The parametric response map (PRM) technique can be used in chronic obstructive pulmonary disease (COPD) to distinguish between disease subtypes, say US researchers.
The technique can characterize the relative contributions of emphysema and functional small-airways disease (fSAD) to overall airway obstruction.
"We believe this offers a new path to more precise diagnosis and treatment planning, and a useful tool for precisely assessing the impact of new medications and other treatments," said lead author Brian Ross (University of Michigan, Ann Arbor, USA) in a press release.
The authors carried out the technique on the computed tomography (CT) scans of 194 COPD patients enrolled in the COPDGene trial.
It involves overlaying CT images of the lungs taken during exhalation and inhalation. A different color is assigned to each small 3D area, known as voxels, according to the difference in the density measured between the two scans, which indicates whether tissue is healthy or diseased.
The authors showed that PRM correlated with standard measures of lung function while also providing information on the relative contribution of emphysema and fSAD to the disease.
For example, they show that in two patients with similar forced expiratory volume in one second (FEV1) measurements (55 and 59%), but different percentages of emphysema (5 and 19%), PRM could distinguish that while the first patient had minimal emphysema, 26% of their lung tissue was affected by fSAD. Meanwhile, progressive emphysema was confirmed in the second patient, but additionally their scan revealed 22% fSAD that would otherwise have been missed by current diagnostic methods.
Additionally, their results show that fSAD appears to precede emphysema and that in more severe cases of COPD (GOLD 3 and 4) lung obstruction is due to combined effects of the two.
CT scans are commonly used in COPD to evaluate the percentage of emphysema and mean lung density. However, the authors say PRM is different in measuring lung function on a voxel-by-voxel basis, allowing for local evaluation of COPD severity and phenotype.
"Being able to distinguish relative contributions of fSAD and emphysema in COPD phenotypes, may serve as a complementary readout to current pulmonary function tests and CT-based metrics that will allow for more accurate diagnosis and improve the treatment management of individual patients," Ross and colleagues write in Nature Medicine.
However, they add that further research will be needed in a larger cohort of patients to validate the use of PRM in the follow up of COPD patients.
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