Diagnosing Lung Nodules on Oncologic MR/PET Imaging: Comparison of Fast T1-Weighted Sequences and Influence of Image Acquisition in Inspiration and Expiration Breath-Hold

Abstract

Objective: First, to investigate the diagnostic performance of fast T1-weighted sequences for lung nodule evaluation in oncologic magnetic resonance (MR)/positron emission tomography (PET). Second, to evaluate the influence of image acquisition in inspiration and expiration breath-hold on diagnostic performance.

Materials and methods: The study was approved by the local Institutional Review Board. PET/CT and MR/PET of 44 cancer patients were evaluated by 2 readers. PET/CT included lung computed tomography (CT) scans in inspiration and expiration (CTin, CTex). MR/PET included Dixon sequence for attenuation correction and fast T1-weighted volumetric interpolated breath-hold examination (VIBE) sequences (volume interpolated breath-hold examination acquired in inspiration [VIBEin], volume interpolated breath-hold examination acquired in expiration [VIBEex]). Diagnostic performance was analyzed for lesion-, lobe-, and size-dependence. Diagnostic confidence was evaluated (4-point Likert-scale; 1 = high). Jackknife alternative free-response receiver-operating characteristic (JAFROC) analysis was performed.

Results: Seventy-six pulmonary lesions were evaluated. Lesion-based detection rates were: CTex, 77.6%; VIBEin, 53.3%; VIBEex, 51.3%; and Dixon, 22.4%. Lobe-based detection rates were: CTex, 89.6%; VIBEin, 58.3%; VIBEex, 60.4%; and Dixon, 31.3%. In contrast to CT, inspiration versus expiration did not alter diagnostic performance in VIBE sequences. Diagnostic confidence was best for VIBEin and CTex and decreased in VIBEex and Dixon (1.2 ± 0.6; 1.2 ± 0.7; 1.5 ± 0.9; 1.7 ± 1.1, respectively). The JAFROC figure-of-merit of Dixon was significantly lower. All patients with malignant lesions were identified by CTex, VIBEin, and VIBEex, while 3 patients were false-negative in Dixon.

Conclusion: Fast T1-weighted VIBE sequences allow for identification of patients with malignant pulmonary lesions. The Dixon sequence is not recommended for lung nodule evaluation in oncologic MR/PET patients. In contrast to CT, inspiration versus expiratory breath-hold in VIBE sequences was less crucial for lung nodule evaluation but was important for diagnostic confidence.

Keywords: Expiration; Inspiration; Lung; MR/PET; PET/MR; PET/MRI; Pulmonary nodule.

Fig. 1. Flow chart of patients.

Diagram showing inclusion of patients evaluated in study (n = number of patients). FDG = fluorodeoxyglucose, PET/MR = positron emission tomography/magnetic resonance, VIBEex = volume interpolated breath-hold examination acquired in expiration, VIBEin = volume interpolated breath-hold examination acquired in inspiration

Fig. 2. Lesion size.

Histogram showing frequency of lesion sizes for all 76 lesions found in 23 patients. Median lesion size was 6 mm; range was between 2 and 86 mm.

Fig. 3. 67-year-old male patient with lung cancer.

A. CT in inspiration. B. VIBE sequence in inspiration. C. Dixon sequence in expiration. In left upper lobe, round atelectasis is shown (arrow), which could easily be identified in CT as well as in VIBE sequence due characteristic comet sign. In Dixon sequence, atelectasis was rated as potentially malignant lesion with confidence rating of 2 (= very likely) leading to false positive lesion. VIBE = volume interpolated breath-hold examination

Fig. 4. 62-year-old male patient with thyroid cancer.

A. Dixon sequence in expiration. B. Fused PET/Dixon. C. VIBEin. D. VIBEex. In fused images, focal uptake was observed in periphery of left lung (arrow), which was rated as lesion based on PET information. In Dixon sequence, there was no correlate to finding while in VIBEin and VIBEex small subpleural nodule was visible, which was rated as potentially malignant with confidence level of 1 (= highly likely). In follow-up imaging, lesion grew in size and was rated as metastasis in standard of reference. PET = positron emission tomography, VIBEex = volume interpolated breath-hold examination acquired in expiration, VIBEin = volume interpolated breath-hold examination acquired in inspiration

Fig. 5. 62-year-old male patient with thyroid cancer.

A. CT in inspiration. B. VIBE in inspiration. In both modalities, lesion in right lung was identified (arrow). In MRI, lesion was assigned to middle lobe, in CT to upper lobe. This was attributed to non-visibility of pulmonary fissure in MRI, which may generally hamper lobe assignment in MRI especially in patients with accessory fissures or fissure distortions. In follow-up imaging, lesion grew in size and was rated as metastasis in standard of reference. VIBE = volume interpolated breath-hold examination

Fig. 6. 76-year-old female patient with thyroid cancer.

A. Dixon sequence in expiration. B. Fused PET/Dixon. C. VIBEin. D. VIBEex. In VIBE images, pulmonary lesion was found both in inspiratory and expiratory scan (arrow). In Dixon sequence, lesion was not adequately visible and scan was rated as negative. Also in fused PET/Dixon images, lesion was not identifiable. Lesion was classified as malignant based on standard of reference due to growth in size in follow-up imaging. PET = positron emission tomography, VIBE = volume interpolated breath-hold examination, VIBEex = volume interpolated breath-hold examination acquired in expiration, VIBEin = volume interpolated breath-hold examination acquired in inspiration