Ultra high spatial and temporal resolution breast imaging at 7T

Abstract

There is a need to obtain higher specificity in the detection of breast lesions using MRI. To address this need, Dynamic Contrast-Enhanced (DCE) MRI has been combined with other structural and functional MRI techniques. Unfortunately, owing to time constraints structural images at ultra-high spatial resolution can generally not be obtained during contrast uptake, whereas the relatively low spatial resolution of functional imaging (e.g. diffusion and perfusion) limits the detection of small lesions. To be able to increase spatial as well as temporal resolution simultaneously, the sensitivity of MR detection needs to increase as well as the ability to effectively accelerate the acquisition. The required gain in signal-to-noise ratio (SNR) can be obtained at 7T, whereas acceleration can be obtained with high-density receiver coil arrays. In this case, morphological imaging can be merged with DCE-MRI, and other functional techniques can be obtained at higher spatial resolution, and with less distortion [e.g. Diffusion Weighted Imaging (DWI)]. To test the feasibility of this concept, we developed a unilateral breast coil for 7T. It comprises a volume optimized dual-channel transmit coil combined with a 30-channel receive array coil. The high density of small coil elements enabled efficient acceleration in any direction to acquire ultra high spatial resolution MRI of close to 0.6 mm isotropic detail within a temporal resolution of 69 s, high spatial resolution MRI of 1.5 mm isotropic within an ultra high temporal resolution of 6.7 s and low distortion DWI at 7T, all validated in phantoms, healthy volunteers and a patient with a lesion in the right breast classified as Breast Imaging Reporting and Data System (BI-RADS) IV.

A 16-Channel Receive, Forced Current Excitation Dual-Transmit Coil for Breast Imaging at 7T

“…no acceleration). The SNR map was calculated by dividing, on a pixel-per-pixel basis, the signal from the phantom image by the standard deviation of a 30 mm×30 mm area at the same location in the noise-only scan [29]. As is customary, the noise-only scans from the individual 16 elements were also used to generate a noise correlation matrix for the array.…”

“…Thus, much of the work in breast imaging has focused on optimizing the RF coil design [23], [24]. Unilateral and bilateral breast receive-only and transmit/receive array coils with up to 30 elements have been presented, working in concert with a variety of transmit coil designs [25]–[29]. We have previously reported the advantages of transmitting with “forced current excitation” (FCE) in the design of a highly homogenous quadrature volume breast coil for 7T [30].…”

Magnetic Resonance Imaging at Ultrahigh Fields

“…Consequently, work carried out at 4 T and higher magnetic fields have been restricted predominantly to the head or the extremities [5] and breast [182] where the object size relative to the RF wavelength employed is manageable. In the torso, the exception was imaging the prostate but with a small endorectal coil positioned immediately adjacent to the tissue of interest [51], [182]. …”

Safety of Ultra-High Field MRI: What are the Specific Risks?

“…This rapid growth in the number of UHF-MRI sites has quickly led to the development of scanning protocols [1][2][3] and the required hardware (brain [4][5][6], cardiac [7,8], body [9,10], spine [11,12], breast [13,14], and eye [15] ) to enable the use of UHF-MRI in both fundamental and clinical research studies as well as in pilot studies for pure clinical use [16][17][18]. A recent review article provides a comprehensive overview of the current status of high field MRI [19 •• ].…”