Local cerebral blood volume determined by three-dimensional reconstruction of radionuclide scan data DE Kuhl, M Reivich, A Alavi, I Nyary and MM Staum Circulation Research 1975, 36:610-619 Circulation Research is published by the American Heart Association. 7272 Greenville Avenue, Dallas, TX 72514 Copyright © 1975 American Heart Association. All rights reserved. Print ISSN: 0009-7330. Online ISSN: 1524-4571
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Local Cerebral Blood Volume Determined by ThreeDimensional Reconstruction of Radionuclide Scan Data By David E. Kuhl, Martin Reivich, Abass Alavi, Istvan Nyary, and Muni M. Staum ABSTRACT We developed a method to determine in man absolute values of local cerebral blood volume (LCBV) localized throughout the brain in three dimensions and presented in a cross-sectional picture format. Previously, absolute values of LCBV have been determined in vivo by stimulated X-ray fluorescence, but these determinations have been limited to one point in the brain at a time. All other previous estimates of LCBV by external emission counting have been contaminated by the significant contribution of blood in the overlying scalp and cranium. In our method, a transverse section scan is made after the injection of 99mTc-labeled red blood cells into a peripheral vein. Data processing then gives a point-to-point estimate of absolute radionuclide concentration analogous to an autoradiograph. After the concentration of blood activity is determined, counting data are converted to a two-dimensional map of LCBV representing a cross section at a known level of the brain. In a series of five baboons, the following equation was obtained for the regression plane that relates LCBV in the center of the brain to arterial carbon dioxide tension (Paco2) and mean arterial blood pressure (MABP): LCBV = 2.88 + 0.049Paco2 - 0.013MABP. In patients, LCBV values ranged from 2 to 4 ml/100 g depending on location; higher values corresponded to regions of cerebral cortex. Differences in blood volumes of focal brain lesions were also quantified. 99m KEY WORDS Tc-labeled red blood cells autoregulation of blood flow CO2 response
• Knowledge of local cerebral blood volume (LCBV) in the living patient would be useful for assessing cerebral hemodynamics. Changes in regional cerebral blood volume have been shown to occur in physiological states such as mentation (1), sleep (2), and alterations in arterial carbon dioxide tension (3) as well as in pathological states such as increased intracranial pressure (4). However, the accurate measurement of LCBV in vivo is difficult. From the Departments of Radiology (Nuclear Medicine) and Neurology, Hospital of the University of Pennsylvania, Philadelphia, Pennsylvania 19104. This investigation was supported in part by U. S. Public Health Service Grants 2R01 GM-16248 and 5 P01 NS-10939 from the National Institutes of Health and by U. S. Atomic Energy Commission Contract AT(ll-l)-3399. Dr. Reivich is a recipient of U. S. Public Health Service Research Career Development Award 5K03-HL-11,896; Dr. Alavi was supported by U. S. Public Health Service Training Grant 3 T01 GM01762. Dr. Nyary was on leave from the Experimental Research Department, University Semmelweiss Medical School, Budapest, Hungary. Please address reprint requests to: David E. Kuhl, M.D., Nuclear Medicine Division, Department of Radiology, Hospital of the University of Pennsylvania, Philadelphia, Pennsylvania 19104. Received June 3, 1974. Accepted for publication February 21, 1975.
