Extravascular Density Research Articles

Changes in Subfoveal Choroidal Thickness in Malignant Hypertension Patients

Purpose: To evaluate changes in subfoveal choroidal thickness in patients with malignant hypertension. Methods: A total of 12 eyes of six malignant hypertension patients were included in the present study. Intraocular pressure, blood pressure (systolic and diastolic), and choroidal thickness were measured before and after blood pressure control. Choroidal thickness was measured using enhanced depth imaging optical coherence tomography (EDI-OCT). The changes in choroidal extravascular density of the EDI-OCT image after blood pressure control were evaluated by comparing brightness values obtained with Adobe Photoshop software. Results: The subfoveal choroidal thickness (SFCT) of malignant hypertension patients was 412.63 ± 66.55 μm (mean ± SD), which was thicker than in normal patients. After blood pressure control, SFCT decreased significantly to 356.96 ± 59.08 μm (mean ± SD) (p = 0.002). The choroidal extravascular density of the EDI-OCT image decreased after blood pressure control (p = 0.002), and the mean change was 17.21 ± 7.56. Conclusions: The choroid is thickened in patients with malignant hypertension, and its thickness decreases after blood pressure control. This suggests that changes in blood pressure may affect choroidal thickness. J Korean Ophthalmol Soc 2014;55(6):840-846

Changes in subfoveal choroidal thickness and choroidal extravascular density by spectral domain optical coherence tomography after haemodialysis: a pilot study

AimsTo examine the changes in the subfoveal choroidal thickness (SFCT) and choroidal density using spectral domain optical coherence tomography (SD-OCT) before and after haemodialysis (HD).Methods28 eyes of 19 patients with...

CT angiography of various superficial femoral artery stents: An in vitro phantom study

To evaluate CT reconstruction parameters to improve stent lumen visualization in vitro. 12 latest superficial femoral artery (SFA) stents were placed in a vessel phantom (diameter 4.7 mm, intravascular attenuation 250 HU, extravascular density 50 HU). Stents were imaged with a 128-slice scanner (SOMATOM Definition Flash, Siemens, Germany) with standard parameters: 120 kV, 200 mAs, collimation 128mm × 0.6mm. Different reconstruction parameters were evaluated: B26f, B30f, B45f, B46f and B60f kernel; slice thickness of 0.6, 2.0 and 5.0mm. To measure visualization characteristics, stent lumen diameter and intraluminal attenuation were assessed. Best stent lumen visualization could be obtained using the B46f kernel (p<0.001). The visible stent lumen ranged from 66.4% to 83.3% with a mean diameter of 77.7 ± 4.6%. Nitinol stents showed a significant improved lumen visibility compared to the cobalt-chromium stent (p = 0.02). The most realistic lumen attenuation was achieved using the B46f kernel with a mean attenuation of 259.3 ± 8.9 HU. The visible lumen diameter in protocols with 5mm slice thickness was significantly lower (70.0 ± 4.9%) compared to thinner slices (p<0.001). CTA of SFA stents should be reconstructed with a slice thickness of 2.0mm and a B46f kernel to achieve best image quality and to become more sensitive to exclude instent restenosis.

Update on multidetector coronary CT angiography of coronary stents: in vitro evaluation of 29 different stent types with dual-source CT

