With 7T MR the increase in signal to noise ratio, the higher BOLD contrast and higher spectral resolution offer additional benefits in the evaluation of brain cortex lesions and the hippocampal subfields and improved presurgical fMRI. High resolution spectroscopic imaging at 7T becomes feasible for tumors and MS. Sodium imaging provides noninvasive quantitative information of cartilage transplant quality. 31P MRS at 7T can differentiate between benign fatty liver disease and NASH. An additional clinical value of PET/MRI in terms of changes in management in up to 16% of cancer patients compared to PET/CT has been reported
Ultra-high field (7 Tesla) MRI
There is growing interest in ultra-high field MRI because of improved clinical results with regard to morphological as well as functional and metabolic capabilities. Moreover recently one 7T MR system was CE certified and FDA approved, which raises the question if the huge investments for the installation of a 7T system provide sufficient added clinical value or can even be a game changer in MR. As the signal-to-noise ratio scales linearly with the field strength (B0) of the scanner, the most obvious application at 7T is to obtain higher spatial resolution in the brain, musculoskeletal system and breast. Of specific clinical interest for neuro-applications is the cerebral cortex at 7T, for the detection of changes in cortical structure as a sign of early dementia, as well as for the visualization of cortical plaques in Multiple Sclerosis (MS) which have a prognostic value and focal cortical dysplasia in epilepsy. In the imaging of the hippocampus, even subfields of the internal hippocampal anatomy and pathology can be visualized with excellent resolution which supports prediction of the clinical outcome of temporal lobe surgery in cases of intractable temporal lobe epilepsy. The dynamic and static blood oxygenation level-dependent contrast increases even supralinearly with the field strength, which significantly improves the presurgical evaluation of eloquent areas before tumor removal. This is important in critical cases with the tumor very close to vital regions of the brain, where a high accuracy and spatial resolution of functional MRI is required. Using susceptibility-weighted imaging, the iron accumulation in multiple sclerosis can be visualized for the first time. The detection rate of chronic MS lesions surrounded by iron rings is significantly higher at 7T compared to 3T and their presence is associated with larger lesions at baseline and increase in lesion volume over several years and therefore helps to detect slowly progressive MS lesions and monitoring of disease. High resolution MR spectroscopic imaging becomes feasible at 7T which allows the additional mapping of pathological processes in MS on a biochemical level and reveals even well-delineated (sub-)cortical MS lesions down to ~3mm. Regions of MyoInositol (mIns) were often larger than on FLAIR and NAA maps, suggesting that mIns increase may be an earlier imaging biomarker for neuroinflammation/lesion development than conventional MR. A further improvement in MRSI at 7T is patch-based superresolution (PBSR) an up-sampling method shown to work better than standard interpolation techniques for MRSI maps. First application of PBSR to glioma measurements, reaches a submillimeter in-plane-resolution resolving tumor metabolism better than ever before in clinical feasible measurement times. The separation of glutamate and glutamine at 7T with the later being a further marker for cancer cells and targeting glutamine metabolism shows promise in anti-cancer therapy. In MR mammography high spatial and temporal resolution are simultaneously feasible at 7T and combined with submillimeter diffusion-weighted imaging improves the differentiation between benign and malignant breast lesions and may help to avoid unnecessary breast biopsies. Multi-nuclear clinical applications, such as sodium imaging for the evaluation of repair tissue quality after cartilage repair therapies and quantification of side effects of drugs on the composition of musculo-skeletal structures and 31P spectroscopy for the differentiation between non-alcoholic benign fatty liver disease and potentially progressive steatohepatitis, are only possible at 7T in clinically reasonable scan times. In comparison studies between 3T and 7T for routine neuro- and MSK imaging the diagnostic confidence for radiologists was significantly higher at 7T due to the higher spatial resolution and higher sensitivity to susceptibility effects at 7T.
PET/MRI
The advantages of PET/MRI include decreased radiation dose, improved motion correction, and the convenience of a combined exam. In head and neck cancer PET/MR provides more specific multiparametric evaluation of pathology with precise anatomic localization and increased conspicuity in cancer of unknown origin compared to PET/CT. In the evaluation of liver metastases the combination of multiphase MR with addition of hepato-biliary contrast agents, which is standard of care, with PET offers higher sensitivity and specificity for liver metastases. The multiparametric MR examination of prostate cancer with the possibilty of detection of lymphadenopathy in morphologically normal appearing pelvic lymph nodes is another benefit of PET/MR. In studies with PET/CT and immediately subsequent PET/MR exminations on the same day, a superiority of PET/MR over PET/CT with regard to clinical management was reported for 16% of cancer patients. This superiority was mainly due to better performance of PET/MR for the detection of brain, bone and liver metastases, while the superiority of PET/CT for the detection of lung metastases had only a minor clinical impact.
Further hardware and method developments in UHF MR and PET/MRI will continuously lead to emerging clinical indications of these high end MR systems.
Prudent et al. Human hippocampal subfields in young adults at 7.0 T: feasibility of imaging. Radiology 2010; 254: 900–906.
Wisse et al Subfields of the hippocampal formation at 7 T MRI: in vivo volumetric assessment. Neuroimage 2012; 61:1043–1049.
Blumcke et al. Defining cliniconeuropathological subtypes of mesial temporal lobe epilepsy with hippocampal sclerosis. Brain Pathol. 2012; 22: 402–411.
Blumcke et al. A new clinico-pathological classification system for mesial temporal sclerosis. Acta Neuropathol. 2007; 113: 235–244.
