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Biomedical subjects

C G Rhodes

Publications and source records attributed to C G Rhodes.

At least 55 records · Page 3Linked to original sources
PubMed1989

Quantification of regional V/Q ratios in humans by use of PET. I. Theory.

With positron emission tomography, quantitative measurements of regional alveolar and mixed venous concentrations of positron-emitting radioisotopes can be made within a transaxial section through the thorax. This allows the calculation of regional ventilation-to-perfusion (V/Q) ratios by use of established tracer dilution theory and the constant intravenous infusion of 13N. This paper considers the effect of the inspiration of dead-space gas on regional V/Q and investigates the relationship between the measured V/Q, physiological V/Q, and V/Q defined conventionally in terms of bulk gas flow (VA/Q). Ventilation has been described in terms of net gas transport, and the term effective ventilation has been introduced. A simple two-compartment model has been constructed to allow for the reinspiration of regional (or personal) and common dead-space gas. By use of this model, with parameters representative of normal lung the effective V/Q ratio for 13N [(VA/Q)eff(13N)] is shown to overestimate VA/Q by 18% when VA/Q = 0.1 but underestimate VA/Q by 68% when VA/Q = 10. For physiological gases, the model predicts that the behavior of O2 should be similar to that of 13N, so that, in terms of gas transport, V/Q ratios obtained using the infusion of 13N closely follow those for O2. Values of the effective V/Q ratio for CO2 [(VA/Q)eff(CO2)] lie approximately halfway between (VA/Q)eff(13N) and VA/Q. These results indicate that dead-space ventilation is far less a confounding issue when V/Q is considered in terms of net gas transport (VAeff), rather than bulk flow (VA).(ABSTRACT TRUNCATED AT 250 WORDS)

Humans↗
PubMed1989

Quantification of regional V/Q ratios in humans by use of PET. II. Procedure and normal values.

Regional measurements of tissue isotope concentration, made using positron emission tomography (PET), allow tracer models to be used in a quantitative manner to provide topographic distributions of many structural and functional parameters, each derived for the same well-defined lung element. In this paper we describe a technique to measure regional ventilation-perfusion ratios (V/Q), in absolute units, by use of PET and the continuous intravenous infusion of an inert gas isotope, 13N, and report on measurements made in 12 normal subjects (4 smokers). Data were obtained from a single lung section (slice thickness, 1.7 cm full width at half-maximum response to a line source) at the level of the right ventricle in the supine posture during quiet breathing. For the 12 subjects, volume-weighted mean values of V/Q, averaged over individual right and left lung fields, ranged from 0.50 to 1.29. Analysis of these means showed no difference between lungs: right, 0.80 +/- 0.23 SD; left, 0.76 +/- 0.20 SD. Topographically, a systematic fall of V/Q in the ventrodorsal direction was observed in eight of the subjects (mean ventrodorsal difference 0.39, range 0.19-0.90), whereas two showed a clear increase toward dependent lung regions (range 0.16-0.26). Seven of the subjects with a falling ventrodorsal V/Q gradient also exhibited discrete regions of low V/Q at the dorsal lung border. We conclude that, in normal subjects, ventilation and perfusion are generally well matched in the supine posture, but isolated mismatching often occurs in dependent lung regions.

Humans↗
PubMed1988

Positron emission tomography in pulmonary edema.

Positron emission tomography (PET) enables the concentration of positron-emitting isotopes to be measured quantitatively in vivo. It is also possible to measure the physical density of the lung with an external source of radiation. Several investigative procedures have been described for studying the distribution of the intravascular and extravascular water pools in the lung with PET. Clinical applications of these procedures have shown that acute hydrostatic pulmonary edema in humans has characteristics similar to experimentally induced hydrostatic edema. In chronic interstitial pulmonary edema, on the other hand, the relationship between the intravascular and extravascular water pools is different, and experimental models of acute pulmonary edema may not be relevant to this category of patients. The possible effects of these differences on lung function, such as gas exchange, may be studied with PET in the future. Microvascular permeability to proteins may also be studied.

Blood Volume↗
PubMed1988

Measures of the inflammatory response in cryptogenic fibrosing alveolitis.