610
baboons man section scanning
Previous estimates by external radionuclide counting have been complicated by the significant contribution of blood in the overlying scalp and cranium (5). To avoid this limitation, we have developed a relatively noninvasive method for the absolute measurement of LCBV localized in three dimensions throughout the brain: the point-topoint concentration of radioactive blood in a transverse section picture of the brain is determined by scanning and compared with the concentration in the peripheral blood. Recently, TerPogossian and his co-workers (6-9) have described their results with a new method for measuring LCBV with three-dimensional resolution; they determined LCBV for a single point in the brain by measuring the stimulated X-ray fluorescence of an iodinated contrast material. Although their method gives accurate data with three-dimensional localization, which previous techniques have lacked, only information from one point in the brain at a time is obtained. Also, the choice of indicators with their method is more limited than it is when radionuclides are used, a feature that may become more important with further extension of the method which we propose. Circulation Research, Vol. 36, May 1975
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LOCAL CEREBRAL BLOOD VOLUME Methods Our method required an injection of labeled (99mTc) red blood cells into a peripheral vein followed by transverse section scanning of the brain. During the scan, a blood sample was taken, and the activity in both red blood cells and plasma was determined. Counting data from scanning were reconstructed and corrected to represent the point-to-point concentration of radioactivity in a cross section of the brain. The measured concentration of blood radioactivity was used to convert the brain activity to LCBV expressed in milliliters of blood per 100 g of brain tissue. The result was a two-dimensional map of LCBV that represented a cross section of the brain at a known level. Scan Procedure.—We used a transverse section scanner that allows visualization of the radioactivity in body organs in the form of a cross-sectional picture (10-13). In such an instrument, radiation detectors are moved in a sequence of tangential passes at regular angular intervals around the head so that any radioactive structure in the selected cross section is viewed from many different directions (Fig. 1). A digital computer then reconstructs these counting data into a cross-sectional picture (14). The MARK III scanner used in this project was designed and constructed in this laboratory (11). Each of the four scintillation detectors' traveled back and forth on one side of a square frame that was positioned to coincide with the level of interest in the brain. The lines of view of the four detectors moved simultaneously through the plane of interest as each detector scanned in a path tangential to the head. The scan cycle was a sequence of six detector passes along the sides of the detector square, each made with the frame rotated 15° from the previous setting. A section scan took 2.5-20 minutes for completion, depending on the counting rates available and the precision required. Counting data were summed and recorded from each detector for each 0.8 cm of detector travel. The resulting 768 values and their corresponding position codes were recorded on perforated paper tape for further processing. Reconstruction of Scan Data.—The counting data, gathered from known directions around the cross section of the head, were used to reconstruct the original distribution of radioactivity by an orthogonal tangent correction (OTC) method (14). The calculations were performed by a minicomputer.2 The OTC method is one of a family of tangent correction processes for transverse section reconstruction from data representing multiple views or projections (15, 16). The reconstruction in the OTC method is accomplished by first using counting data from a single orthogonal pair of tangent passes and forming a matrix distribution that reflects the proportionality that exists between them. The matrix data are then redistributed to conform to the proportionality of each succeeding orthogonal pair of tangent lines. Our final matrix was a 64 x 64 array of picture elements, which represented accurately the relative activity distribution in the cross section of the head. The entire matrix could be multiplied by an experimentally 1 Each detector had a 2 x 0.5-inch Nal (Tl) crystal and a focused collimator. 2 Varian 620/L which has 20K words of core memory with an added 1.17 million words of disk memory.
FIGURE 1
Transverse section scanning. The MARK III scanner had four collimated scintillation detectors. The scan cycle was a series of detector movements tangential to the plane of interest, and the angular displacement between scan paths was 15°. Counting data were reconstructed by computer to produce a cross-sectional picture of brain radioactivity. With knowledge of the concentration of "mTc-labeled red blood cells in the peripheral venous blood, it was possible to convert the brain radioactivity into a two-dimensional map of LCBV that represented a cross section of the brain at a known level. A = anterior, P = posterior, L = left, and R = right.