The aim of this study was to test a large sample of the latest coronary artery stents using four image reconstruction approaches with respect to lumen visualization, lumen attenuation, and image noise in dual-source multidetector row CT (DSCT) in vitro and to provide a CT catalogue of currently used coronary artery stents. Twenty-nine different coronary artery stents (19 steel, 6 cobalt-chromium, 2 tantalum, 1 iron, 1 magnesium) were examined in a coronary artery phantom (vessel diameter 3 mm, intravascular attenuation 250 HU, extravascular density -70 HU). Stents were imaged in axial orientation with standard parameters: 32 x 0.6 collimation, pitch 0.24, 400 mAs, 120 kV, rotation time 0.33 s. Image reconstructions were obtained with four different convolution kernels (soft, medium-soft, standard high-resolution, stent-dedicated). To evaluate visualization characteristics of the stent, the lumen diameter, intraluminal density, and noise were measured. The stent-dedicated kernel offered best average lumen visualization (54 +/- 8.3%) and most realistic lumen attenuation (222 +/- 44 HU) at the expense of increased noise (23.9 +/- 1.9 HU) compared with standard CTA protocols (p < 0.001 for all). The magnesium stent showed the least artifacts with a lumen visibility of 90%. The majority of stents (79%) exhibited a lumen visibility of 50-59%. Less than half of the stent lumen was visible in only six stents. Stent lumen visibility largely varies depending on the stent type. Magnesium is by far more favorable a stent material with regard to CT imaging when compared with the more common materials steel, cobalt-chromium, or tantalum. The magnesium stent exhibits a lumen visibility of 90%, whereas the majority of the other stents exhibit a lumen visibility of 50-59%.

Characterization of PET partial volume corrections for variable myocardial wall thicknesses

Limited scanner resolution and cardiac motion contribute to partial volume (PV) averaging of positron emission tomography (PET) images. An extravascular (EV) density image, created from the subtraction of a blood pool (BP) from a transmission (TX) image can be used to estimate PV losses in the myocardium (MYO). A phantom emulating the left ventricle myocardium, with a variable wall thickness (5 mm to 25 mm), was used to characterize the method for use in 3-D PET /sup 18/FDG studies. At a myocardial thickness of 5 mm, 40% recovery of the activity was obtained. At myocardial thicknesses greater than 20 mm, full recovery was seen. Prior to EV image creation, the morphological operators dilation/erosion were applied to the BP and TX images to account for the presence of the phantom's plastic walls, which would otherwise bias the EV values. Dividing MYO by the EV values improved the recovery to 95% at 5 mm, however, TX ring artifacts and the anisotropic nature of dilation/erosion contributed to errors in the EV image. Instead of using dilation/erosion, a second method, involving placement of adjusted ROIs on the BP and TX images was investigated. Use of these new EV values, as well as placement of the cardiac phantom in a chest phantom to reduce TX ring artifacts, allowed for 90% recovery of the activity at 5 mm. These results show that the EV density image can correct for PV averaging with 3-D PET over a range of myocardial thicknesses applicable to patient studies. However, in the thinnest regions, the method was found to be sensitive to errors in both the blood pool and transmission images.

64-slice multidetector coronary CT angiography: in vitro evaluation of 68 different stents

The purpose of this study was to test a large sample of different coronary artery stents using four image reconstruction approaches with respect to lumen visualization, lumen attenuation, and image noise in 64-slice multidetector-row computed tomography (MDCT) in vitro and to provide a catalogue of currently used coronary artery stents when imaged with state-of the-art MDCT. We examined 68 different coronary artery stents (57 stainless steel, four cobalt-chromium, one cobalt-alloy, two nitinol, four tantalum) in a coronary artery phantom (vessel diameter 3 mm, intravascular attenuation 250 HU, extravascular density -70). Stents were imaged in axial orientation with standard parameters: 32 x 0.6 collimation, pitch 0.24, 680 mAs, 120 kV, rotation time 0.37 s. Four different image reconstructions were obtained with varying convolution kernels and section thicknesses: (1) soft, 0.6 mm, (2) soft, 0.75, (3) medium soft, 0.6, and (4) stent-optimized sharp, 0.6. To evaluate visualization characteristics of of the stent, the lumen diameter, intraluminal density and noise were measured. The high-resolution kernel offered significantly better average lumen visualization (57% +/-10%) and more realistic lumen attenuation (222 HU +/-66 HU) at the expense of increased noise (15.3 HU +/-3.7 HU) compared with the soft and medium-soft CT angiography (CTA) protocol (p<0.001 for all). Stents with a lumen visibility of more than 66% were: Arthos pico, Driver, Flex, Nexus2, S7, Tenax complete, Vision (all 67%), Symbiot, Teneo (70%), and Radius (73%). Only ten stents showed a lumen visibility of less than 50%. Stent lumen visibility largely varies depending on the stent type. Even with the improved spatial resolution of 64-slice CT, a stent-optimized kernel remains beneficial for stent visualization when compared with the standard medium-soft CTA protocol. Using 64-slice CT and high-resolution kernel, the majority of stent products show a lumen visibility of more than 50% of the stent diameter.

