Performance of free-breathing contrast-enhanced and unenhanced stack-of-spirals ultrashort echo time MRI for lung follow-up in patients with malignancies at 1.5T

Introduction

Chest computed tomography (CT) remains the first choice for lung imaging; however, its reliance on repeated scans with cumulative X-ray ionizing radiation doses for patients with malignant tumors raises significant concerns, particularly regarding the potential increased risk of cancer development (1,2). This risk is especially high for vulnerable groups such as pregnant women, young individuals, and cancer patients, who need frequent follow-up. While magnetic resonance imaging (MRI) is a radiation-free alternative, lung imaging with MRI faces challenges due to low proton density, rapid T2/T2* decay, and artifacts from cardiac and respiratory motion (3).

The development of ultrashort echo time (UTE) techniques (4-12) has enabled their widespread use with high sensitivity in assessing various lung diseases (8-13) and even airway visibility in infants (14). However, while UTE combined with radial k-space sampling has been explored for lung imaging (15), its inefficient k-space coverage (6,7,16) often results in prolonged scan times, increasing motion artifacts and hindering high-quality clinical imaging.

Recently, stack-of-spirals UTE (spiral-UTE) has been introduced, which uses a spiral trajectory with adaptive ultrashort TE to minimize T2* decay and employs nonselective radiofrequency (RF) pulses and variable-duration slice encoding (17,18). Unlike three-dimensional (3D) radial trajectories, spiral-UTE uses Cartesian sampling for slice selection and spiral sampling for in-plane encoding, achieving efficient “stack-of-spirals” k-space coverage. This method reduces the k-space coverage time, enabling faster scans and more efficient coverage than radial trajectories do (11,19-24), while maintaining image quality (17,22,25,26). Previous studies have evaluated image quality and lung lesion detection via contrast-enhanced (20,24) or noncontrast spiral-UTE (26,27) compared with CT in healthy volunteers or patients with lung abnormalities. However, a comprehensive comparison of contrast-enhanced and noncontrast spiral-UTE for lung imaging remains unexplored. An interpolated 3D T1-weighted gradient-echo (GRE) sequence referred to as volumetric interpolated breath-hold examination (VIBE) has been described (28), which is optimized for short acquisition time and achieves resolution improvements through the use of asymmetric k-space sampling and interpolation. As it is efficient and can form high-spatial-resolution images, this sequence has already been routinely applied in magnetic resonance (MR) scans of various body parts.

This study had two main objectives: first, to evaluate the image quality of both contrast-enhanced and noncontrast spiral-UTE compared with that of routine breath-hold contrast-enhanced T1WI (VIBE) for lung follow-up in patients with malignancies; second, to compare the nodule detection capabilities of spiral-UTE with and without contrast injection as a CT reference. We present this article in accordance with the STARD reporting checklist (available at https://qims.amegroups.com/article/view/10.21037/qims-2025-1871/rc).

Methods

Study population

This study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The current prospective study was approved by Medical Ethics Committee of Tongji Medical College, Huazhong University of Science and Technology (approval No. 2023122). All the enrolled participants agreed to the additional acquisition of spiral-UTE of the lungs, and written informed consent was obtained.

From January 2022 to February 2023, consecutive oncology patients who had undergone chest CT with suspected brain metastases were required clinically to get the gadolinium-based contrast-enhanced MR head scan within 7-day interval. The additional sequences [unenhanced spiral-UTE (UTEu), contrast-enhanced spiral-UTE (UTEe) and enhanced-VIBE] were prospectively acquired for the patients who were willing to be enrolled into the current study. The exclusion criteria were patients who refused to scan the additional three MR sequences, patients’ chest CT was beyond 7-day interval with MR scans, those with non-MR compatible pacemakers, metallic implants, and contraindications for contrast-enhanced MRI [known prior adverse reactions to the contrast agent, claustrophobia, pregnancy in women, severe dyspnea, continuous cough, inability to establish intravenous access, or acute or chronic severe renal impairment with estimated glomerular filtration rate (eGFR) <30 mL/min/1.73 m²].

