We are writing to emphasize the clinical implications of using ultra-high field (UHF) magnetic resonance imaging (MRI) (7 Tesla (7T) MRI) for the detection and management of systemic malignant tumors such as brain metastases (BMs). Our two recent systematic reviews, based on 89 studies of BMs from breast and lung cancers, have indicated the critical role of conventional and advanced MRI in diagnosing and monitoring BMs [1, 2].

Conventional sequencing may miss subtle lesions, particularly those with minimal enhancements. UHF MRI has revolutionized the field by offering an unprecedented level of detail. 7T MRI provides high-resolution images that allow for a more detailed visualization of tumor structures and margins [3]. In addition, the high signal-to-noise ratio in 7T MRI, together with greater spatial and contrast resolutions, allows the visualization of fine anatomical details. This enables the detection of small metastases that may be missed by standard low-field-strength scanners [4, 5].

Tumors often have higher metabolic rates than healthy tissues. The improved signal-to-noise ratio of 7T MRI allows for the more sensitive detection of metabolites, such as choline, which is a marker of increased cell turnover often associated with tumors, such as BMs [5]. Additionally, UHF MRI reveals subtle changes in tissue structure, such as small variations in water diffusion, allows better visualization of small blood vessels, and has great potential to map functional connections in the brain. These abilities help clarify early tumor infiltration, identify early tumor angiogenesis, and elucidate how tumors disrupt brain networks [3, 5, 6].

7T MRI exploits magnetic susceptibility phenomena, such as those relating to susceptibility-weighted imaging (SWI), functional MRI, and magnetic resonance spectroscopy (MRS) [6, 7]. Magnetic susceptibility effects scale linearly with B0, enabling 7T MRI to create new contrasts based on small differences in susceptibility [8]. This higher sensitivity to susceptibility opens new frontiers for SWI, allowing the contrasting of tissues containing different paramagnetic and diamagnetic substances [7]. Higher sensitivity to deoxyhemoglobin improves the detection of neurovascular coupling in functional MRI and thus allows for higher spatiotemporal resolution and sensitivity [7]. In addition, MRS at 7T separates neurochemicals and increases detectability relative to measurements made with weaker magnetic fields [9]. The high signal-to-noise ratio in 7T MRI results in minimal averaging and thus a short scan time and improved overall imaging quality [9].

A growing body of literature, including studies on conventional and advanced MRI techniques, has recognized the potential of 7T MRI for the early detection of BMs as a promising area of research [6, 10-13]. It is known that 3T MRI has higher sensitivity than 1.5T MRI [14], and the increased susceptibility effects at 7T provide greater sensitivity for metastatic lesions in SWI, such as in the case of hemorrhage [15]. A primary study found that 7T MRI detected 20% more cerebral microhemorrhages in BMs than 1.5T MRI [9]. As bleeding is common in BMs, 7T SWI may identify additional small or early metastatic lesions that are not seen in standard MRI [11]. In addition, vessel wall imaging benefits from the high resolution possible at 7T, enabling the assessment of vessel wall thickness and small vessel abnormalities [9, 11]. UHF MRI is thus unparalleled in providing high-resolution imaging with smaller voxel volumes and in providing novel neuroimaging biomarkers for characterizing neuroinflammation and neurovascular alterations in BMs.

In general, SWI may not be sufficiently sensitive for the detection of BMs. Recent studies have investigated the use of quantitative susceptibility mapping in obtaining information about the microstructure and composition of brain tissue [7, 9]. Quantitative susceptibility mapping is a post-processing method that creates a susceptibility map from (magnitude and phase) SWI data, enabling high-resolution imaging in a UHF [16].

Early detection of BMs is crucial for timely intervention and thus improves patient prognosis by enabling treatment before the tumor has the chance to grow and spread further. In this regard, identifying the specific characteristics of tumors through markers such as increased metabolic activity or angiogenesis can assist in the more effective tailoring of treatment plans. 7T MRI is a promising tool for monitoring a patient's response to treatment in a non-invasive way. By visualizing changes in metabolic activity, blood flow, and microstructural features, doctors can gauge the effectiveness of treatment and make adjustments if necessary. This approach can avoid the unnecessary side effects of ineffective therapies. Moreover, subtle changes in the tumor captured by 7T MRI might signal early recurrence before the recurrence becomes clinically apparent, allowing for prompt intervention when a tumor is potentially more manageable. Ultimately, earlier detection and better treatment selection can minimize the damage caused by BMs, potentially leading to an improved quality of life for patients.

While acknowledging the potential drawbacks of 7T MRI, such as its high cost and limited availability, and technical issues, such as signal inhomogeneity and geometric distortions [17], we emphasize that these challenges are not insurmountable. Technology and data analysis are continuously improving, and the clinical value of UHF MRI is increasingly being recognized.

AUTHOR CONTRIBUTIONS

Sadegh Ghaderi and Sana Mohammadi contributed to the conception and design of the study, Sadegh Ghaderi and Sana Mohammadi contributed to data collection, and Sadegh Ghaderi and Sana Mohammadi contributed to the drafting of the manuscript. Sadegh Ghaderi and Sana Mohammadi revised all sections. The final version was approved by all authors.

CONFLICT OF INTEREST STATEMENT

The authors declare no financial or other conflicts of interest.

ETHICS STATEMENT

There were no human or animal subjects in this study.

INFORMED CONSENT

Not applicable.