determined calibration factor to convert the counts in each picture element to absolute values of radioactivity in units of microcuries per 100 g of brain tissue. In like manner, an additional multiplier, dependent on the measured activity of the blood sample, could be used to convert the matrix values to LCBV in units of milliliters of blood per 100 g of brain tissue. For accurate estimates of brain activity by this method, the volume of tissue must fill the field of view of the detector. With the MARK III scanner, the diameter of the field of view was approximately 2 cm. Therefore, even though smaller spatial detail was resolved in these section pictures, the activity estimate for each point or picture element represented a cube of tissue measuring approximately 2 x 2 x 2 cm. Red Cell Labeling.—Ow method for labeling baboon erythrocytes was similar to that of Atkins et al. (17). After centrifuging 10 ml of a mixture (4:1) of whole blood in a special ACD solution (Squibb), we removed 2.0 ml of packed red blood cells. We added the desired amount of 99m Tc in a volume not exceeding 1.0 ml of eluate from a 99m Tc generator (New England Nuclear). After 5 minutes of gentle agitation, we added 0.1 ml of a solution of stannous chloride in ACD solution equivalent to not more than 50 /xg of stannous ion. The stannous chloride had been previously filtered through a 0.22^ membrane filter. After another 5 minutes of gentle agitation, we washed the red blood cells successively with two 10-ml portions of saline by centrifuging and decanting the supernatant solution each time to remove excess stan-
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612 nous ions and radioactivity not firmly bound to the cells. Subsequent washings removed less than 2% of the activity per wash. The overall yield was approximately 65%. We resuspended the labeled red blood cells in saline and injected an accurately measured volume containing the desired amount of radioactivity into the subject. A sample of the injectate was retained as a reference standard. Calculation of LCBV.—LCBV was calculated as the ratio of the 99mTc activity per weight of brain tissue, determined by scan, to the 99mTc activity per milliliter of cerebral whole blood, predicted by assay of a peripheral venous blood sample. We assumed that the brain was normal and that the minimum volume of brain tissue quantified by this method ( 2 x 2 x 2 cm) was sufficiently large that its average hematocrit could be considered equal to the average vessel hematocrit for the whole brain (0.85 x peripheral venous hematocrit [7]). Thus,
LCBV
x 100, 0.85Hct Crbc + (1 - 0.85Hct) Cplas
(1)
where Cbrain is the activity per gram of brain determined by scan, 0.85 is the cerebral hematocrit correction, Hct is the peripheral venous hematocrit, Cr4c and Cp(os are the activities per milliliter of red blood cells and plasma, respectively, and 100 converts LCBV to standard units of milliliters per 100 g.
Results STUDIES IN BABOONS
Validity of Label.—To have a short scanning time and an acceptable radiation exposure, we chose short-lived 99mTc (half-life 6 hours) as a label rather than the more well-established red blood cell label 61Cr (half-life 28 days). Although the physical characteristics of 99mTc are superior to those of 51Cr for this purpose, red blood cell labeling methods for 99m Tc are not as efficient and are still being improved (17, 18). When baboon red blood cells were labeled with M Cr, an insignificant amount of radioactivity appeared in the plasma during the several hours after injection. However, when baboon red blood cells were labeled with 99mTc by the method described previously (17), approximately 14% of the red blood cell activity appeared in the plasma almost immediately after injection, even though substantially all of the unbound activity had been removed from the sample prior to injection. After this, the red blood cell concentration of 99mTc declined at a rate of about 5%/hour. During the first hour, the plasma concentration of 99mTc declined by 30% and then fell at a rate of 5%/hour. The composition of this unwanted plasma activity was not determined; only a portion could be accounted for later in the liver and the spleen, presumably sequestered there as red cell fragments. For a valid prediction of LCBV by Eq. 1, the
brain activity concentration (C6rain) can represent activity that is in plasma as well as red blood cells, but it must not include extravascular activity. If the assumed relation between local cerebral hematocrit and peripheral venous hematocrit (0.85:1) is correct and if Crbc and Cptas are known, LCBV will be predicted correctly by Eq. 1. Any extravascular 99m Tc will falsely increase the calculated value of LCBV. To estimate the magnitude of such an error, we compared LCBV determinations in baboons when 99m Tc and 61Cr red blood cell labels were used simultaneously. Separate red cell samples from six baboons were labeled with 99mTc and 51Cr and reinjected into a peripheral vein when the animal's arterial carbon dioxide tension (Pco2) was approximately 37 mm Hg. In the six baboons, four brain biopsies were made less than 1 hour after the injection and four were made 4-5 hours after the injection under conditions of extremely high and low values of LCBV. The concentration of 99mTc and 51Cr was measured in the brain and blood samples. Separate values of LCBV were calculated for each label (ratio of activity per weight of tissue to activity per milliliter of blood). Figure 2 shows the correlation of in vitro determinations of LCBV by the99mTc-labeled red blood cell method and the 51Cr-labeled red blood cell method.