Phantom Studies Investigating Extravascular Density Imaging for Partial Volume Correction of 3-D PET&amp;lt;tex&amp;gt;$^18$&amp;lt;/tex&amp;gt;FDG Studies

Limited scanner resolution and cardiac motion contribute to partial volume (PV) averaging of cardiac PET images. An extravascular (EV) density image, created from the subtraction of a blood pool scan from a transmission image, has been used to correct for PV averaging in H/sub 2//sup 15/O studies using 2-D imaging but not with 3-D imaging of other tracers such as /sup 18/FDG. A cardiac phantom emulating the left ventricle was used to characterize the method for use in 3-D PET studies. Measurement of the average myocardial activity showed PV losses of 32% below the true activity (p<0.001). Initial application of the EV density correction still yielded a myocardial activity 13% below the true value (p<0.001). This failure of the EV density image was due to the 1.66 mm thick plastic barrier separating the myocardial and ventricular chambers within the phantom. Upon removal of this artifact by morphological dilation of the blood pool, the corrected myocardial value was within 2% of the true value (p=ns). Spherical ROIs (diameter of 2 to 10 mm), evenly distributed about the myocardium, were also used to calculate the average activity. The EV density image was able to account for PV averaging throughout the range of diameters to within a 5% accuracy, however, a small bias was seen as the size of the ROIs increased. This indicated a slight mismatch between the emission and transmission image resolutions, a result of the difference in data acquisitions (i.e., span and ring difference) and default smoothing. These results show that the use of EV density image to correct for PV averaging is possible with 3-D PET. A method of correcting barrier effects in phantoms has been presented, as well as a process for evaluating resolution mismatch.

Correction of partial volume effects in myocardial SPECT

Background. Marked partial volume effects occur in myocardial single photon emission computed tomographic (SPECT) studies because of limited resolution in imaging the myocardial wall and contractile motion of the heart. Little work has been undertaken to develop correction techniques for SPECT except for efforts to improve the reconstructed resolution. Our purpose was to examine the extent of the problem and propose a correction method. Methods and Results. A potential correction method, developed initially for positron emission tomography, involved estimation of extravascular density by means of subtracting vascular density derived in a blood pool study from total density derived from a transmission study. Provided partial volume errors are the same for transmission and emission data, activity per gram of extravasclar tissue can be obtained by means of dividing the perfusion regional data by extravascular density for the same region. Simulations were designed to assess the importance of partial volume errors and the use of extravascular density to correct the errors. Recovery coefficients for the myocardium were estimated by means of simulation of the beating heart on the basis of published values for ventricular dimensions. Resolution for transmission with a scanning line source system was compared with emission resolution. The effect of spillover on measured partial volume losses was assessed, and a method for matching spillover for emission and extravascular density was demonstrated. Correction for partial volume effects was demonstrated for a phantom with variable wall thickness. Significant variation in recovery coefficient was demonstrated between posterior and septal walls for individual patients independent of heart size. Filtering was necessary to account for the difference in transmission resolution measured in the axial direction. Spillover effects had a significant influence on the measured recovery for small objects; however, for a specific reconstruction algorithm and defined region size, correction was implemented to match the spillover effects for emission and extravascular density. Use of extravascular density for correction of partial volume loss, for ordered subsets expectation maximization reconstruction with compensation for resolution, was demonstrated to be accurate to within 10%. Conclusions. The feasibility of correcting partial volume effects with extravascular density was demonstrated. Correction is effective provided care is taken to match both resolution and spillover for emission and extravascular density.