Cryptogenic fibrosing alveolitis (CFA) is characterized by interstitial fibrosis and parenchymal inflammation. Eleven patients with CFA (10 proved by lung biopsy) were followed over 2 yr using clinical symptoms, radiographic change, and pulmonary function tests to adjust their treatment. Lung lavage, positron camera (PET) measurements of regional extravascular lung density (Dev), pulmonary blood volume (Vb), and the metabolic rate for 18F-deoxyglucose (MRglc), clearance of 99mTc-diethylenetriaminepentacetate (99mTc-DTPA) aerosol, and lung uptake of 67Ga were measured initially and at the end of the first year to give a profile of the inflammatory response. Compared with normal subjects, there was an increased percentage of neutrophils and eosinophils in the lung lavage, increased Dev (p less than 0.002) with no significant difference in Vb, increased MRglc (p less than 0.02), 99mTc-DTPA clearance (p less than 0.002), and 67Ga uptake (p less than 0.02). The smallest increases in Dev were seen in the two patients with most destruction shown by lung biopsy. There were inverse correlations between Dev and both FVC and TLC, but a direct correlation between Vb and transfer factor. 99mTc-DTPA clearance changed concordantly with clinical status and radiographic and respiratory function changes during the first year. If glucose utilization (MRglc) remained in the normal range between the initial and first yearly assessment, the patient improved or remained stable during the second year as shown by clinical status and radiographic and respiratory function measurements. If it rose or remained high, the patient's condition deteriorated.(ABSTRACT TRUNCATED AT 250 WORDS)

Aged↗
PubMed1987

Glucose utilization in vivo by human pulmonary neoplasms.

Neoplastic tissue in general shows a high rate of glucose consumption under both anaerobic and aerobic conditions. Using positron emission tomography (PET) we measured the rate of uptake of the glucose analogue 18fluoro-2-deoxy-D-glucose (18FDG) in 12 patients with carcinoma of the lung. The tumor types were six squamous cell, two large cell, two oat cell, one adenocarcinoma, and one undifferentiated carcinoma. In each patient a transaxial plane was selected that contained the bulk of the tumor tissue. Regional density and blood volume were measured. Following the intravenous injection of 18FDG, the rates of uptake in the tumor and normal lung tissue were assessed from sequential scans over 1 hour. In each patient the rate of uptake of 18FDG in the tumor tissue was significantly increased relative to normal lung tissue. For the group the rate of uptake by the tumor was 211.4 +/- 69.4 ml/100 g/hr (mean +/- SD) compared to 31.9 +/- 13.2 in the contralateral lung (P less than 0.05). The tumor-to-normal tissue ratio of 6.6 (range, 2.7 to 14.6) was higher than previously reported ratios for brain and liver tumors. In contrast to brain tumors there was little correlation between tumor type and rate of 18FDG uptake. Measurements of glucose metabolism taken in vivo in human pulmonary tumors may lead to advances in screening, staging, and therapy.

Adult↗
PubMed1987

Contribution of the positron camera to studies of regional lung structure and function.

Positron emission tomography is a major technological advance in the characterisation of structure-function relationships within and between regions in normal and abnormal lungs (Hughes et al. 1985). The measurements are noninvasive and relatively exact since the geometric conditions are precisely defined. Regional expansion, flow (ventilation, perfusion), oxygen concentration (from VA/Q) and glucose metabolism can be measured in absolute terms per cubic centimetre of thorax or per gram of extravascular lung. Examples of structure-function relationships in normal subjects, emphysema, bronchitis and sarcoidosis are briefly presented.

Blood Glucose↗
PubMed1987

Regional lung density and blood volume in nonsmoking and smoking subjects measured by PET.

Regional lung density (DL) and regional fractional pulmonary blood volume (VB) were measured quantitatively during tidal breathing in 30 healthy supine subjects (15 smokers and 15 nonsmokers) in a 1.7-cm-thick midthoracic cross section using positron emission tomography (PET) and 11CO (inhaled)-labeled erythrocytes. Regional alveolar volume (VA), extravascular lung density (DEV), and relative alveolar size (Valv = VA/DEV) were calculated. For the nonsmokers, mean values (+/- SD between subjects) for the right lung were as follows: DL, 0.28 +/- 0.03 g/cm3; DEV, 0.10 +/- 0.02 g/cm3; and Valv, 7.1 +/- 1.9 ml/g lung tissue. In the smoking subjects DEV (right plus left lung) was 16% higher. No significant difference in VB between smokers and nonsmokers was found. The differences in DEV and VB between right and left lung were not significant. Mean values (+/- SD) of the dorsal-to-ventral ratios calculated for the right lung in the nonsmokers were as follows: DL, 1.34 +/- 0.16; VA, 0.90 +/- 0.05; VB, 1.52 +/- 0.26; DEV, 1.10 +/- 0.17; and Valv, 0.85 +/- 0.19. Almost identical ratios were found in the smokers. The influence of overall thoracic expansion was investigated in one subject restudied during voluntary hyperinflation and during positive end-expiratory pressure.