Chest CT protocol

Clinically indicated unenhanced chest CT scans served as the reference standard. Scans were conducted on one of two CT scanners (Definition AS+ and Force, Siemens Healthcare, Forchheim, Germany) with patients in breath-hold and arms raised. The CT parameters included a 120 kV tube voltage, a modulated tube current, a patient-adapted field of view, a pitch of 1.2, and 1 mm slice thickness reconstructions via B31f (mediastinal window) and B70f (lung window) kernels. Scans were performed at end-inspiration.

MRI protocols

All scans were conducted on a 1.5T MR scanner (Magnetom Aera, Siemens Healthcare). Clinically indicated contrast-enhanced head MR scans were performed to assess intracranial metastases. A combined 18-channel matrix coil and 32-channel spine coil covered the lung area. Before the contrast material was administered, a 3D free-breathing spiral-UTE (UTEu) research protocol was added precontrast. Then intravenous gadobutrol (Gadovist; Bayer Schering Pharma, Berlin, Germany) was injected at 0.1 mL/kg body weight (1.5 mL/s injection rate), a contrast-enhanced head sequence would be scanned first (finished ~3 minutes postinjection), followed by axial and coronal enhanced VIBE with breath holding, and 3D free-breathing UTEe. The scans of VIBE and UTEe sequences did not need additional injection of any contrast material. The detailed parameters are listed in Table 1. In spiral-UTE, automatic respiratory gating was used to help achieve free-breathing mode, and slice partial Fourier 7/8 was applied to decrease acquisition times.

Image analysis

Quantitative assessment of MR lung signal intensity

Region-of-interest (ROI)-based analyses were performed for all 3 MR sequences. The apparent signal-to-noise ratio (SNR) and apparent contrast-to-noise ratio (CNR) were calculated to represent the quantitative assessment of lung signal intensity with the following formulas, which have been introduced previously (6,16,29): apparent SNR=SI_lung/SI_airway, apparent CNR=(SI_lung − SI_airway)/SI_vessel.

SI_lung represents the average signal intensity of the pulmonary parenchyma and is measured at three axial levels: the aortic arch, carina, and inferior pulmonary veins. ROIs were placed in the anterior and posterior lung parenchyma at each level, at least 2 cm away from the lung periphery to avoid partial volume effects, yielding 12 SIs per patient, with their average defined as the SI_lung. The SI_airway was calculated as the average SI of the ROIs in the trachea, left, and right main bronchi. SI_vessel was derived from ROIs in the pulmonary trunk and the left and right main pulmonary arteries. The measurements followed those of Dournes et al. (6), with values averaged across two radiographers (X.N.M. and A.D.X.) with 5 and 9 years of experience, respectively. The measurements of each ROI in the three MR sequences were acquired at the same locations. Interobserver variability was assessed for quantitative measurements.

Qualitative assessment of MR image quality

Two board-certified radiologists with 12 and 14 years of thoracic imaging experience (J.W.W. and X.S.) reviewed the chest CT images in consensus, recording pulmonary nodules >3 mm or tumors. Disagreements were resolved through discussion to establish a reference standard. Both mediastinal (width: 400 HU; level: 20 HU) and lung (width: 1,500 HU; level: −700 HU) window settings were used. A third radiologist with 30 years of experience (Chuan-Sheng Zheng) acted as an arbiter if needed, ensuring consensus on nodule presence, size (mm), lobar location, and attenuation type [solid, part-solid, ground-glass opacity (GGO)].

At least one month after CT, the MR lung images were evaluated by two blinded readers (J.W.W. and X.S.). They reviewed one sequence type (UTEu, UTEe, or enhanced-VIBE) at a time in random order, with a 2-week interval to prevent memory bias. While the MR sequence type was identifiable, other clinical information was blinded to ensure impartiality. Image quality was subjectively scored independently via a 5-point scale (4: excellent; 3: good; 2: fair; 1: poor; 0: not recognizable) (6) to assess fissures, airways, vessels, lung signal homogeneity, motion artifacts, lesion conspicuity, and overall image quality, with CT as the reference. The fissures evaluated included the left oblique fissure and right horizontal/oblique fissures. The airways assessed included the trachea, main bronchi, lobar bronchi, and 18 bronchopulmonary segments (30,31). The pulmonary arteries were similarly classified. Scores for coronal and axial enhanced VIBE were averaged for comparison. Interobserver agreement was analyzed, and reviewers could use multiplanar reconstructions from CT/MR 3D software as needed.