RESPONSE TO SURFACTANT THERAPY IS INDEPENDENT OF EXTRAVASCULAR WATER ACCUMULATION IN NEWBORN RESPIRATORY DISTRESS SYNDROME. 2041

To evaluate whether elevated pulmonary vascular permeability is associated with extravascular water accumulation and lack of response to surfactant replacement therapy in newborn Respiratory Distress Syndrome(RDS),we measured the pulmonary transcapillary escape rate(PTCER)for gallium-68 labelled transferrin and the extravascular density(EVD)with the quantitative, non-invasive nuclear medicine imaging technique of positron emission tomography(PET).Three groups of infants were studied: 1)NORMAL:infants without respiratory symptoms;2)RDS-NO SURF:preterm infants with RDS who did not require surfactant replacement;3)RDS-SURF:preterm infants with RDS who required surfactant.The NORMAL infants underwent one PET scan within the first 8 days of life.All but one RDS infant underwent 2 PET scans at least 6 hours apart within the first 24 hrs of life.The RDS-SURF infants underwent scans before and 6-8 hours after surfactant administration.The mean PTCER and EVD for all infants with RDS(131±48 10-4/min and 33±6 gms/100ml lung respectively) were significantly greater than for NORMAL infants(78±27 10-4/min and 28±6 gms/100ml lung respectively,p=0.02 and 0.04 respectively;Table).There was a correlation between PTCER and EVD(p=0.02,r=0.48).Initial PTCER and EVD were not significantly different between the two RDS subgroups and did not change between the two scans.The oxygen index (OI) decreased by 47% in the RDS_SURF subgroup.We conclude: 1)infants with RDS have increased pulmonary vascular permeability and extravascular water accumulation in the lungs;2)response to surfactant replacement therapy is independent of changes in the PTCER or EVD.

Distribution of Regional Density and Vascular Permeability in the Adult Respiratory Distress Syndrome

Although recent X-ray computed tomography (CT) data show that ventral-dorsal gradients in density are common in the adult respiratory distress syndrome (ARDS), we hypothesized that the ventral-dorsal gradient of pulmonary vascular permeability would be evenly distributed in patients with ARDS. We tested this hypothesis by analyzing previously reported data (Am Rev Respir Dis 1991; 143:150–154) obtained by the nuclear medicine imaging technique of positron emission tomography (PET). Measurements of regional lung density (LD), extravascular density (EVD), and the pulmonary transcapillary escape rate (PTCER; an index of vascular permeability) were obtained in eight patients with ARDS and in 10 normal subjects. The data from four contiguous tomographic slices were organized into 20 anatomically oriented “bins” from the ventral (Bin #1) to dorsal (Bin #20) position. Mean overall LD, ERD, and PTCER were all significantly higher (p < 0.05) in the ARDS patients than in normal subjects. Regression slopes of LD ver...

Bronchial and reverse pulmonary venous blood flow protect the lung from ischemia-reperfusion injury