Adult↗
PubMed1987

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.

Acute Disease↗
PubMed1987

Quantification of regional ventilation in humans using a short-lived radiotracer--theoretical evaluation of the steady-state model.

The accuracy of the steady-state measurement of ventilation by means of a short-lived insoluble inert gas tracer rests with the validity of the steady-state flow equation. This has previously been applied to the qualitative assessment of regional ventilation using krypton-81m, but may potentially be used for the calculation of regional alveolar ventilation per unit alveolar gas volume--(VA/VA)cal--from measurements of the alveolar concentration of the tracer. The steady-state alveolar tracer concentration was calculated for the course of a breathing cycle, using a lung model featuring airways dead space and tidal gas flow. The calculations were made by computer simulations of a lung, characterized by predefined values of parameters describing the lung structure and the mode of ventilation. In the normal lung of supine man at rest (specific alveolar ventilation, ranging from 1.0 to 3.5 min-1) the errors of (VA/VA)cal relative to the predefined true values range from an overestimation by some 3% in the low ventilation regions to an underestimation by 8% in the best ventilated regions. The errors mainly result from ventilation of the airways dead space, which will influence the distribution of tracer in the lung by the transfer of tracer between regions by way of the common dead space and by the decay of tracer during its transport through the bronchial tree.

Humans↗
PubMed1986

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.

Adult↗
PubMed1985

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.

Densitometry↗
PubMed1984

Measurement of cerebral blood flow using bolus inhalation of C15O2 and positron emission tomography: description of the method and its comparison with the C15O2 continuous inhalation method.

This article describes a rapid method for the regional measurement of cerebral blood flow using a single breath of C15O2 and positron emission tomography. The technique is based on the bolus distribution principle and utilises a reference table for the calculation of flow. Seven subjects were studied using both this method and the C15O2 continuous inhalation steady-state technique. The single-breath method gave flow values 20% higher than those obtained using the steady-state method. A simulation study was performed in an attempt to define the reasons for the difference between the two techniques. Estimations were made of identified sources of error in the measurement of regional cerebral blood flow using the single-breath technique and compared with results from a similar study previously described for the steady-state technique. However, further comparative studies will be necessary to satisfactorily explain the difference between both techniques.

Adult↗
PubMed1984

Regional pulmonary blood volume in patients with abnormal blood pressure or flow in the pulmonary circulation.

We studied the effects of a chronic increase in flow and of chronic hypertension on regional pulmonary blood volume and extravascular lung density (lung tissue and interstitial water per unit thoracic volume) in one group of patients with intracardiac, left-to-right shunt and in another group with Eisenmenger's syndrome or primary pulmonary hypertension. We used positron computerized tomography to measure regional lung density (transmission scans) and blood volume (labelling with 11CO). The distribution of pulmonary blood volume was more uniform in patients with a chronic increase in pulmonary blood flow than in normal subjects. There were also indications of an absolute increase in intrapulmonary blood volume. In patients with chronic pulmonary arterial hypertension, the regional distribution of blood volume was abnormally uniform, but there were no indications of substantial abnormalities in overall intrapulmonary blood volume.

Adolescent↗
PubMed1984

Regional extravascular density and fractional blood volume of the lung in interstitial disease.

Regional lung density (g lung/ml thoracic volume) and fractional pulmonary blood volume (ml blood/ml thoracic volume) have been measured by positron tomography in 10 patients with interstitial disease. From the measurements regional extravascular lung density (g tissue and interstitial water/ml thoracic volume) was derived, providing a non-invasive measurement of the interstitial reaction. Extravascular lung density was increased and large regional variations were observed. Fractional blood volume was reduced in patients with pulmonary fibrosis. In two patients with sarcoidosis, a reduction in extravascular lung density occurred after treatment with oral prednisone. Abnormalities in extravascular lung density and fractional blood volume correlated with abnormalities shown by tests of overall pulmonary function.

Adult↗