to improve more sophisticated applications of MRI such as functional imaging (spectroscopy or perfusion imaging and the like).
In conclusion, high-field MR imaging has both advantages and disadvan- tages which the user must be aware of when switching to this new technol- ogy.
14.1 Tissue Contrast
Higher field strengths alter the T1 and T2 relaxation times of biological tis- sues. T1 is usually longer at 3 T compared with 1.5 T while T2 is shorter.
This means that one has to adjust the TRs and TEs of different pulse se- quences when they are to be used on 3-T scanners. For spin echo and fast spin echo sequences, a longer TR is needed at 3 T to achieve a similar con- trast as with 1.5 T. Conversely, TE should be somewhat shorter in order to compensate for the longer T1 relaxation times at 3 T.
14.2 Magnetic Susceptibility
Susceptibility effects (▶ Chapter 13.4) increase in proportion to the field strength of the magnet. As a result, image distortion may increase and de- grade image quality especially when GRE sequences are used. Conversely, the stronger susceptibility effects may be advantageous in conjunction with MR techniques such as perfusion imaging (▶ Chapter 11.2) where they con- tribute to image contrast.
14.3 Chemical Shift
Chemical shift in Hz increases in proportion to the magnetic field strength.
The larger chemical shift is advantageous in spectroscopy where the spectral lines are spread farther apart. This improves spectral resolution and discrim- ination of the peaks of fat and water, which in turn enables better calibration of the frequency-selective RF pulse for fat suppression. MR spectroscopy at 3 Tesla can be performed with a smaller scan volume, thereby reducing contamination of the spectrum from outside the area of interest.
14 High-Field Clinical MR Imaging
14.4 Radiofrequency (RF) Absorption
The amount of energy deposited in the body by an RF field is proportional to the square of the field strength and is thus significantly greater for high- field scanners. The threshold for energy absorption in the body (primarily in the form of heat), defined as the specific absorption rate (SAR), is there- fore more easily reached. This limits the scan times that are theoretically feasible on high-field scanners as the possible succession of pulses must be slowed down to prevent overheating. These limitations must be borne in mind when sequences optimized for 1.5 T are used on 3-T scanners. Spe- cifically, four times as much RF energy needs to be applied per unit time to achieve the same flip angle at 3 T as at 1.5 T. A sequence with a pulse dura- tion and amplitude optimized so that energy deposition is just below the SAR threshold at 1.5 T will exceed the upper limit at 3 T. This limits the use of sequences with high SAR values such as SE and FSE sequences.
Various strategies are available to minimize the overall SAR. A promising approach is to use a series of variable flip angles (VFA), which differ both in size and temporal spacing. The VFA strategy is associated with less en- ergy exposure because the shorter intervals between two refocusing pulses reduce overall scan time while the resulting MR signal remains the same.
Another promising technique is parallel imaging (▶ Chapter 10), which re- duces RF energy deposition by applying fewer refocusing pulses per echo train while echo time is kept constant.