Henry et al. Hippocampal sclerosis in temporal lobe epilepsy: findings at 7 T(1). Radiology 2011; 261: 199–209. Stefanits et al. Seven-Tesla MRI of Hippocampal Sclerosis: An In Vivo Feasibility Study With Histological Correlations. Invest Radiol. 2017 Nov;52(11):666-671
de Graaf et al.Clinical application of multi-contrast 7 T MR imaging in multiple sclerosis: increased lesion detection compared to 3 T confined to grey matter. Eur. Radiol. 2013; 23: 528–540.
Tallantyre et al 3 and 7 Tesla MRI of multiple sclerosis cortical lesions. J. Magn. Reson. Imaging 2010; 32: 971–977.
Bluestein et al. Detecting cortical lesions in multiple sclerosis at 7 T using white matter signal attenuation. Magn. Reson. Imaging 2012; 30: 907–915.
Beisteiner et al. Clinical fMRI:evidence for a 7 T benefit over 3 T. Neuroimage 2011; 57:1015–1021.
Bagnato et al. Tracking iron in multiple sclerosis: a combined imaging and histopathological study at 7 Tesla. Brain 2011; 134: 3602–3615.
Tallantyre et al. A comparison of 3 T and 7 T in the detection of small parenchymal veins within MS lesions. Invest. Radiol. 2009; 44: 491–494.
Mistry et al. Central veins in brain lesions visualized with high-field magnetic resonance imaging: a pathologically specific diagnostic biomarker for inflammatory demyelination in the brain. JAMA Neurol. 2013; 70: 623–628.
Tallantyre et al. Ultra-high-field imaging distinguishes MS lesions from asymptomatic white matter lesions. Neurology 2011; 76: 534–539
Dal-Bianco et al. Slow expansion of multiple sclerosis iron rim lesions: pathology and 7 T magnetic resonance imaging. Acta Neuropathol. 2017 Jan;133(1):25-42
Hangel et al. Ultra-high resolution brain metabolite mapping at 7 T by short-TR Hadamard-encoded FID-MRSI. Neuroimage. 2016 Nov 4. pii: S1053-8119(16)30601-2. doi: 10.1016/j.neuroimage.2016.10.043.
Povozan et al. Simultaneous mapping of metabolites and individual macromolecular components via ultra-short acquisition delay 1 H MRSI in the brain at 7T. Magn Reson Med. 2018 Mar;79(3):1231-1240
Heckova et al. Real-time Correction of Motion and Imager Instability Artifacts during 3D γ-Aminobutyric Acid-edited MR Spectroscopic Imaging. Radiology. 2018 Feb;286(2):666-675
Gruber et al. Mapping an Extended Neurochemical Profile at 3 and 7 T Using Accelerated High-Resolution Proton Magnetic Resonance Spectroscopic Imaging. Invest Radiol. 2017 Oct;52(10):631-639
Altman et al. From Krebs to Clinic: Glutamine Metabolism to Cancer Therapy Nat Rev Cancer 2016 Nov;16(11):749
Korteweg et al. Feasibility of 7 Tesla breast magnetic resonance imaging determination of intrinsic sensitivity and high-resolution magnetic resonance imaging, diffusion weighted imaging, and H-1-magnetic resonance spectroscopy of breast cancer patients receiving neoadjuvant therapy. Invest.Radiol. 2011; 46: 370–376.
Pinker et al. Clinical application of bilateral high temporal and spatial resolution dynamic contrast-enhanced magnetic resonance imaging of the breast at 7 T. Eur. Radiol. 2014; 24: 913–920.
Gruber et al.. Dynamic contrast enhanced magnetic resonance imaging of breast tumors at 3 and 7 T: a comparison. Invest. Radiol. 2014; 49: 354–362.
Trattnig et al. Na-23 MR imaging at 7 T after knee matrix-associated autologous chondrocyte transplantation: preliminary results. Radiology 2010; 257: 175–184.
Zbyn et al. Evaluation of native hyaline cartilage and repair tissue after two cartilage repair surgery techniques with Na-23 MR imaging at 7 T: initial experience. Osteoarthr. Cartil. 2012; 20: 837–845.
Chang et al. Improved assessment of cartilage repair tissue using fluid-suppressed Na-23 inversion recovery MRI at 7 Tesla: preliminary results. Eur. Radiol. 2012; 22: 1341–1349.
Schmitt et al. Cartilage quality assessment by using glycosaminoglycan chemical exchange saturation transfer and Na-23 MR imaging at 7 T. Radiology 2011; 260: 257–264.
Krusche-Mandl et al. Long-term results 8 years after autologous osteochondral transplantation: 7 T gagCEST and sodium magnetic resonance imaging with morphological and clinical correlation. Osteoarthr. Cartil. 2012; 20: 357–363.
Szendroedi et al. Abnormal hepatic energy homeostasis in type 2 diabetes. Hepatology 2009; 50: 1079–1086.
Valkovic et al. Application of localized P-31 MRS saturation transfer at 7 T for measurement of ATP metabolism in the liver: reproducibility and initial clinical application in patients with non-alcoholic fatty liver disease. Eur.Radiol. 2014; 24: 1602–1609.
Springer et al. Comparison of routine brain imaging at 3 and 7T. Invest Radiol 2016; 51(8): 469-82
Springer et al. Comparison of routine knee magnetic resonance maging at 3T and 7T. Invest Radiol 2017; 52(1): 42-54
Ehman et al. PET/MR: Where might it replace PET/CT. J Magn Reson Imaging 2017; 46:1247
Catalano et al. Clinical Impact of PET/MR Imaging in patients with cancer undergoing same-day PET/CT: Initial experience in 134 patients - a hypothesis-generating exploratory study. 2013; 269: 857-869
Catalano et al. Comparison of CE-FDG-PET/CT with CE-FDG-PET/MR in the evaluation of osseous metastases in breast cancer patients. Br J Cancer 2015; 112: 1452-1460