Detection of pulmonary nodules on MR

Two chest radiologists with 8 and 11 years of lung imaging experience (Z.K.L. and H.H.L.) reviewed MR images together to identify pulmonary nodules and were blinded to the CT results. A third radiologist with 30 years of experience (Chuan-Sheng Zheng) acted as an arbiter if a consensus was not reached. Pulmonary nodules, defined as rounded or irregular opacities >3 mm in diameter, were evaluated on the basis of prior studies and clinical guidelines for nodules requiring medical attention (11,32-34).

Statistical analysis

Continuous variables are expressed as the mean ± standard deviation (SD). Subjective scores were analyzed via Kruskal-Wallis one-way analysis of variance (ANOVA) with Bonferroni correction. Interobserver variability for quantitative measurements was assessed via Bland-Altman plots (35,36). Intraclass correlation coefficients (ICCs) were calculated for interobserver agreement and categorized as poor (<0.5), moderate (0.5 to <0.75), good (0.75 to <0.9), or excellent (>0.9). Statistical analyses were conducted via SPSS (IBM SPSS 22, IBM Corp., Armonk, NY, USA), with P<0.05 considered statistically significant.

Results

All the patients successfully underwent MR scans without any allergic reactions. A total of 76 oncology patients with confirmed malignant tumors (34 males, 42 females; aged 56.2±12.3 years) were enrolled in this single-center prospective study. Patients included those with lung cancer (n=40) and extrathoracic malignancies (n=36), who underwent follow-up CT for lung metastases and contrast-enhanced MR. Primary tumors included lung cancer (n=40), gastrointestinal cancer (n=17), cervical cancer (n=5), breast cancer (n=4), liver cancer (n=4), uterine endometrial cancer (n=2), ovarian cancer (n=1), esophageal cancer (n=1), nasopharyngeal cancer (n=1), and pancreatic cancer (n=1).

Quantitative assessment

With respect to the apparent SNR and apparent CNR, there were significant differences among UTEu, UTEe and enhanced VIBE (SNRs: 1.38±0.34, 1.78±0.43 and 0.42±0.16, P<0.001; CNRs: 0.05±0.03, 0.07±0.03, 0.03±0.01, P<0.001; Figure 1). Notably, UTEe obtained the best apparent SNR and apparent CNR, with significant differences compared with UTEu and enhanced-VIBE, and enhanced-VIBE yielded the lowest apparent SNR and apparent CNR among the three sequences.

Figure 1 Paired comparisons of apparent SNRs and apparent CNRs between UTEu, UTEe and enhanced VIBE in the 76 oncology patients (Kruskal-Wallis one-way ANOVA and Bonferroni correction were used). ANOVA, analysis of variance; CNR, contrast-to-noise ratio; SNR, signal-to-noise ratio; UTE, ultrashort echo time; UTEe, enhanced spiral-UTE; UTEu, unenhanced spiral-UTE; VIBE, volumetric interpolated breath-hold examination.

Qualitative assessment of image quality

Spiral-UTE significantly outperformed enhanced-VIBE in terms of all the metrics (P<0.001, Table 2). There was no significant difference between UTEu and UTEe in terms of fissure visibility; airway visibility in the trachea, main bronchus or lobar bronchus; signal homogeneity; motion artifacts; or lesion conspicuity, with P values >0.05 (Figure 2). Spiral-UTE consistently visualized bronchi up to the fourth level, whereas enhanced-VIBE provided visibility only up to the first and second levels. Notably, UTEe outperformed in depicting pulmonary vessels, surpassing both UTEu and enhanced VIBE.

Figure 2 A representative patient with breast cancer. Lung CT (A) revealed fissures (arrows) and a bulla (circles) in the left lung, which were not visible in enhanced-VIBE (B, score 0). Unenhanced (C) and enhanced (D) spiral-UTE clearly depicted those structures with scores of 3 and 4, respectively, in agreement with (A). CT, computed tomography; spiral-UTE, stack-of-spirals ultrashort echo time; VIBE, volumetric interpolated breath-hold examination.

