Magnetic resonance imaging (MRI) uses radiofrequency (RF) waves and a strong magnetic field to provide extremely detailed pictures of organs and tissues not visible using other imaging modalities. The technique has proven very valuable for the diagnosis of a range of conditions such as heart and vascular disease, cancer, and joint and musculoskeletal disorders. RF coils, a key component of MRI systems, can be likened to antennae that transmit and receive the signals used to generate images. An MRI coil can be thought of as a microscope in that you can only see what the coil lets you see.
Over the past two decades, MRI coils have been developed and refined for use at magnetic field strengths of 1.5 Tesla (T) or less. However, the coil technologies used at these field strengths are not optimal for recently developed MRI scanners that operate at higher magnetic field strengths (3T or 7T). Responding to this need at NYU, we are designing RF coils that are better suited for high field MRI, allowing us to develop novel ways of imaging the human body at 3T and 7T. We have already designed and built several innovative MRI coils capable of generating anatomical images of unprecedented resolution. More importantly, in doing so, we have made great progress in overcoming certain major technological hurdles that have previously hindered the development of high field MRI. We present three highlights of our research to offer a window into the exciting field of MRI coil development and the important progress being made by the NYU Department of Radiology.
PARALLEL IMAGING IN HYPERPOLARIZED HELIUM LUNG MRI
Hyperpolarized helium (3He) MRI is an innovative means to generate images of the airways and airspaces of the lungs by having the patient inhale hyperpolarized 3He, as a contrast agent, during an MRI. It is a safe, non-invasive technique that does not expose the patient to ionizing radiation and can generate both morphologic and functional data. However, the widely used single-channel RF coil has made it impossible to perform parallel imaging in conjunction with this technique, resulting in lengthy scan times requiring longer breath holds in sick patients. We have developed and implemented a multichannel (2ch transmit/24ch receive) 3He coil (FIGURE 1) that can be combined with parallel imaging to significantly improve temporal and spatial resolution at no cost in terms of the signal-to-noise ratio (SNR). Parallel imaging, by reducing the number of phase encoding steps, reduces the SNR in conventional, thermally polarized imaging. However, hyperpolarized gas imaging draws signal from a fixed pool of polarized molecules that do not recover magnetization. If the number of phase encoding steps is decreased, flip angles can be increased commensurately to make full use of that fixed pool. Thus, reduction of scan time does not result in SNR loss. As seen in FIGURES 2 AND 3, the SNR and resolution remain the same with and without applying parallel data acquisition. Equipped with the phased array coil in FIGURE 1, and utilizing parallel data acquisition, one can scan the entire lung in a single breath hold (13s) with integrated parallel acquisition techniques (iPAT), which is impossible with a single-channel coil (FIGURE 4).
Figure 1. 2ch transmit/24ch receive phased array system. (A) external view of the overall
mechanical fixture; (B) top shell internal views; (C) bottom shell internal views.
FIRST IN THE WORLD – 7T BREAST MRI
MRI has emerged as an important tool for the screening and detection of breast cancer, the second leading cause of cancer-related mortality for women in the U.S. Unlike mammography, MRI has high sensitivity, lacks radiation, is unaffected by breast density and size, and is sensitive to all tumor types. Despite these advantages, especially its high sensitivity for detection of abnormalities, conventional clinical MRI at lower field strengths (1.5T or 3T) has only moderate specificity, which often leads to a false positive diagnosis and unnecessary biopsy. Since the SNR in MR is proportional to the magnetic field strength, and the additional SNR at higher fields can be translated into higher spatial, temporal, or spectral resolution, it is logical to expect that the relatively poor specificity at 1.5T or 3T will be significantly improved when moving to a higher field strength, such as 7T. However, this transition also results in a much higher magnetic resonance frequency of ~ 300MHz, which precludes the use of the conventional, wellestablished coil designs developed for use at 1.5T. To perform high field-strength breast MRI it is first necessary to develop and optimize 7T breast coil technology.
We have successfully built and implemented the first 7T breast coil (FIGURE 5). In vivo imaging with this coil at 7T demonstrates a threefold SNR increase over 3T imaging without contrast agents. At 7T, the morphology of glandular tissue and ducts is clearly visualized (FIGURE 6), and the detailed structure, size, and margins of tumors are better defined (FIGURE 7). When performing dynamic contrast enhancement imaging with the 7T coil, the difference in the rate of contrast enhancement between benign and malignant tumors is much larger than at 3T (FIGURE 8 AND TABLE 1), a feature which improves the ability to distinguish between different tumor types. In addition, the contrast between tumor and glandular tissues is more than doubled due to the longer T1 at 7T (FIGURE 9).
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Table1: COMPARISON BETWEEN ABSOLUTE WASH-IN RATE DIFFERENCE AND RELATIVE WASH-IN RATE DIFFERENCE AT 3T AND 7T
|
3T |
7T |
|||
Benign |
Malignant |
Benign |
Malignant |
|
| Gd concentration (uM/mL) | 1.25 |
2.5 |
1.25 |
2.5 |
 S (%) |
260 |
349 |
1028 |
1848 |
| Wash-in rate W= S/ t (%/min) |
260 |
349 |
1028 |
1848 |
Absolute Difference |
Absolute Difference |
|||
| W(malignant) –W(benign)(%/min) | 89 |
820 |
||
Relative Difference |
Relative Difference |
|||
| [W(malignant) –W(benign)]/W(benign)(%) | 34 |
80 |
||