9m T ( .
label
a, N
I Slope = 1.02 - . 0 2 Y Intercept = 2 2 ± . 0 7
5' Cr
label
LCBV ml blood/IOO g FIGURE 2
Correlation of in vitro determinations of LCBV in the baboon brain by the MmTc-labeled red blood cell method and the "Cr-labeled red blood cell method. Brain samples were taken less than 1 hour after injection (open circles) or 4-5 hours after injection (solid circles). The solid line represents the calculated linear regression line through these data and the broken lines are the 95% confidence limits. Extrauascular leak of "mTc was considered responsible for the difference of 0.22 ml blood/100 g, which was constant throughout the range of LCBV values. Circulation Research, Vol. 36, May 1975
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The regression line and the 95% confidence intervals are shown. Note that the slope is 1.02 ± 0.02, but the y intercept is 0.22 ± 0.07 ml blood/100 g and not zero. We interpreted these data as indicating an error of +0.22 ml blood/100 g when the preparation of 99m Tc-labeled red blood cells was used as an indicator for LCBV determinations in baboons. This error was constant throughout the range of LCBV values. These results could be explained by a small consistent percent (approximately 7%) of labeled red blood cell activity moving into the extravascular space very quickly after injection. This extravascular concentration then remains constant throughout the subsequent 4-5 hours and is independent of any subsequent changes in LCBV. Because of these results, in all subsequent studies in normal baboons reported in this paper, LCBV values determined by 99mTc-labeled red blood cell counting were corrected by subtracting 0.22 ml blood/100 g from all measured values. Accuracy and Reproducibility of Measuring LCBV by Section Scan.—In a series of four baboons, approximately 15 me of 99mTc-labeled red blood cells was injected into each animal. Four hours later, peripheral venous blood samples were obtained, and the baboons were killed. Section scans were then made of the frozen intact heads. The total counts of each scan averaged approximately 60,000-100,000. We then used the scan data to calculate brain activity concentration (MC/100 g) and LCBV (ml blood/100 g) for a 2 x 2-cm region in the center of the baboon brain. From each frozen head, we sawed a section corresponding to the scan cross section, and we removed a brain tissue sample from the center of each section. Blood samples and brain tissue samples were weighed and counted; brain activity concentration and LCBV were determined. There was good agreement between the section scan results and the results from in vitro counting; the coefficient of variation of the data was 6.7% for brain activity concentration and 8.1% for LCBV. The reproducibility of the scan method over time is illustrated in Figure 3, which is a plot of scan-determined LCBV in a 2 x 2-cm section centered in a baboon brain measured 60, 82, 108, 129, and 139 minutes after injection. At the times of measurement, the baboon's arterial Pco 2 was 33.9 ± 0.7 mm Hg and his mean arterial blood pressure was 122 ± 4 mm Hg. Each scan required 7.5 minutes and contained 250,000 counts. The mean of the scan-determined LCBV values was 3.22 ml blood/100 g with a coefficient of variation of 6.5%.
LCBV (ml blood /lOOg) 4r-
60
80
IOO
120
140
Time after injection ( min) FIGURE 3
Reproducibility of repeated scan determinations of LCBV in a 2 x 2-cm section centered in a baboon brain. The baboon was maintained in a steady state with an arterial Pco2 of 33.9 ±0.7 mm Hg and a mean arterial blood pressure of 122 ± 4 mm Hg. The mean value of LCBV was 3.22 ml blood/100 g with a coefficient of variation of 6.5%.