Interobserver agreement

For quantitative measurements, Bland-Altman plots depicting interobserver agreement for the apparent SNR and apparent CNR measurements are shown in Figure 3. The repeatability coefficient and limits of agreement are summarized in Table S1. Interobserver reliability for apparent SNR measurement revealed an interobserver bias of −0.064±0.297 for UTEu, −0.016±0.517 for UTEe and −0.178±1.162 for VIBE; for apparent CNR measurement, the bias was −0.001±0.021 for UTEu, 0.007±0.152 for UTEe and −0.001±0.023 for VIBE, indicating a small interobserver bias of less than 0.178 for the apparent SNR and less than 0.007 for the apparent CNR. For the subjective scores, the ICCs depicting interobserver agreement between the two radiologists were rated as moderate to excellent, ranging from 0.744 to 0.965 (Table S2).

Figure 3 Bland-Altman plots depicting the reliability and agreement of the quantitative apparent SNR and apparent CNR measurements between the two observers. The dashed lines denote the limits of agreement, and the solid lines denote the means. The X-axis represents the average apparent SNR or apparent CNR values of the two observers, and the Y-axis represents the difference in the apparent SNRs or apparent CNRs of the two observers. CNR, contrast-to-noise ratio; SD, standard deviation; SNR, signal-to-noise ratio; UTE, ultrashort echo time; UTEe, enhanced spiral-UTE; UTEu, unenhanced spiral-UTE; VIBE, volumetric interpolated breath-hold examination.

Detection of pulmonary nodules

A total of 130 pulmonary nodules (≥3 mm in diameter) were identified in 51 patients via CT. The mean dose-length product was 196.12±52.41 mGy·cm, and the mean effective dose was 2.75±0.85 mSv. The time interval between the lung CT and MR scans was 1.29±1.20 days (range, 0–7 days).

Among the 130 nodules detected via CT, 100 were detected via both UTEu and UTEe, with a detection rate of 76.9%, and 62 were detected via VIBE, with a detection rate of 47.7%. The detection rates of pulmonary nodules are summarized in Table 3 according to size, location and attenuation type. The detection rates of UTEu and UTEe for any category were better than those of VIBE, regardless of nodule size, lobar location, or attenuation type.

As the nodule size category increased, the detection rate of the three sequences increased. The detection rates of UTEu, UTEe and VIBE were only 47.5%, 50.0% and 15.0%, respectively, for nodules 3–5 mm; 90.0% (81/90), 88.9% (80/90) and 66.7% (60/90), respectively, for nodules ≥5 mm; and 98.5% (64/65), 98.5% (64/65), and 84.6% (55/65), respectively, for nodules ≥7 mm; all 100.0% for nodules ≥10 mm (Figures 4,5). Three nodules identified on VIBE, one on UTEu, and one on UTEe—all located near the diaphragm or heart—were misclassified as small bronchi or vessels and thus were considered false-positive cases; therefore, the sensitivity and positive predictive value (PPV) of UTEu were 76.9% and 99.0% (100/101), those of UTEe were 76.9% and 99.0% (100/101), and those of VIBE were 47.7% and 95.4% (62/65).

Figure 4 Representative axial images of lung CT (A), enhanced VIBE (B), unenhanced spiral-UTE (C) and enhanced spiral-UTE (D) with three solid pulmonary nodules (arrows; ranging from 3–10 mm in maximum diameter) from a 55-year-old female patient with breast cancer. Nodule 1, which was 4.3 mm in diameter in the left upper lobe, is clearly shown in (A1,C1,D1) but was missed by (B1); nodule 2, which was 7.3 mm in the left upper lobe, and nodule 3, which was 10.5 mm in diameter in the right lower lobe, are clearly shown in all the images (A2-D2,A3-D3). CT, computed tomography; spiral-UTE, stack-of-spirals ultrashort echo time; VIBE, volumetric interpolated breath-hold examination.

