Magnetic relaxation in blood and blood clots

Abstract

Nuclear magnetic relaxation rates are measured for whole blood, blood plasma, whole blood clots, and plasma clots in vitro. Relaxation rates are linear in the hematocrit and transverse relaxation rates are significantly greater than longitudinal relaxation rates. Longitudinal relaxation rates measured from 0.01 to 42 MHz for proton Larmor frequencies are found to decline monotonically with increasing magnetic field strength; however, the dispersion curves do not follow a simple Lorentzian behavior, which is anticipated in a suspension of particles in a solution of proteins having a distribution of molecular weights. The transverse relaxation rate is a function of the acquisition parameters, in particular, the choice of TE in either Hahn echo experiments or in echo-train experiments. The origin of this dependence of T2 on TE or the interpulse spacing in an echo train is identified with the exchange of water from inside the red blood cell to the outside and is only an important relaxation mechanism in the case where the blood cell membrane is intact and the cell contains deoxygenated hemoglobin. The dependence of the apparent transverse relaxation rate on the interpulse spacing in a Meiboom-Gill-Carr-Purcell pulse sequence provides the estimate that the mean residence time of water inside the blood cell is about 10 ms. These data provide a sound basis for understanding the dependence of magnetic images on magnetic field strength and the choices of the image acquisition parameters, TE and TR.

Characterizing Brain Oxygen Metabolism in Patients with Multiple Sclerosis with T2-Relaxation-Under-Spin-Tagging MRI

“…T2-relaxation-under-spin-tagging MRI estimates global Yv through its effects on the T2-relaxation time in a large intracranial venous sinus (i.e., superior sagittal sinus). The T2-relaxation time is more accurate than T2* regarding their interrelationship with blood oxygenation (Bryant et al, 1990;Gomori et al, 1987;Thulborn et al, 1982;Wright et al, 1991) with high degree of precision and reliability. The Yv converted from T2 using TRUST has been shown in good agreement with PET studies .…”

Cerebral blood volume measurements by rapid contrast infusion and T‐weighted echo planar MRI

“…The relatively long TR of 3000 msec was chosen to minimize T 1 effects in arteries since this would compromise measurement of the ratio of signal intensity change between tissue and artery. The T 1 of blood before administration of contrast is approximately 1 sec (35), so that the TR of 3000 ms, at 3 times that value, should avoid significant T 1 -shortening effects of the contrast agent and produce mainly T* 2 effect. In support of this, preliminary studies (data not shown) of S 0 vs. TR showed that relative baseline signal of blood vessels without any contrast reached a maximum at or just above 2500 msec.…”

Inverse correlation between cerebral blood flow measured by continuous arterial spin-labeling (CASL) MRI and neurocognitive function in children with sickle cell anemia (SCA)

“…We obtained and processed the images per Oguz et al, but substituted specific values for the spin-lattice relaxation time (T1) of blood at 1.5 T based on packed-cell volume (PCV). 11,12 The vascular distributions of the major cerebral arteries 13 were independently outlined by 2 investigators and used to calculate regional CBF to gray matter. Blood studies, including PCV, white blood cell (WBC) count, and platelet count, were performed using an automated analyzer (Sysmex SE 9500; Roche, Indianapolis, IN) when the patients were in their usual state of health.…”