Localized pH changes have been suggested to occur in the brain during normal function. However, a lack of methods for non-invasively measuring pH with high spatial and temporal resolution limits current knowledge of brain pH dynamics. Here I report that a magnetic resonance imaging (MRI) strategy named T1 relaxation in the rotating frame (T1ρ) is sufficiently sensitive to detect widespread pH changes in the mouse and human brain evoked by systemically manipulating carbon dioxide (CO2) or bicarbonate (HCO3). Moreover, T1ρ detected changes suggesting a localized acidosis in the human visual cortex induced by a flashing checkerboard. Lactate measurements and pH-sensitive 31P spectroscopy at the same site also suggest a localized acidosis. Consistent with the established role for pH in blood flow recruitment, T1ρ correlated with blood oxygenation level dependent contrast (BOLD), although T1ρ was directly sensitive to blood oxygen content. These observations provide the strongest evidence thus far for localized pH fluctuations in the human brain during normal function. Furthermore, they suggest a novel functional imaging strategy based on pH that is independent of traditional fMRI contrast mechanisms. Possible sources of acidosis include local metabolism, which is likely to correlate with the degree of stimulation and the associated changes in local neural activity. Therefore, we hypothesized that T1ρ and pH changes would increase with increasing stimulation frequency. To test this hypothesis, we used a full-field visual flashing checkerboard and varied the frequency between 1, 4, and 7Hz. The response was imaged with T1ρ, BOLD, and 31P spectroscopy. Supporting our hypothesis, we found that increasing stimulation frequency increased responses measured by all three imaging modalities. The activation area detected by T1ρ overlapped to a large degree with that detected by BOLD, although the T1ρ response area was significantly smaller. 31P spectroscopy detected a greater acidosis with the higher stimulation frequencies. These observations suggest that, similar to the BOLD response, the magnitude of the T1ρ and pH response depends on stimulation frequency and is thus likely to be activity-dependent. Brain acidosis is the end product of energy metabolism. Metabolically active cells lower local pH, the detection of which could help pinpoint regions activated by sensory stimuli, emotion, or cognitive task. fMRI mostly relies on BOLD changes in the venous system while arterial spin labeling (ASL) enables changes in tissue perfusion resulting from local cerebral blood flow (CBF) changes. BOLD contrast can be significantly distant from the actual site of neuronal activity because it relies on changes of the local magnetic field within veins. The venous contribution results in a loss of spatial specificity and spatial resolution of the BOLD response. In addition, the hemodynamic response to brief periods of neural activity is delayed. However, ASL contrast originates predominantly from tissue and capillaries. Even though functional signal changes detected by ASL have superior spatial and temporal resolution as compared to BOLD contrast, ASL contrast still suffers from poor temporal resolution due to delays in the hemodynamic response resulting from neurovascular coupling. Therefore, the ability to measure pH dynamics may provide a more localized and direct measure of brain activity. We hypothesized that pH-sensitive T1ρ response in the visual cortex will temporally precede the hemodynamic response measured by functional imaging including BOLD and ASL contrast since local acidosis evoked by neural activity may drive the hemodynamic response. To test this hypothesis, dynamic imaging was performed using T1ρ, BOLD, and ASL while viewing a phase-encoded expanding ring stimulus which induces travelling waves of neural activity in the visual cortex. We calculated the phase maps for the eccentricity across their occipital cortices for each of functional signal and compared the T1ρ temporal resolution with the hemodynamic response. This study suggests that T1ρ signal has a higher temporal resolution as compared to the hemodynamic response. This is further evidence that the T1ρ signal is not sensitive to blood oxygenation or other blood factors that might alter T1ρ. In conclusion, T1ρ imaging has the potential to provide a new functional imaging marker that may be more specific to the area of brain activity. Therefore, it is possible that by non-invasively detecting pH dynamics in the human brain, T1ρ MRI could offer a novel, more direct approach to map brain function. A number of psychiatric and neurological disorders could potentially benefit from the ability to study dynamic pH changes.

Spin-lattice relaxation in the rotating frame (T1ρ) is a relaxation parameter measured in nuclear magnetic resonance studies. This parameter has been found to be sensitive to chemical exchange processes occurring in diseased tissue associated with abnormal metabolism when measured in magnetic resonance imaging (MRI). Metabolic changes in tissue affected by abnormal metabolism can be quantified with good spatial and temporal resolution using T1ρ, better than a similar method of assessment known as CEST and current methods such as spectroscopic (1H- and 31P-MRS) and nuclear medicine (PET) methods used in clinical settings. Though T1ρ has these advantages, there is no consensus as to which metabolic changes T1ρ is most sensitive. The metabolic changes may be pH related, or due to changes in concentration of metabolites like glucose and glycogen. This work is tries to identify which metabolite evokes the greatest change in T1ρ by studying the response of three spin relaxation measures (T1ρ, T2 and T1) at different temperatures. It was found that T1ρ is more sensitive to pH changes than glucose and lysine at 3T. Also at body temperature, the pH results showed an exponential decay trend for T1ρ signifying the limited range of sensitivity in the pH range of 6.9 to 7.5. The T2 results can be used to explain this trend.

T1rho MRI, spin-lattice relaxation in the rotating frame, is postulated to be sensitive to early biochemical changes within articular cartilage that may lead to osteoarthritis. This means that it has potential as a non-invasive, early biomarker for disease progression. However, T1rho has been primarily studied in a research setting. Therefore, the main question posed in this work is: Can T1rho MRI be used in an at-risk population (ACL-rupture patients) and translated to a clinical setting? To answer this question, two tools (Relaxometry program, Line Profile Analysis) were created and validated for measuring T1rho within living subjects. These tools were used to answer the following sub-questions: 1) Is there a quantifiable difference between healthy (normal subject)and injured cartilage (ACL-rupture patient) in living subjects? 2) Is there some measure which can be tracked over time and used as a meaningful pre-screening procedure for post-traumatic osteoarthritis development at 3.0T MRI? 3) Is it possible to obtain the same or similar quality data from 1.5T T1rho (predominant MRI field strength found in clinics) as found from 3.0T T1rho images? These questions were answered affirmatively, and the author concludes that T1rho could be translated and applied into a clinical setting across the nation.

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