The lungs have quite literally been a black hole for MRI for many years. Now advances in hardware and imaging techniques are helping lung MRI move into the clinical arena.
There are several reasons to use MRI for lung studies, Prof. Hans-Ulrich Kauczor, director of diagnostic radiology at the University Hospital in Heidelberg, Germany, told delegates at Sunday's New Horizons session. The modality can provide a comprehensive structural and functional assessment of the lungs in a single examination and has the potential to yield quantitative measurements. MRI's chief advantage, however, over CT is the absence of ionizing radiation.
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| Prof. Hans-Ulrich Kauczor |
Although lung MRI started about 25 years ago, with the first studies being conducted in the 1980s, researchers were unable to show any clinical value. The techniques were revived in the 1990s by select groups who explored the idea of hyperpolarized gas imaging, but once again the modality failed to make the transition into clinical practice. Over the past 10 years, however, the development of MR-specific contrast agents and the launch of next-generation systems offering parallel imaging have improved the quality of lung MRI.
Will the removal of technological barriers lead to broad clinical acceptance and applications in the forthcoming years? Kauczor followed this question with a provocative vision: MRI can simplify the current diagnostic workup for patients presenting with suspected pulmonary disease.
"Pulmonary lung function test, lab test, chest x-ray, echo, CT, bronchoscopy, scintigraphy... Why not try to solve all the stuff that is addressed nowadays by four or five imaging techniques with a single imaging technique, namely MRI?" he said.
The wide variety of lung MRI techniques was set out by Dr. Jim Wild, a medical physicist at the University of Sheffield in the U.K. His checklist included standard proton MRI, oxygen-enhanced imaging, contrast-enhanced pulmonary angiography, perfusion imaging, and hyperpolarized gas MRI (helium and xenon).
Wild acknowledged that some of the methods he presented would only ever be of value to radiology researchers. Others could make the transition into clinical practice, so long as the methods were simplified.
"Overall, the future looks bright for lung MRI," he said.
There is definitely light at the end of the tunnel for lung MRI, according to Dr. Jürgen Biederer, a radiologist at the University Hospital in Kiel, Germany. Biederer drew on examples from published literature to show the clinical merits of breath-hold and free-breathing sequences when seeking out neoplasms or evaluating inflammatory lung disease.
In the case of small nodular lesions, for example, MRI was shown to rival CT in terms of sensitivity. CT can detect 65% to 85% of nodules sized between 5 and 10 mm in diameter. Depending on the sequence, MRI has a sensitivity of ~75% to ~90% for 3 to 5-mm nodules, rising to ~95% to ~100% for 6 to 10-mm nodules.
"It is easier to read lung nodules on MRI than CT because they really stand out on a black background," Biederer said.
Lesion detection with MRI is not always trouble-free, though. Calcified metastases will be difficult to spot, and there can also be problems with artifacts.
Different sequences may be put together to make a comprehensive lung MRI protocol, he said. This protocol should be able to address a number of clinical questions, including pneumonia, solid lesions, pulmonary embolism, chronic airway disease, and tumor staging (mediastinal and chest wall invasion).
He showed a selection of 10 T1- and T2-weighted sequences that could be combined in various ways to make such a protocol. The ideal in-room time for such a protocol is 15 minutes. Some of the suggested combinations fit into this time window, others need an extra five or 10 minutes.
Speaking in the same session, Dr. Sebastian Ley, a radiologist at the University Hospital in Heidelberg, explained how steady-state free precession and contrast-enhanced MRA sequences can both be used to visualize the pulmonary arteries. The SSFP technique has the advantage of being free-breathing, whereas MRA is a breath-hold imaging technique, requiring patient cooperation. The two can be combined to make a seven-minute protocol for pulmonary embolism screening.
Ley predicted that CT angiography would typically be preferred in the acute setting, owing to the logistics of MRI. Either MRI or CT could be used when assessing patients with chronic pulmonary vascular disease. In the case of congenital abnormalities, MRA and MR perfusion imaging are the best choice, he said.
Note: a version of this article appeared in the 2010 ECR Today newspaper.
