Multislice CT provides boost for COPD patients, but dose concerns persist

March 9, 2009

Multislice CT continues to spawn new applications in lung imaging, such as software techniques that allow depiction of airway morphology abnormalities in the prevalent chronic obstructive pulmonary disease. But radiologists are still not doing enough to cut the radiation dose while using the latest imaging tools, resulting in unnecessary and potentially hazardous excess exposure, according to some experts.

Multislice CT continues to spawn new applications in lung imaging, such as software techniques that allow depiction of airway morphology abnormalities in the prevalent chronic obstructive pulmonary disease. But radiologists are still not doing enough to cut the radiation dose while using the latest imaging tools, resulting in unnecessary and potentially hazardous excess exposure, according to some experts.

A refresher course on Tuesday morning will feature practical tips for dose optimization and new thinking on how CT should best be used in the classification of COPD, which affects an estimated 340 million people worldwide and is the fourth leading cause of death, according to the International COPD Coalition.

Increasingly, there is recognition that patients can have different underlying causes for their disease -- emphysema or irreversible reduction in the caliber of small airways (airway remodeling). Some have a combination of both conditions. Regardless of the cause, patients may have the same physiological profile and show similar degrees of airflow obstruction on pulmonary function tests, said Prof. Philippe Grenier, chairman of radiology at Pitié-Salpêtrire Hospital in Paris.

Finding the root cause is important because therapeutic options will differ. For example, those with very severe emphysema may be candidates for lung volume reduction surgery to improve function. Others have panlobular disease, which affects the secondary pulmonary lobule and is linked to genetic deficiencies that could be specifically targeted with therapies. Some other patients do not have emphysema but have abnormalities located on the small airways.

Using new software, radiologists are starting to use CT to segment and quantify the small airways, for example in measuring the walls of the proximal airways of the bronchi and in detecting inflammatory changes manifesting as abnormal thickening.

Radiologists have many options for reducing radiation dose without any degradation of image quality. In this optimised chest CT scan with dose-length product (DLP) of 176 mGy/cm in a standard patient (70 Kg ± 4 kg), the optimised dose was less than 40% of the reference dose levels (close to 500 mGy/cm) used worldwide. (Provided by D. Tack)

"We are able to quantify accurately both lungs, lobe by lobe. This was impossible before MDCT," Grenier said.

 

Treatment of patients with airway-predominant disease is typically pharmacological, involving prescription of inhaled steroids. New drugs are also in development and have the potential to expand the number of available treatments for this patient group. In the meantime, patients diagnosed with this type of COPD may participate in clinical drug trials.

Quantifying emphysema with CT is a complex task, and up to now a variety of techniques have been used with little standardization. However, a consensus among experts is emerging. Grenier and other expert members of the Fleischner Society for Thoracic Imaging and Diagnosis are in the process of writing recommendations on the best methods for classifying patients with COPD. He will provide an update on their progress during Tuesday's refresher course.

Though valuable for directing patient care, increased use of pulmonary imaging continues to cause concern about radiation exposure. During the course, Prof. Christian Herold, chairman of the radiology department at the Medical University of Vienna, will address this issue in the context of managing incidentally detected pulmonary nodules. He will review guidelines and explore a variety of criteria, such as patient age and nodule size, which may be used to manage workup. In many cases, very small nodules do not warrant a full follow-up, particularly in patients who are young and more vulnerable to effects of radiation dose and/or who have no risk factors for lung disease.

Statistics of average dose delivered suggest that dose optimization has been slow to materialize throughout this decade, said Dr. Denis Tack, a radiologist at Hôpital Réseau Hospitalier de Médecine Sociale (R.H.M.S.) in Baudour, Belgium, and the third speaker at the refresher course.

"We still perform too many examinations in many countries in Europe as well as the U.S. The dose problem has still not been resolved," he said. "Radiologists are overloaded with clinical problems, and diagnostic accuracy comes first. Care for dose reduction has no real place in everyday practice in CT. Very few radiologists know how to do it," he said.

To prevent needless exposure, departments should analyze literature regularly to identify indications that are actually validated, he advised. Lung perfusion imaging for pulmonary embolism using dual-energy MSCT is an example of a study with questionable benefits that may be overused.

"If we perform dual-phase acquisition perfusion studies to anybody undergoing CT angiography for pulmonary embolism, we deliver 40% more dose, but we still don't know if that brings progress in terms of diagnosis or prognosis," Tack said.

The predictive value of CT pulmonary angiography for pulmonary embolism is already close to 98%, so there is not a lot of room for improvement in accuracy, he added.

In association with national health authorities, Tack has been visiting imaging sites in Luxembourg and educating professionals about dose optimization. On average, these facilities have been able to cut dose by 40%, with acceptable reductions in image quality.

"Dose can be reduced, while the diagnosis is unchanged," he said.