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Blood volume (VB) in tissue

The fraction of vascular volume in tissues is usually determined with[15O]CO PET [Martin et al 1987]; see measurement of blood volume. Since carbon monoxide labels only red blood cells, a certain value for regional hematocrit is assumed.

Blood volume in the brain

Normal value for grey matter is 5.2 ± 1.4 %, for white matter 2.7 ± 0.6 %, and for cerebellum 4.7 ± 2.0 % [Leenders et al 1990]; a ratio of 0.85 for small vessel versus large vessel hematocrit was used in this study. Blood volume in grey and white matter was found to decrease with age approximately 0.50 % per year [Leenders et al 1990]. Blood volume can change during hypercapnia and hypocapnia, from 5.5 ± 0.6 % to 6.9 ± 1.2 and 5.1 ± 0.8 %, respectively, in cortical grey matter, and from 2.1 ± 0.5 % to 2.8 ± 0.8 and 2.0 ± 0.5 % in central white matter [Rostrup et al 2005]. Blood volume may also be lower in central grey matter (4.2 ± 1.0 %) than in cortical grey matter [Rostrup et al 2005].

Arterial fraction (fA) of blood volume (VB)

In full kinetic modelling, the vascular volume in the tissue volume must be taken into consideration. Sometimes, it is even necessary to separate the arterial and venous volumes of tissue vasculature.

Venous cerebral blood volume changes are much less (approximately 50%) than arterial blood volume changes [Schaller 2004]. Arterial or arterial and capillary blood volumes (Va or V0), can be estimated as one of the compartment model parameters from dynamic [15O]H2O or [15O]O2 studies, and those have even been considered to be more reliable hemodynamic parameters reflecting changes in cerebral arterial blood volume than VB (CBV) estimated from [15O]CO study [Okazawa et al 2001].

Estimates of regional arterial and venous blood volume fractions

Arterial fraction of cerebral blood volume in humans has been estimated using PET to be about 30 % [Ito et al 2001]. This is in line with the MRI measurement where the arterial blood volume fraction was found to be 29 ± 7 % in the total cerebral blood volume in the rat brain [Duong and Kim 2000]. This fraction is similar to that of the systemic circulation, but much higher than that (16 or 17 %) widely used for the measurement of cerebral metabolic rate of oxygen (CMRO2) using PET. The venous plus capillary fraction of cerebral blood volume was 63-70 % [Ito et al 2000].

Blood volume in the myocardial tissue

The regional intracapillary myocardial blood volume in humans (six CAD patients) has been measured to be 12.9% [Wacker et al. 2002]. This represents almost the entire intramyocardial blood volume (about 90%) because of the relatively low arterial and venous (about 5%) vascular volumes in myocardium [Wacker and Bauer, 2003]. Thus, we can estimate that the total vascular volume is about 14%.

Correcting PET data for vascular radioactivity

Please note that the peak radioactivity concentration in venous blood is substantially lower than in arterial blood. Arterial fraction of cerebral blood volume in humans has been estimated using PET to be about 30% (Ito et al. 2001). Therefore, the common assumption that arterial or venous blood curve represents total blood volume in tissue may lead to negative tissue concentrations during the blood peak and introduces a bias in results.

A certain blood volume fraction can be subtracted from the dynamic PET image data withimgcbv or from regional time-activity curves withdftcbv.

PET image

Program imgcbv requires the following command-line arguments:

  1. Dynamic [15O]O2 image file
  2. Pre-processed arterial blood datafile (if times are in seconds, program will give a warning)
  3. VB as a fraction (0-1)
  4. VB-corrected dynamic image file (output)

Regional TACs

Program dftcbv requires the following command-line arguments:

  1. Regional dynamic TACs (*.dft file)
  2. Pre-processed arterial blood datafile (times in minutes)
  3. VB as a fraction (0-1)
  4. VB-corrected regional dynamic TAC file (output)

References:

Duong TQ, Kim S-G. In vivo MR measurements of regional arterial and venous blood volume fractions in intact rat brain. Magn Reson Med 2000; 43: 393-402.

Ito H, Kanno I, iida H, Hatazawa J, Shimosegawa E, Tamura H, Okudera T. Arterial fraction of cerebral blood volume in humans measured by positron emission tomography. Ann Nucl Med 2001; 15: 111-116.

Leenders KL, Perani D, Lammertsma AA, Heather JD, Buckingham P, Healy MJR, Gibbs JM, Wise RJS, Hatazawa J, Herold S, Beaney RP, Brooks DJ, Spinks T, Rhodes C, Frackowiak RSJ, Jones T. Cerebral blood flow, blood volume and oxygen utilization. Normal values and effect of age. Brain 1990; 113: 27-47.

Martin WRW, Powers WJ, Raichle ME. Cerebral blood volume measured withinhaled C15O and positron emission tomography. J Cereb Blood FlowMetab 1987; 7: 421-426.

Okazawa H, Yamauchi H, Sugimoto K, Toyoda H, Kishibe Y, Takahashi M. Effects of acetazolamide on cerebral blood flow, blood volume, and oxygen metabolism: a positron emission tomography study with healthy volunteers. J Cereb Blood Flow Metab 2001; 21: 1472-1479.

Rostrup E, Knudsen GM, Law I, Holm S, Larsson HBW, Paulson OB. The relationship between cerebral blood flow and volume in humans. Neuroimage 2005; 24: 1-11.

Schaller B. Physiology of cerebral venous blood flow: from experimental data in animals to normal function in humans. Brain Res Rev 2004; 46: 243-260.

Wacker CM, Bauer WR. Neue Ansätze der Magnetresonanztomographie zur Beschreibung myokardialer Mikrozirkulationsparameter am Menschen. Herz 2003; 28(2): 74-81.

Wacker CM, Wiesmann F, Bock M, Jakob P, Sandstede JJW, Lehning A, Ertl G, Schad LR, Haase A, Bauer WR. Determination of regional blood volume and intra-extracapillary water exchange in human myocardium using Feruglose: first clinical results in patients with coronary artery disease. Magn. Reson. Med. 2002; 47: 1013-1016.


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