Accuracy and precision of MR blood oximetry based on the long paramagnetic cylinder approximation of large vessels

Abstract

An accurate noninvasive method to measure the hemoglobin oxygen saturation (%HbO 2 ) of deep-lying vessels without catheterization would have many clinical applications. Quantitative MRI may be the only imaging modality that can address this difficult and important problem. MR susceptometry-based oximetry for measuring blood oxygen saturation in large vessels models the vessel as a long paramagnetic cylinder immersed in an external field. The intravascular magnetic susceptibility relative to surrounding muscle tissue is a function of oxygenated hemoglobin (HbO 2 ) and can be quantified with a field-mapping pulse sequence. In this work, the method's accuracy and precision was investigated theoretically on the basis of an analytical expression for the arbitrarily oriented cylinder, as well as experimentally in phantoms and in vivo in the femoral artery and vein at 3T field strength. Errors resulting from vessel tilt, noncircularity of vessel cross-section, and induced magnetic field gradients were evaluated and methods for correction were designed and implemented. Hemoglobin saturation was measured at successive vessel segments, differing in geometry, such as eccentricity and vessel tilt, but constant blood oxygen saturation levels, as a means to evaluate measurement consistency. The average standard error and coefficient of variation of measurements in phantoms were <2% with tilt correction alone, in agreement with theory, suggesting that high accuracy and reproducibility can be achieved while ignoring noncircularity for tilt angles up to about 30°. In vivo, repeated measurements of %HbO 2 in the femoral vessels yielded a coefficient of variation of less than 5%. In conclusion, the data suggest that %HbO 2 can be measured Key words: magnetic resonance oximetry; blood oxygen saturation; phase imaging; demagnetizing field; magnetic resonance susceptometry Accurate noninvasive measurement of hemoglobin oxygen saturation (%HbO 2 ) of deep-lying vessels inaccessible to pulse oximetry would have many clinical applications. Among these are the quantification of global cerebral metabolic rate of oxygen (CMRO 2 ) and the assessment of congenital heart defects in pediatric patients. Generally, both currently require catheterization procedures, which are invasive and carry risks. In the case of jugular bulb oximetry, the risks (1) include arterial puncture (between 1% and 4.5% has been reported in the literature), jugular vein occlusion from misplacement of catheter, and subclinical thrombosis, whose incidence rate is as high as 40%. Furthermore, accuracy is highly dependent on the position of the catheter against the vessel wall, and simultaneous measurement of both the left and right sides is not possible but important due to asymmetric venous drainage (2).Noninvasive quantification of blood oxygenation can be achieved with MRI by measuring T 2 (3) or T * 2 (4), or from a measurement of the relative susceptibility (5-7) of intravascular blood since all three quantities depend on the volume fraction of oxygenated hemoglobi...

Cortical Cerebral Blood Flow, Oxygen Extraction Fraction, and Metabolic Rate in Patients with Middle Cerebral Artery Stenosis or Acute Stroke

“…When the length of the vessel exceeds 6ϫ its diameter, it can be fitted to the longcylinder model in MR susceptometry. 8 Consequently, we selected veins that had a length of at least 6ϫ their diameter. The orientation of the vein in the gray matter was verified as parallel to B 0 by tracing the vessel through several adjacent axial, sagittal, and coronal sections.…”

“…5,6 MR susceptometry uses gradient-echo phase maps to estimate oxygen saturation in segments of the jugular and gray matter veins. 7,8 This technique requires blood vessels to have a substantially greater length than the diameter. 7,8 Susceptibility-weighted imaging is a type of MR susceptometry that can obtain measurements of venous oxygen saturation by using the phase difference between the venous blood and surrounding tissue.…”

“…7,8 This technique requires blood vessels to have a substantially greater length than the diameter. 7,8 Susceptibility-weighted imaging is a type of MR susceptometry that can obtain measurements of venous oxygen saturation by using the phase difference between the venous blood and surrounding tissue. 9,10 ASL is a noninvasive technique that can measure CBF.…”

Imaging Brain Oxygenation with MRI Using Blood Oxygenation Approaches: Methods, Validation, and Clinical Applications

“…Possible errors resulting from vessel tilt, noncircularity of vessel cross-section, and induced magnetic field gradients have been evaluated. 67,68 These effects are generally low, and methods for correction have already been designed and implemented. The main drawback of susceptometry-based oximetry remains that only medium-to large-sized vessels can be targeted, providing mainly global brain oxygen information, though a recent study has proposed extending the technique to smaller veins with favorable orientations.…”

Transcranial Imaging of Functional Cerebral Hemodynamic Changes in Single Blood Vessels using in vivo Photoacoustic Microscopy

“…Functional magnetic resonance imaging offers multiparametric and noninvasive measurements of blood flow, and relative changes in blood oxygenation, blood volume, and oxygen metabolism (Davis et al, 1998;Mandeville et al, 1998) without depth limitations. However, it is still challenging to isolate pure hemoglobin oxygen saturation (SO 2 ) changes without administration of exogenous contrast agents, especially when measuring a transient functional hemodynamic response in single cerebral vessels (Langham et al, 2009).…”