Responses to Arterial CO2 Tension and Mean Arterial Blood Pressure.—Five baboons were anesthetized with Sernylan (1 mg/kg), paralyzed with Flaxedil (5 mg/kg), and artificially ventilated with a Harvard large-animal respirator (inspired gas was 30% O2, 70% N 2 ). End-tidal Pco 2 was monitored with a Beckman infrared CO2 analyzer. Catheters were" placed in both femoral arteries for blood sample collection and blood pressure recording. The core temperature of the baboons was monitored and kept constant (37 ± 0.2°C) by an infrared heating lamp. After the completion of surgery, a red blood cell sample from each baboon was labeled with approximately 15 me of 99mTc and injected intravenously. Alterations in arterial Pco 2 were induced by both hyperventilation and CO2 inhalation (8% CO2, 30% O2, 62% N 2 ). When blood pressure and end-tidal Pco2 had stabilized, arterial blood samples were drawn for blood gas determinations. Repeated transverse section scans were made through the head over the next 4-5 hours under normocapnic, hypocapnic, and hypercapnic conditions. Depending on time and dose, each scan required 2.5-20 minutes for collection of approximately 50,000 counts/picture. In most instances, after at least 15 minutes with the baboon under stable conditions, a series of four section scans was performed with a scan time of 2.5 minutes for each. From these data
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and the blood sample data, LCBV for each scan study was then calculated for a 2 x 2-cm section centered in the baboon's head. The primary data were then reduced by averaging corresponding values of LCBV and mean arterial blood pressure for the same arterial Pco2 level (Table 1). Since there was a wide range of mean arterial blood pressures (69 to 153 mm Hg), we performed a bivariate analysis of our data (arterial Pco 2 and mean arterial blood pressure were the TABLE 1
Experimental Conditions for Scan-Determined
Baboon
N
Range
LCBV
Arterial Pco 2 (mmHg)
MABP (mmHg)
LCBV (ml blood/ 100 g)
42.4 37.8 27.2 27.8 70.5 69.9 71.1 66.6 45.1
94 97 84 87 131 131 149 130 81
4.23 4.08 3.53 3.28 5.49 5.26 5.22 5.05 4.54
38.9 38.3 64.1 67.1 31.3 27.1
116 111 125 129 119 95
2.62 2.63 2.72 3.11 2.25 2.35
32.7 33.9 34.4 47.0 53.6 35.2 33.9
114 134 119 124 121 88 69
3.34 3.17 3.20 4.66 4.46 3.14 3.16
39.1 62.6 24.4 34.6
138 132 142 116
2.40 3.82 2.43 2.21
39.9 40.9 24.7 21.4 67.9 73.8 42.6
135 140 130 137 142 153 121
3.00 3.28 2.49 2.27 4.10 3.85 3.17
33 21.4-73.8
33 69-153
33 2.21-5.49
Pco 2 = carbon dioxide tension, MABP = mean arterial blood pressure, and LCBV = local cerebral blood volume, N = number of measurements.
independent variables, and LCBV was the dependent variable). The equation of the calculated regression plane was: LCBV = 2.88 + 0.049 P a co 2 - 0.013MABP,
(2)
where P a co 2 = arterial Pco2 and MABP = mean arterial blood pressure. The overall fit was significant at the P < 0.01 level (F ratio test). Also, both slopes were significant: the arterial Pco 2 slope at the P < 0.001 level and the mean arterial blood pressure slope at the P < 0.05 level (£-test). These data agree well with the few similar results known from the literature (Table 2). The equation of the normalized regression plane for scan-determined LCBV is shown in Figure 4 (plane A). This equation was obtained by first calculating the regression plane for LCBV in each baboon and determining the value of LCBV under normal conditions, i.e., arterial Pco2 = 37 mm Hg and mean arterial blood pressure = 120 mm Hg. This value was then made unity and all other LCBV values for the baboon were related to it. The data for all of the baboons were pooled and regression plane A was calculated. A normalized regression plane was calculated in a similar manner from external head counting data that were obtained without benefit of section reconstruction (Fig. 4, plane B). These counting data were recorded during the section scans in each lateral pass, only when the collimated detector was directed at the brain. They represent the counting data that one would expect to obtain by applying a collimated radiation detector to the head to estimate cerebral blood volume without use of our section reconstruction method. Note that the slopes of plane B do not agree with those of plane A. In fact, the mean arterial blood pressure slope of plane B is opposite in sign to what would be TABLE 2
Comparison of the Alterations in LCBVin Response to Changes in Arterial Pco2 and Mean Arterial Blood Pressure Found in the Present Experiments and in Previously Published Investigations
Animal Goat Rhesus monkey Rhesus monkey Baboon
Arterial Pco2 slope
MABP slope
Reference
-0.015 -0.013
3 7 8 Present paper
0.043 0.049 0.049
Pco 2 = carbon dioxide tension and MABP = mean arterial blood pressure. Circulation Research, Vol. 36, May 1975
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LOCAL CEREBRAL BLOOD VOLUME LCBV Qiormollztd]
IOO
ACOgQnmHq)
Plane A LCBV • 0.667 + 0.011 P - 0.0009 MABP normal ACO2 Plane B L C B V 0.751 + 0.0038 P „ +T 0.0010 MABP »co 2 normal MABP [mmHg] FIGURE 4
Response of LCBV to arterial Pcot (PAco,) end mean arterial blood pressure (MABP). Plane A = external counting data with section reconstruction and plane B = external counting data without section reconstruction. The slopes are different for the two normalized regression planes. The mean arterial blood pressure slope of plane B is opposite in sign to what would be expected with autoregulation. External counting without section reconstruction is an insensitiue indicator of LCBV.