We used an intact in vivo canine model of pulmonary ischemia-reperfusion injury to evaluate whether the bronchial circulation or reverse pulmonary venous blood flow would protect the lung from injury during 2 h of unilateral pulmonary arterial (PA) occlusion and lung deflation. Serial measurements of regional extravascular density and transcapillary protein flux were made after reperfusion by using the quantitative imaging technique of positron emission tomography. Twenty-one animals were divided into four experimental groups. In all experimental groups, the left PA was clamped and the left lung was allowed to collapse and remain unventilated for a period of 2 h. In addition, in group I (n = 5) the left bronchial circulation was disrupted and the left pulmonary veins were clamped, in group II (n = 5) the bronchial circulation and the pulmonary veins were left intact, in group III (n = 6) the bronchial circulation was left intact but the pulmonary veins were clamped, and in group IV (n = 5) the bronchial circulation was disrupted but the pulmonary veins remained patent. The rate of protein flux in the left lung was increased only in group I (complete ischemia with lung deflation) [mean 195 x 10(-4) min-1 (range 85-453 x 10(-4) min-1) at 0.25 h and 114 x 10(-4) min-1 (range 22-200 x 10(-4) min-1) at 3 h] after reventilation and PA reperfusion (normal = 49 +/- 31 x 10(-4) min-1). Extravascular density increased significantly from 0.25 to 3 h only in group I.(ABSTRACT TRUNCATED AT 250 WORDS)

Modifying pulmonary ischemia-reperfusion injury by altering ventilatory strategies during ischemia.

We used an intact in vivo canine model of pulmonary ischemia-reperfusion (IR) injury to evaluate the differential effects of alveolar hypoxia and ventilation during 2 h of unilateral warm lung ischemia. Serial measurements of regional pulmonary blood flow, extravascular density (EVD), and transcapillary protein flux were made after reperfusion with the quantitative imaging technique of positron emission tomography. Twenty-seven animals were divided into five experimental groups: VENT O2 (n = 5) in which the left lung was ventilated with 40% O2 during ischemia, STATIC O2 (n = 4) in which the left lung was statically inflated with 40% O2 during ischemia, VENT N2 (n = 5) in which the left lung was ventilated with 100% N2 during ischemia, VENT N2/CO2 (n = 5) in which the left lung was ventilated with 95% N2-5% CO2 during ischemia, and STATIC N2 (n = 8) in which the left lung was statically inflated with 100% N2 during ischemia. These groups were compared with a control group (CONT, = 3) that was studied previously. Protein flux was significantly increased in the previous ischemic lung only for the STATIC N2 group [median 175 x 10(-4) min-1 (range 53-1,217) for the STATIC N2 group vs. 50 x 10(-4) min-1 (range 40-56) for the CONT group] 0.25 h after reperfusion and did not change over 3 h. EVD also increased but not significantly. Protein flux and EVD in the other groups were not different from CONT.(ABSTRACT TRUNCATED AT 250 WORDS)

Inflammation and oxygen free radical formation during pulmonary ischemia-reperfusion injury.

In a companion study, we showed that 2 h of warm unilateral lung ischemia followed by reperfusion resulted in bilateral tissue injury, indicated by increases in extravascular density (EVD) and permeability, measured as the pulmonary transcapillary escape rate (PTCER) for radiolabeled transferrin. EVD and PTCER measurements were obtained with the quantitative imaging technique of positron emission tomography (PET). In the current study, we evaluated this increase in EVD histologically and correlated EVD and PTCER with measurements of oxidant-reactive sulfhydryls (RSH) in plasma as a marker of oxygen free radical (OFR) formation. Histologically edema, leukocyte infiltration, and hemorrhage were all present on the ischemic side, but only after reperfusion, whereas only neutrophil infiltration was observed on the nonischemic side. Histology scores correlated with EVD (r = 0.81) and PTCER (r = 0.75), but permeability was abnormal at times even in the absence of neutrophil infiltration. Plasma RSH concentration from the ischemic lung decreased significantly (P less than 0.05) during pulmonary ischemia (i.e., before reperfusion) and returned to baseline on reperfusion. The degree of RSH oxidation did not correlate with the severity of injury as measured by PET or histology. Thus pulmonary ischemia-reperfusion injury is characterized by inflammation, hemorrhage, edema, and OFR formation. Injury occurred after reperfusion, not after ischemia alone. In addition, injury to the contralateral nonischemic lung suggests a neutrophil-independent circulating mediator of injury.

Injury in nonischemic lung after unilateral pulmonary ischemia with reperfusion.