Figure 5 Representative axial images of lung CT (A), enhanced-VIBE (B), unenhanced spiral-UTE (C) and enhanced spiral-UTE (D) with solid pulmonary nodules (arrows). Images of a 64-year-old male patient with identified lung cancer and an irregular solid nodule with a maximum diameter of 28.6 mm in the right upper lobe (A1) is clearly shown in (C1) and (D1); however, it was only shown as an increased signal-intensity patch in (B1), which limited the definite diagnosis. Images of a 64-year-old male patient with confirmed lung cancer; a solid tumor with a maximum diameter of 32.2 in the left upper lobe (A2) was clearly detected in (B2-D2). CT, computed tomography; spiral-UTE, stack-of-spirals ultrashort echo time; VIBE, volumetric interpolated breath-hold examination.

The detection rate was lower for both lower lobes (UTEu, 69.5%, 41/59; UTEe, 67.8%, 40/59; VIBE, 42.4%, 25/59) than for both upper lobes (UTEu, 80.6%, 50/62; UTEe, 82.3%, 51/62; VIBE, 51.6%, 32/62). For different attenuation types, the detection rate for solid nodules was higher than that in the part-solid and GGO groups for UTE (Figure 6).

Figure 6 Representative images of axial images of lung CT (A), enhanced-VIBE (B), unenhanced spiral-UTE (C) and enhanced spiral-UTE (D) with solid, part-solid and ground-glass opacity pulmonary nodules. A solid nodule with a maximum diameter of 25.4 mm (A1, long arrow) was clearly shown in (B1-D1, long arrows), and a part-solid nodule with a maximum diameter of 14.6 mm (A1, arrowhead) was missed in (B1) but clearly shown in (C1 and D1, arrowheads). A ground-glass opacity in (A2; long arrow) was missed in (B2-D2). CT, computed tomography; spiral-UTE, stack-of-spirals ultrashort echo time; VIBE, volumetric interpolated breath-hold examination.

Discussion

In this work, we evaluated the performance of free-breathing spiral-UTE for lung follow-up in a cohort of patients with malignant tumors. Comparative analysis demonstrated that spiral-UTE (both enhanced and nonenhanced spiral-UTE) significantly outperformed routine VIBE for visualizing anatomical structures and detecting pulmonary nodules. Both enhanced and unenhanced spiral-UTE showed comparable capabilities in terms of pulmonary nodule size, lobar distribution and attenuation type, providing evidence for proper choice in the clinical setting.

With respect to image quality, our study demonstrated that free-breathing spiral-UTE outperformed enhanced-VIBE in depicting anatomical lung structures and improving the assessment of signal uniformity, motion artifacts and lesion conspicuity. This is consistent with previous findings (37), which compared the detection of lung lesions between free-breathing spiral UTE and breath-hold VIBE. Furthermore, enhanced VIBE proved inadequate for visualizing fissures and bronchi, particularly at the segmental level, whereas spiral-UTE could provide acceptable to excellent diagnostic image quality across all the morphological assessments, confirming previous scoring results (20,37).

To our knowledge, prior studies investigating the clinical use of spiral-UTE have focused primarily on either contrast-enhanced spiral-UTE (20,24) or unenhanced spiral-UTE (27,37,38) in comparison to lung CT. The current study expands on the literature by including a direct comparison between enhanced and unenhanced spiral-UTE with routine VIBE, using lung CT as a reference. Our results reinforce previous findings that enhanced and unenhanced spiral-UTE are comparable in terms of fissure visibility, segmental bronchus visibility, signal homogeneity, motion artifacts and lesion conspicuity. However, enhanced spiral-UTE outperformed unenhanced spiral-UTE in visualizing pulmonary vessels, likely attributable to the increased signal intensities of the pulmonary vasculature following gadolinium contrast agent administration. Additionally, enhanced spiral-UTE achieved the highest apparent SNR and apparent CNR, followed by unenhanced spiral-UTE and enhanced-VIBE, which performed the worst. This observation is consistent with prior reports indicating that a higher SNR in the lung parenchyma and vessels is obtained after contrast material application (39,40). In contrast, enhanced VIBE exhibited limited tissue contrast between the pulmonary parenchyma and bronchus.