expected in the presence of autoregulation. External counting without section reconstruction is an insensitive indicator of cerebral blood volume, as would be expected, since there is no discrimination against the effect on the data of extracerebral circulation. INITIAL STUDIES IN MAN
Transendothelial Passage of Label.—In extending these studies to man, we replaced the 99mTc labeling method of Atkins et al. (17) with the method of Cutkowski and Dworkin (18) in which red blood cells are pretreated with stannous glucoheptenate before labeling. With this modification, the procedure was simplified, labeling efficiency was increased, and label loss in vivo was reduced. About 7% of the red blood cell activity still appeared in the plasma at the end of 15 minutes. Over the next hour, the plasma concentration declined by 30%, but the red blood cell concentration declined by less than 2%. We have not yet quantified the effect of possible transendothelial passage of label on the measurement of LCBV in the normal or the diseased brain of man. If there is any undesired extravascular activity in the normal brain, then an even greater concentration may be present when lesions of the blood-brain barrier occur. The effect would be (1) to mask small reductions in LCBV that might
coincide with lesions of the blood-brain barrier or (2) to suggest LCBV increases in the damaged brain when there are none. Our more recent studies in man suggest that this problem is not serious. For example, the scans shown in Figure 7 are of a brain tumor that had a low LCBV and a damaged blood-brain barrier. As expected, 99mTc-pertechnetate scans showed a high concentration of activity localized in the tumor (over a 3:1 increase compared with normal brain tissue). But, 99mTc-red cell scans showed almost no activity localized in the tumor. This result would not have occurred if a significant amount of activity had entered the extravascular space. The unwanted plasma activity may largely be bound to albumin or to larger fragments of red blood cells, neither of which will readily pass the endothelium. Regional Variations in Hematocrit.—Rosenblum (19) has cautioned that, since measurement of regional cerebral blood flow by the 133Xe method depends on regional hematocrit, interpretation may be incorrect if one assumes a constant regional hematocrit in health and disease states. Because of plasma skimming, regional cerebral hematocrits may be altered in cerebral infarction or other conditions characterized by endothelial damage and during hyperviscosity states. This possibility must also be considered in these measurements of LCBV, since they also depend on regional hematocrit (Eq. 1). We have not yet clarified the importance of regional differences in brain hematocrit nor the inconstancy of regional hematocrit in different pathological states. The error introduced in the LCBV measurement will depend on the magnitude of the hematocrit difference and the volume of brain tissues involved. With our present scanning method, activity concentrations are quantified in minimum volumes of brain tissue measuring 2 x 2 x 2 cm. The hematocrit of such a region is probably not very different from the average cerebral hematocrit. A small error in LCBV due to regional variations in hematocrit could become larger, however, when improvements in technique permit study of smaller tissue volumes. The section method proposed in this paper may be useful in clarifying this potential problem of variations in regional hematocrit. For example, if one uses a tracer that is strictly limited to red blood cells (or plasma), the section method measures local cerebral red blood cell volume (or local cerebral plasma volume) without requiring assumptions concerning local cerebral hematocrit. This fact suggests that section scans of one level in
