Quantification of regional extravascular lung water in dogs with positron emission tomography, using constant infusion of 15O-labeled water

Lung water density is increased in patients at risk of heart failure and is largely independent of conventional cardiovascular magnetic resonance measures

Aims

Non-invasive methods to quantify pulmonary congestion are lacking in clinical practice. Cardiovascular magnetic resonance (CMR) lung water density (LWD) mapping is accurate and reproducible and has prognostic value. However, it is not known whether LWD is associated with routinely acquired CMR parameters.

Methods and results

This was an observational cohort including healthy controls and patients at risk of heart failure. LWD was measured using CMR with a free-breathing short echo time 3D Cartesian gradient-echo sequence with a respiratory navigator at 1.5 T. Associations were assessed between LWD, lung water volume and cardiac volumes, left ventricular (LV) mass and function, myocardial native T1, and extracellular volume fraction. In patients at risk for heart failure (n = 155), LWD was greater than in healthy controls (n = 15) (30.4 ± 5.0 vs. 27.2 ± 4.3%, P = 0.02). Using receiver operating characteristic analysis, the optimal cut-off for LWD was 27.6% to detect at-risk patients (sensitivity 72%, specificity 73%, positive likelihood ratio 2.7, and inverse negative likelihood ratio 2.6). LWD was univariably associated with body mass index (BMI), hypertension, right atrial area, and LV mass. In multivariable linear regression, only BMI remained associated with LWD (R 2 = 0.32, P < 0.001).

Conclusion

LWD is increased in patients at risk for heart failure compared with controls and is only weakly explained by conventional CMR measures. LWD provides diagnostic information that is largely independent of conventional CMR measures.

Noninvasive Imaging Methods for Quantification of Pulmonary Edema and Congestion: A Systematic Review

Quantification of pulmonary edema and congestion is important to guide diagnosis and risk stratification, and to objectively evaluate new therapies in heart failure. Herein, we review the validation, diagnostic performance, and clinical utility of noninvasive imaging modalities in this setting, including chest x-ray, lung ultrasound (LUS), computed tomography (CT), nuclear medicine imaging methods (positron emission tomography [PET], single photon emission CT), and magnetic resonance imaging (MRI). LUS is a clinically useful bedside modality, and fully quantitative methods (CT, MRI, PET) are likely to be important contributors to a more accurate and precise evaluation of new heart failure therapies and for clinical use in conjunction with cardiac imaging. There are only a limited number of studies evaluating pulmonary congestion during stress. Taken together, noninvasive imaging of pulmonary congestion provides utility for both clinical and research assessment, and continued refinement of methodologic accuracy, validation, and workflow has the potential to increase broader clinical adoption.

The evaluation of lung function with PET

In many ways, the lung is an ideal organ for study with positron emission tomography (PET). First, structure-function relations are homogeneous over larger areas than in other organs (reducing problems associated with otherwise relatively poor spatial resolution and partial-volume averaging). Second, many physiologic and metabolic processes can be studied, including pulmonary blood flow, ventilation, vascular permeability, endothelial receptor and enzyme function, among others. A variety of radiotracers have been used to evaluate pulmonary blood flow with PET, including 68Ga- or 11C-albumin microspheres administered intravenously, H2 15O administered by i.v. infusion, and 13N-N2 administered by inhalation. Pulmonary ventilation has been evaluated with both 13N-N2 and 19Ne gas, also administered by inhalation. In general, the relative advantage of one approach over another depends on site-specific cyclotron capacity and experience, and on the nature and timing of concomitant studies with other positron-emitting radiopharmaceuticals. The various blood flow methods have been used primarily in studies of pulmonary gas exchange, in both experimental animals and in humans. Acute lung injury is usually defined by both an increase in extravascular water (pulmonary edema) and an increase in the permeability of the pulmonary endothelium to protein. Both processes can easily be evaluated with PET. Extravascular water is measured by a combination of scans with i.v. H2 15O and C15O. The latter is administered by inhalation to label the blood pool (to calculate intravascular water concentrations). Pulmonary vascular permeability has been evaluated with dynamic sequential imaging after either 68Ga-transferrin or 11C-methylalbumin infusions. The rate of uptake of either tracer into the pulmonary extravascular space is an index of "leakiness" of the pulmonary endothelium, and is quantified as the pulmonary transcapillary escape rate, or PTCER. PTCER appears to be a highly sensitive index of acute lung injury. Two receptor/ enzyme systems that have been evaluated include the beta-adrenergic receptor system (using 11CGP-12177 as the ligand) and angiotensin converting enzyme (using 18F-fluorocaptopril). In each case, the object is to measure Bmax, or the maximum binding-capacity for the ligand in question. Changes in Bmax can be used to infer changes in protein expression of the receptor or enzyme, or can be used to quantify adequacy of therapy with inhibitor drugs. Given the highly active nature of the pulmonary endothelium, it is likely that many other pulmonary receptor or enzyme systems can be studied in a similar fashion.

Remote processing, delivery and injection of H2[15O] produced from a N2/H2 gas target using a simple and compact apparatus

We report here a simple apparatus for remote trapping and processing of H2[15O] produced from the N2/H2 target. The system performs a three step operation for H2[15O] delivery at the PET imaging facility which includes the following: (i) collecting the radiotracer in sterile water; (ii) adjusting preparation pH through removal of radiolytically produced ammonia, while at the same time adjusting solution isotonicity; and (iii) delivery of the radiotracer preparation to the injection syringe in a sterile and pyrogen-free form suitable for human studies. The processing apparatus is simple, can be remotely operated and fits inside a Capintec Dose Monitoring Chamber for direct measurement of accumulated radioactivity. Using this system, 300 mCi of H2[15O] (15 microA of 8 MeV D+ on target) is transferred from target through 120 m x 3.18 mm o.d. Impolene tubing to yield 100 mCi of H2[15O] which is isotonic, neutral and suitable for human studies. A remote hydraulically driven system for i.v. injection of the H2[15O] is also described. The device allows for direct measurement of syringe dose while filling, and for easy, as well as safe transport of the injection syringe assembly to the patient's bedside via a shielded delivery cart. This cart houses a hydraulic piston that allows the physician to "manually" inject the radiotracer without directly handling the syringe.

Current methodology for oxygen-15 production for clinical use

The radionuclide oxygen-15, half-life 2.05 min, is used in simple chemical forms to study oxygen metabolism, blood flow and blood volume in man, using the technique of positron emission tomography (PET). The production of 15O and the preparation of [15O]O2, [15O]CO2, [15O]CO and [15O]H2O is now well established in several PET centres in Europe, North America and Japan. In order to provide a practical design and operational data base for others intending to make use of these techniques an EEC task group representing four European laboratories routinely using 15O in PET studies was set up to review the current practice of 15O production purification and quality control of the clinically useful products.