In 5 patients with lung cancer, UTE-MRI was used to derive pulmonary R2* in lung and establish its repeatability. Plausible R2* values were obtained only when using TEs of 0.08 and 0.2 ms: higher TEs produced implausible mean negative R2* within individual patients and a cohort R2* not significantly different from zero, due to lack of signal decay beyond TE=0.2ms. Pulmonary R2* values derived using TEs of 0.08 and 0.2ms were higher than prior reports where longer echo-times were employed. Test-retest limits-of-agreement were +90.5%to -47.5% indicating that a 90% increase in R2*is required post-radiotherapy to reliably demonstrate radiation-induced change.
5 patients (55 to 78 years old) with previous radiation therapy for lung cancer were scanned twice within 1 week (interval between scans 1 to 7 days). Data was acquired using a UTE protocol that utilized 3D isotropic k-space filling, with successive separate acquisitions at increasing TE. MRI was performed during free breathing on a Philips 3T Achieva, using a SENSE XL phased array torso coil. 3-D isotropic radial acquisition was performed (TR 10ms, 1 NSA, matrix 192 x 192; FOV 350mm; reconstructed to 192 slices of 3.5mm thickness in the transverse plane; voxel size 3.5 x 3.5 x 3.5mm), imaging from the dome of diaphragm to lung apices. The sequence was run seven times using TE values of 0.08, 0.2, 0.3, 0.4, 0.5, 0.75 and 1.0ms during shallow respiration. Morphological imaging was provided by coronal breath-held T1W e-THRIVE and transverse respiratory-triggered T2W TSE sequences. Total imaging time was approximately 40 minutes.
Regions of interest (ROI) were drawn within lung contralateral to tumour and therefore away from the radiation field. R2* values (= 1/T2*) were derived using voxel-wise linear regression analysis of the logarithmic plot of signal intensity decay with increasing TE (ADEPT software, ICR, Sutton) so as to generate R2* intensity maps using the following different combinations of echo times: (a) TE = 0.08 and 0.2 ms (points 1 to 2 of the signal intensity decay curve with increasing TE); (b) TE = 0.08, 0.2, 0.3, 0.4, 0.5, 0.75 and 1.0 ms (points 1 to 7) (c) TE = 0.2 and 0.3 ms (points 2 to 3); (d) TE = 0.2, 0.3 and 0.5 ms (points 2 to 5); and (e) TE = 0.2, 0.3, 0.4, 0.5, 0.75 and 1.0 ms (points 2 to 7). For each echo time combination, R2* values were also generated from a circular ROI placed in free air outside the patient as a comparator, on the same image slice as the pulmonary ROIs. For all analyses, R2* was calculated to include both positive and ‘physically implausible’ negative voxels values, within which signal decay did not occur with increasing TE. Following this, test-retest repeatability was calculated for R2* derived using the TE combination that provided consistently meaningful R2* values.
1. Egger C, Gerard C, Vidotto N, Accart N, Cannet C, Dunbar A, et al. Lung volume quantified by MRI reflects extracellular-matrix deposition and altered pulmonary function in bleomycin models of fibrosis: effects of SOM230. American journal of physiology Lung cellular and molecular physiology. 2014;306(12):L1064-77.
2. Bydder G. The Agfa Mayneord lecture: MRI of short and ultrashort T2 and T2* components of tissues, fluids and materials using clinical systems. The British journal of radiology. 2014.
3. Togao O, Tsuji R, Ohno Y, Dimitrov I, Takahashi M. Ultrashort echo time (UTE) MRI of the lung: assessment of tissue density in the lung parenchyma. Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine / Society of Magnetic Resonance in Medicine. 2010;64(5):1491-8.
4. Ohno Y, Nishio M, Koyama H, Yoshikawa T,
Matsumoto S, Seki S, et al. Pulmonary 3 T MRI with ultrashort TEs: influence of
ultrashort echo time interval on pulmonary functional and clinical stage
assessments of smokers. Journal of magnetic resonance imaging : JMRI.
2014;39(4):988-97.