To date, the combination of spiral-UTE with the SPIRiT (iterative self-consistent parallel imaging reconstruction) algorithm (41) in our study has been successfully applied in detecting metastatic solid nodules in breast cancer patients, demonstrating its ability to achieve a reasonable scan duration, higher readout efficiency, satisfactory image quality (20,24), and fewer artifacts than the radial acquisition method (26). In our study, the integration of automatic respiratory synchronization into spiral-UTE enabled free-breathing scanning, making it particularly advantageous for patients who have difficulty with breath-holding and eliminating the need for external belt placement (26). Additionally, preexamination respiratory training played a crucial role in reducing scan time by encouraging a rhythmic breathing pattern, which facilitated sufficient data acquisition within a shorter time frame.

Numerous studies have explored lung UTE as a promising alternative to CT, with a high pulmonary nodule detection rate ranging from 81% to 93% (8,11,20,24,27,38,42). In our study, the overall nodule detection rate for spiral UTE was 76.9%, which was in accordance with these previous studies and supports the potential of spiral-UTE as a viable alternative to chest CT, particularly for patients requiring repeated imaging without the risks associated with X-ray radiation exposure. Our results revealed that the detection sensitivity of spiral-UTE varied with nodule size, increasing progressively with increasing nodule size. Specifically, spiral-UTE achieved a sensitivity of 68.0% for nodules 5 to <7 mm in diameter, 95.0% for nodules 7 to <10 mm in diameter and 100.0% for nodules ≥10 mm in diameter. These findings are consistent with UTE-related previous reports, such as those of Burris et al. (71% for nodules ≥4 to <6 mm, 83% for nodules ≥6 to <8 mm, 93% for nodules ≥8 to <10 mm and 100% for nodules larger than 10 mm; 1.25 mm³ isovoxel; 8 patients, unenhanced UTE) (8); Ohno et al. (94.3% for nodules 6–8 mm, 100% for nodules 8–15 mm and 100% for nodules >15 mm; 1 mm³ isovoxel; 52 patients, unenhanced UTE) (11); Cha et al. (100% for nodules ≥5 to <7 mm, ≥7 to <10 mm and larger than 10 mm; 1.5 mm³; 32 patients, unenhanced UTE) (24); Cha et al. (89.7% for nodules ≥5 to <7 mm, 100% for nodules ≥7 to <10 mm and 100% for nodules larger than 10 mm; 0.98 mm³ isovoxel; 29 patients, enhanced UTE) (20); Huang et al. (80% for nodules ≥6 to <8 mm, 100% for nodules ≥8 to <10 mm and 100% for nodules larger than 10 mm; 1.56 mm³ isovoxel; 71 patients, unenhanced UTE) (38); and Jiang et al. (27) (82.61% for diameter ≤6 mm, 90.91% for nodules 6–10 mm, 89.19% for nodules ≥10 mm with 3 mm slice thickness; 52 patients, unenhanced UTE).

However, the detection rate for nodules measuring 3–5 mm in diameter was only 50.0% in our study, which largely mirrors some previous studies reporting relatively low detection rates for nodules smaller than 5 mm via various MR methods (8,38,43-45). This value was lower than the rates reported by Cha et al. (20,24), who reported detection rates of 76.7% and 88.1% for small nodules 2–5 mm in diameter, and several factors may account for this discrepancy. First, our study included a broader range of nodule types, including part-solid, GGO, and solid nodules, whereas Cha et al. (20) focused exclusively on solid nodules. Second, the lower isometric spatial resolution (≈1.6 mm³) in our study, combined with the higher proportion of nodules in the 3–5 mm size range, may have contributed to the reduced detection rate for smaller nodules. Overall, our findings support previous studies indicating that while spiral-UTE has limited sensitivity for detecting nodules less than 5 mm, it has high sensitivity for detecting larger nodules, reaching 90.0% for nodules larger than 5 mm, 98.5% for those larger than 7 mm and 100.0% for those larger than 10 mm.