We developed an in vivo intact canine model to study pulmonary ischemia-reperfusion (IR) injury. The surgical approach simulates that of unilateral lung transplantation but is free of technical difficulties and other factors related to lung preservation. Serial measurements of regional pulmonary blood flow (rPBF), extravascular density (EVD), and transcapillary protein flux were made with the quantitative imaging technique of positron emission tomography. Eleven experimental and six control animals were studied. After 2 h of warm ischemia followed by reperfusion, no significant change occurred in rPBF despite significantly increased EVD, which was greater on the ischemic than on the nonischemic side. Protein flux, measured as a rate constant, was also greater on the ischemic than on the nonischemic side (median 181 x 10(-4)/min, range 104-619, vs. median 90, range 33-132) immediately after reperfusion. Both sides were also significantly different from control values (median 37, range 21-57). On both sides, protein flux decreased over time and at 5 h after reperfusion was not different from that of controls. Data from the control animals showed that these findings in the experimental animals were not due to surgical technique, deterioration in the surgical preparation, or hyperperfusion of the nonischemic lung. Thus IR injury of one lung can lead to similar, but less severe, injury in the contralateral lung. Because injury in the nonischemic lung develops only after reperfusion of the ischemic lung, injury to the nonischemic lung is probably humorally mediated. The model is a useful and relevant method for studying the physiological consequences of pulmonary IR injury.

Physical Aspects of Cardiac Scanning with a Block Detector Positron Tomograph

Physical aspects relating to cardiac scanning are described for an eight ring (15 plane) positron tomograph consisting of BGO block detectors (CTI/Siemens 931-08/12). Performance parameters were derived from a cylindrical heart phantom having a "myocardial" wall of thickness varying from 3 mm to 27 mm. This phantom was inserted into a chest phantom consisting of simulated chest wall, lungs, and arms. Recovery coefficients for myocardial thicknesses of 10 mm and 15 mm were 0.75 and 0.9, respectively. Division by the transmission minus "blood pool" (extravascular density) image was found to give a variation of corrected myocardial counts within +/- 5% when transmission data were smoothed. The on-line dead time correction algorithm was found to be accurate to within 5% up to 20 mCi (740 MBq) in the axial field of view (FOV) (10.8 cm) in the central chamber of the heart phantom. However, the correction factor at this rate is approximately 3, which would imply poor use of administered dose. Counts in the image due to scatter are approximately 2% in the (cold) central cavity of the heart phantom relative to counts/pixel in the active myocardium. The presence of phantom arms in the FOV was found to have only a small effect on mean pixel counts and noise in the heart phantom image, as did movement of the arms within a reasonable range.

A Positron Emission Tomographic Comparison of Pulmonary Vascular Permeability during the Adult Respiratory Distress Syndrome and Pneumonia

We measured extravascular density (EVD) and the pulmonary transcapillary escape rate (PTCER) for 68Ga-transferrin using positron emission tomography in 14 normal volunteers and 29 patients with radiographic infiltrates, including six patients with congestive heart failure (CHF), eight patients with the adult respiratory distress syndrome (ARDS), and 15 patients with focal pneumonia. Contralateral, radiographically normal regions were also evaluated in the patients with focal pneumonia. Mean EVD was elevated in the patients with CHF, ARDS, and pneumonia in regions of radiographic infiltrate compared with values from normal subjects (p less than 0.05), but it was not significantly different among the three patient groups. PTCER in normal subjects and in patients with CHF was not significantly different (21 +/- 11 versus 44 +/- 16 x 10(-4) min-1, respectively, p = NS). PTCER was elevated in regions of infiltrate because of either pneumonia (173 +/- 99) or ARDS (170 +/- 79). PTCER was also elevated in regions contralateral to those with focal infiltrate during pneumonia, even though these regions were radiographically normal and had normal EVD values. These results suggest that PTCER is a sensitive but nonspecific index of abnormal pulmonary vascular permeability, which may be useful for classifying patients in clinical studies of pulmonary edema.