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the brain, after separate labeling of red cells and plasma, should yield local cerebral hematocrit values in the transverse section format. With these kinds of data, one might assess the validity of assuming a constant cerebral vessel hematocrit under different conditions, as we have assumed in this paper. Initial Results.—Patient scans with the MARK III scanner were begun 15 minutes after the injection of a 15-mc dose of 99mTc-labeled red blood cells into an antecubital vein. This dose delivers approximately 200 mrad to the total body. The total scanning time was 20 minutes. A blood sample was taken from the antecubital vein in the arm opposite to the injection site before and after scanning, and the mean values of C rbc and Cpia9 from these samples were used in Eq. 1. The counts from each section scan totaled 25,000-50,000. In estimating LCBV, no corrections were made for extravascular label or possible variations in regional brain hematocrit. In normal patients, LCBV values ranged from about 2 to 4 ml/100 g, depending on the location within the cross section. The higher blood volumes coincided with regions containing cortex. The following two studies illustrate the potential usefulness of the method in evaluating the patient with a brain tumor. The first patient (E.K.) was a 72-year-old female with a history of diabetes, hypertension, and a clinical episode 3 months earlier which was thought to be due to a cerebrovascular accident. Conventional scintillation camera imaging with 99mTc pertechnetate (Fig. 5) demonstrated abnormal uptake in the right hemisphere. The appearance of this abnormal scan suggested to us a cerebral infarction in the distribution of the right middle cerebral artery. Figure 6 shows the transverse section scans done first with 99mTc pertechnetate and, on the following day, with 99mTc-labeled red blood cells. Like the conventional scintillation camera results, the section scan of 99m Tc pertechnetate showed a local alteration in the blood-brain barrier suggestive of cerebral infarction (13). Unexpectedly, the red cell or LCBV scan indicated that the lesion had a local blood volume that was three times that of normal brain tissue, a finding that would not be expected in a 3-month-old cerebral infarction. Subsequently, a cerebral arteriogram showed an area of markedly increased abnormal vascularity in the region of high pertechnetate uptake that had been noted on the radionuclide scan. At autopsy a malignant glioma was found which
B
Post
Ant Lt
Rt
FIGURE 5
Conventional scintillation camera views made with "mTc pertechnetate demonstrated an altered blood-brain barrier in the distribution of the right middle cerebral artery of patient E.K. Section studies of this patient are shown in Figure 6.
.••>-•
2
1
6 cm
FIGURE 6
Section scans of a malignant glioma in patient E.K. previously thought to be a cerebral infarction. The section scan of 9amTc pertechnetate (top left) demonstrates the local alteration in the blood-brain barrier, and the section scan of °""Tc-labeled red blood cells (bottom left) demonstrates the LCBV. Corresponding quantitative data are shown at the right. The lesion had three times the normal blood volume. At autopsy, the tumor was found to be much more highly vascular than was the adjacent normal brain tissue.
