Methemoglobin Suppression in a 0.3 Tesla Magnet. An In Vitro and In Vivo Study

Rationale and Objectives: The aim of this study was to design a pulse sequence that suppresses methemoglobin and is applicable in a 0.3-T permanent magnet. Materials and Methods: Blood samples were collected from six healthy volunteers. Magnetic resonance imaging was performed with a 0.3-T scanner until the typical signal intensities of methemoglobin were obtained. Each blood sample was then scanned using repetition times of 300, 600, 900, 1200, 1500, and 1800 ms and a constant echo time of 20 ms. All other parameters (field of view, slice thickness, matrix, and number of signal averages) were kept constant for all six sequences. Signal intensities and repetition time data were used to calculate the T1 relaxation time of extracellular methemoglobin. T1 was determined by ordinary least square regression according to the equation S = k(1 - e^{-repetition time/T1}), where S is signal intensity and k is the proportional constant. A short tau inversion recovery sequence with an inversion time calculated from the T1 value of methemoglobin was used on fat, water, and methemoglobin blood samples and in 11 patients diagnosed with subacute brain hemorrhages. The inversion time was calculated as ln2 T1 (tissue), where T1 is the relaxation time of extracellular methemoglobin to be suppressed. Results: The T1 relaxation time of extracellular methemoglobin was determined to be 231.24 ± 9.068 ms, and inversion time was calculated to be 160 ± 6.67 msec. Application of an inversion time of 160 ms showed complete suppression on extracellular methemoglobin blood samples and all of the 11 subjects. Conclusion: A methemoglobin suppression technique using an inversion time of 160 ± 6.67 ms is applicable in a 0.3-T permanent magnet.