Regional extravascular density of the lung in patients with acute pulmonary edema.

The regional distribution of extravascular lung density (lung tissue and interstitial or alveolar fluid per unit thoracic volume) and fractional pulmonary blood volume (volume of blood per unit thoracic volume) was measured in five patients with acute interstitial pulmonary edema and two patients with acute alveolar edema. We found a uniform increase in extravascular lung density in the patients with acute interstitial edema but a preferentially dependent distribution in the patients with alveolar edema. Fractional blood volume had an abnormally uniform distribution in patients with interstitial edema. In alveolar edema, there was marked redistribution of blood volume away from severely edematous regions. The results are in agreement with previous experimental work with animal models. The distribution of extravascular lung density and fractional blood volume in subjects with acute interstitial edema is, however, different from that found in subjects with chronic interstitial edema, suggesting that the pathophysiological characteristics of the two conditions may be different.

Effect of changes in inflation and blood volume on regional lung density--a PET study: 2.

Data obtained during positron emission tomographic (PET) imaging of the lung are expressed in terms of lung volume, not lung weight. Therefore changes in PET data may simply reflect changes in regional lung inflation. Normalizing PET data with regional measurements of lung density (rLD) made with positron transmission tomography may be useful, but how rLD is altered by changes in either regional inflation or blood volume has not been evaluated. In 12 supine dogs the mean rLD was 0.32 +/- 0.06 g/ml lung and was not significantly changed by doubling the tidal volume. The mean LD decreased with increasing levels of positive end-expiratory pressure (PEEP) (5-20 cm H2O, but regional changes were consistently present only at levels greater than 5 cm H2O. After sufficient hemorrhage to reduce cardiac output by half, mean rLD only decreased from 0.33 +/- 0.07 to 0.30 +/- 0.06 g/ml lung, while regional extravascular density remained unchanged. Thus, large changes in tidal volume, modest amounts of PEEP, and significant decreases in blood volume produced small, although measurable, decreases in rLD. Therefore, normalization of emission data with transmission derived measurements of LD should not be necessary for interventions that cause rLD to decrease.

Regional lung hematocrit in humans using positron emission tomography.

Regional lung hematocrit ratio (R) was measured in five normal subjects and five patients (2 with pneumonia, 2 with nephrotic syndrome with anemia, and 1 with pancreatitis) using positron emission tomography, a red cell marker 11CO, and a plasma marker [methyl-11C]albumin). The measurements were made in a transaxial thoracic section at midheart level with the subject in supine posture and with a spatial resolution of 1.7 cm. The normal regional hematocrit ratio (means +/- SE) calculated for the lung was 0.90 +/- 0.014, 0.94 +/- 0.023 for the thoracic wall, and 1.00 +/- 0.003 for the heart chambers. The regional lung hematocrit ratio in the patients ranged between 0.81 and 0.86. No correlation was found among the regional lung hematocrit ratio and regional blood volume, lung extravascular density, and the peripheral hematocrit (obtained from venous blood samples). To the extent that 70% of the pulmonary blood in the field of view is in larger vessels with normal hematocrit, the hematocrit in the capillary bed is approximately two-thirds that of the peripheral venous value. Blood volume measurements on the basis of single vascular tracers need to take account of these results.

Positron emission tomography in the lung.

Positron emission tomography using the ECAT II scanner to image and measure regional lung function is outlined. The combined use of transmission and emission imaging provides quantitative information about regional lung structure (density, extravascular density, and vascular volume) and function (ventilation, perfusion, ventilation-perfusion ratios, glucose metabolic rate). Clinical applications in asthma, chronic obstructive lung disease, pulmonary vascular disease, interstitial lung disease, and squamous cell carcinoma are presented. Future prospects for PET are discussed.