involved the frontal, parietal, and temporal lobes and had infiltrated the subarachnoid space. The center of the lesion was necrotic, and it was not possible to evaluate the number of blood vessels there. However, in the more peripheral and viable areas, especially in the subarachnoid space, the vascular component of the neoplasm was extremely prominent. The blood vessels (arteries and venCirculation Research, Vol. 36, May 1975
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ules) were extremely congested with red blood cells, their lumens were dilated, and their number appeared definitely increased compared with the number in adjacent normal brain tissue. The second patient (G.D.) was a 66-year-old female with a small malignant glioma in the right parietal lobe. An arteriogram revealed that peritumor edema had expanded the surrounding brain and displaced the right anterior and posterior cerebral artery to the left. Two months later, after steroid therapy, the edema had subsided and clinically the patient had improved. Initial and subsequent section studies, first with 99mTc pertechnetate and then with 99mTc-labeled red blood cells, are shown in Figure 7. The initial section scan of 99mTc pertechnetate demonstrated the small tumor by identifying its local alteration of the blood-brain barrier. The section scan of 99mTclabeled red blood cells showed in the same location a much larger region of reduced LCBV, probably a result of compression of the local microcirculation by peri tumor edema. Corresponding studies were made 2 months later after steroid therapy. This section scan of 99mTc pertechnetate showed a
-6-4-2
0
2
4
6D
reduction in tumor uptake of radioactivity, probably a result of steroid effect on tumor vessel endothelium. As an incidental finding, this section also showed an increased uptake in the adjacent convexity due to a burr hole that had been required in the interval for biopsy. However, the most interesting feature of the 99mTc-red cell scan was a more normal LCBV in the region of the tumor. Presumably, the steroid treatment had diminished the edema and relieved compression of the peritumor microcirculation. Note in Figures 6 and 7 the correspondence of normal LCBV values in the left hemisphere. Also note the usefulness of the actual quantitative data for side-to-side comparisons. Discussion
Our knowledge of the status of cerebral hemodynamics in patients would be enhanced by a reliable means of measuring absolute values of LCBV without the need for intracarotid injection of indicator. Until now, the only method approaching these requirements has been the X-ray fluorescence technique of TerPogossian et al. (6-9). All previous
-6-4-2
0
2
4
6 cm
FIGURE 7
Section scans of a malignant glioma surrounded by edema in patient G.D. At the top of the figure, section scans of "mTc pertechnetate show the alteration of the blood-brain barrier. At the bottom, section scans of '""Tc-labeled red blood cells show LCBV. Initial studies are to the left, and studies made 2 months later, after steroid therapy, are to the right. The section scans of '""Tc-labeled red blood cells initially showed a large region of reduced LCBV due to compression of local microcirculation by edema. After steroid therapy, peritumor edema decreased and peritumor LCBV returned toward normal. Circulation Research, Vol. 36, May 1975
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KUHL. REIVICH. ALAVI. NYARY. STAUM
in vivo methods are flawed by a lack of quantification and an inability to discriminate the effects of extracerebral circulation when the indicator is injected intravenously. With the method that we have introduced in this paper, certain of the restrictions of previous approaches are avoided. Reconstruction of one or more transverse section scans after intravenous injection of a radioactive indicator permits an absolute measurement of LCBV localized in three dimensions. With this approach, each part of the brain can be studied without regard to contaminating data from the overlying scalp or skull or from other regions of the brain. The method is an improvement over the existing X-ray fluorescence technique in that the quantitative data are presented as one or more cross-sectional pictures, the attentuation effects by the human skull on gamma emissions (140-500 kev) in radionuclide scanning are less severe than those with the X-rays of lower energy which are encountered in stimulated fluorescence (9), and the use of a labeled radioactive indicator permits a wider choice of alternative compounds which may be administered to the patient in extremely small amounts. Several improvements in our method would lead to better results. Red blood cell labeling should be refined so that radioactivity appears nowhere but in circulating red blood cells. We hope to realize about a tenfold increase in detection sensitivity with our new section scanner (MARK IV) which is now under construction. This increase should permit faster scans with better counting statistics and enable scans of multiple levels in the brain to be made in a relatively brief period of time. Improvements in spatial resolution are also possible. The collimated detectors used in this study view a slice of brain about 2 cm thick, and the method quantifies activity within volumes that are about 2 cm in diameter. Better spatial resolution in the detection system would permit visualization and quantification of smaller structures. As a general method, three-dimensional reconstruction of radionuclide scan data should become more important in the study of the brain. Ideally, such a technique would enable precise measurements of radionuclide concentration to be made within functional structural units of the brain in man; such measurements have not been possible up until now. The same study principles might be applied to other tracers enabling measurements of local cerebral blood flow and local metabolic rates for various metabolites within the brain in man. Measurements of these kinds of regional functions
with three-dimensional resolution throughout the brain and without the necessity for intracarotid injection of indicator would constitute a significant advance over the presently available methods. Acknowledgment We thank Dr. Nicholas Gonatas, Dr. Robert Zimmerman, Mr. Roy Q. Edwards, Mr. Anthony R. Ricci, Mr. Nelson L. Marin, and Ms. Deborah A. Long for assistance in this project.
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