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MRI has come a long way in the last two decades, according to an update presented at RSNA 2008. "In 2008, we use 1.5 and 3 Tesla, 200 T/m/sec, with parallel processing and MRI ventilation, perfusion, DWI, and so forth," stated Charles S. White, MD, Chief of Thoracic Radiology, Department of Radiology at the University of Maryland, Baltimore, Maryland, who presented the update. "Today, MRI has multiple thoracic applications."

MRI evaluation of the pulmonary nodule uses static MR imaging with an ultrafast spin echo sequence (< 1 sec/slice to freeze lung motion).  With high T2-relaxivity, T2 weighted images lead to higher signal for neoplasm and no signal from blood, unlike MDCT where vessels and nodules often have identical density. A sample protocol is TR/TE: 2 R-R intervals/23 milliseconds; Flip angle: 160°; an effective slice thickness: 5 mm, no; interslice gaps; matrix size, 158 x 256; Spatial resolution, 2.4 x 1.3 mm2. For a dynamic assessment, analogous to CT enhancement nodule evaluation, there is more enhancement in malignant nodules and differences in absolute upslope between benign and malignant nodules; but there are challenges with the overlaps with active infection. "Regarding diffusion weighted imaging (DWI), which relies on ability of H2O protons to diffuse into tissue," Dr. White explained, continuing, “it is less likely to be inhibited in cancer (tissues less intact), and thus, the technique may be able to distinguish benign from malignant nodules." In an MRI pulmonary nodule study of 54 nodules (36 malignant), with a 1-5 point signal intensity scale used (c/w spinal cord), the study found these average nodule characteristics: Malignant – 4, Benign – 2.5 (with statistical significance of p<0.01), but overlaps included small lesions and active infection.

MRI of the thorax, is also useful for lung cancer staging, such as the described in the RDOG study (Webb Radiology 1991; 178:705) where CT and MRI were shown to be largely equivalent. However, MRI may be useful for Pancoast tumors because of outstanding contrast resolution to assess invasion through the lung apex and MRI diffusion weighted imaging may distinguish benign from malignant mediastinal lymph nodes. As for use of MRI in physiologic lung imaging, MRI ventilation uses 3-He, 129-Xe (hy perpolarized) oxygen enhanced, and MRI pulmonary perfusion employs contrast enhancement with gadolinium or, alternatively, arterial spin labeling.

For MRI ventilation, He-3 and Xe-129 (after polarization) act as lung contrasts agents that rapidly diffuse to fill lung airspaces. “We can use this to assess the static and dynamic of ventilation with DWI, including apparent diffusion coefficient (ADC) and quantify asthma, COPD, Cystic fibrosis, and bronchiolitis obliterans syndrome after lung transplantation,” Dr. White told MedicExchange.  Oxygen-enhanced MRI uses T1 shortening of the weak paramagnetic effect of O2 for enhancement of lung parenchyma and blood by oxygen. A T1-weighted MR sequence is capable of acquiring signal from lung parenchyma. A change of T1 reflects change in the oxygen concentration in tissue. MRI lung perfusion is done with first pass imaging using bolus tracking after gadolinium injection.  Multiple images acquired with an ultrafast (time-resolved) T1W MRI sequence.  This approach allows assessment of pulmonary vascular looking for pulmonary embolism in a dynamic fashion.  Another approach, arterial spin labeling, allows evaluation of lung perfusion without contrast in a breathhold acquisition; however, only steady-state images are obtained.

Finally, MRI is a valuable adjunct to MDCT to assess the aorta.  Multiple sequence options include different types of dark blood imaging, while for bright blood imaging, steady state free-precesssion sequences (i.e. true FISP, Fiesta, FFE) are typically used.  Phase contrast imaging allows acquisition of directional and flow information.  For MR angiography, time resolved approaches can be used to assess aortic pathology such as aneurysm, dissection or congenital heart disease dynamically.  Newer 3D options are increasingly available.

“MRI has multiple thoracic applications,” Dr. White concluded, “Some are standard, but mainly 2nd line such as evaluation of Pancoast tumor and vascular conditions, while others are emerging such as lung perfusion.”