In terms of lobar localization, the detection rate of spiral-UTE was superior to that of VIBE. The detection rate for pulmonary nodules in the lower lobes tended to be lower than that for nodules in the upper lobes. This finding was similar to that of a previous report (20) that evaluated the detection rate of solid pulmonary nodules. The potential causes of this observed phenomenon may be attributed to two principal factors. First, nodules located in the lower lobes of the lungs are more susceptible to being obscured by cardiac pulsation artifacts and respiratory motion artifacts, which degrade image quality and reduce nodule visibility. Second, as patients are routinely positioned in the supine posture for scanning procedures, nodules situated in the posterior aspects of the bilateral lower pulmonary lobes have a higher tendency to be impacted by gravitational atelectasis, with a subsequent reduction in their visibility. In our study, the detection rates for the upper lobes (82.3%) and lower lobes (69.5%) via spiral-UTE were slightly lower than those reported by Cha et al. (20), who reported detection rates of 91.8% for the upper lobes and 86.1% for the lower lobes. This can also be explained by the lower spatial resolution and the inclusion of part-solid and GGO nodules in our study.

Unlike some prior studies that explored free-breathing spiral-UTE for pulmonary nodule assessment, Cha et al. (20) focused primarily on detecting solid breast cancer metastatic nodules. Their analysis compared contrast-enhanced spiral-UTE with CT. Separately, Jiang et al. (27) evaluated the diagnostic performance of free-breathing spiral-UTE against free-breathing radial VIBE. In our study, by contrast, we compared the performance of contrast-enhanced and unenhanced free-breathing spiral-UTE. Our results demonstrated that, regardless of pulmonary nodule size, lobar distribution or attenuation type, the nodule detection capability of spiral-UTE (with or without contrast) was comparable when using CT as a reference. This finding provides new insights into the clinical utility of spiral-UTE for lung imaging, particularly in scenarios where contrast agent administration may not be feasible or necessary. To our knowledge, this is the first study to directly compare the performance of enhanced and unenhanced spiral-UTE in a comprehensive manner, offering valuable evidence that both approaches yield similar diagnostic accuracy for pulmonary nodule detection. This finding significantly advances the understanding of spiral-UTE lung imaging and underscores its potential as a radiation-free alternative to CT for patients requiring longitudinal follow-up.

There are several limitations in this study that should be acknowledged. First, this was a single-center study. Second, the capacity of spiral-UTE for differentiating malignant from benign nodules was not evaluated because of the lack of histologic confirmation for all detected nodules. Third, while we initially tested spiral-UTE at a higher resolution (1.2×1.2×1.2 mm³), the substantial computational load required for image reconstruction and occasional reconstruction failure necessitated an adjustment to a voxel size of ≈1.6×1.6×1.6 mm³. This adjustment may cause some bias when comparing spiral-UTE with VIBE, which uses slightly different voxel sizes. Fourth, the five radiologists involved in this study were highly experienced in chest imaging, which may have led to an overestimation of the detection rate if less experienced radiologists were involved. Fifth, although the blinded mode was applied, the MR sequence type was quite easily recognizable to radiologists due to visual differences, which may introduce bias despite the interval between readings. Sixth, the delay time of the postcontrast scans may cause some bias affecting the measurements of the apparent SNR and apparent CNR but would not change our comparative results. Seventh, the ability to detect newly occurring metastatic nodules for each patient in terms of the time dimension was not explored in the current study but will be explored in our next step. Eighth, even though true SNR and true CNR both should be normalized with the SD of the noise-only region, apparent SNR and apparent CNR values are more accessible in a clinical setting, which we believe would not affect the results of the current study. Ninth, a potential limitation of this study is the inconsistency in flip angle settings between the spiral-UTE and VIBE sequences. The flip angle for UTE (4°) was selected according to the calculated Ernst angle for lung parenchyma (approximately 4.2° at 1.5T, assuming T1≈1,200 ms), to optimize signal from short-T2 tissues without contrast enhancement. In contrast, the VIBE sequence used a higher flip angle of 9°, which was originally optimized for abdominal post-contrast imaging and inadvertently applied to the non-contrast scans. This difference may have influenced the relative signal and contrast between lung parenchyma and focal lesions. Future studies will aim to harmonize the acquisition parameters and optimize flip angles specifically for non-contrast lung imaging. Finally, the prototype spiral-UTE is a vendor-specific sequence that may not be acquired on other platforms.

Conclusions

Free-breathing spiral-UTE offers superior visualization of pulmonary anatomical structures and lesion depiction than does routine VIBE. Both contrast-enhanced and unenhanced spiral-UTE demonstrate comparable image quality and high sensitivity for detecting pulmonary nodules larger than 5 mm, proving to be potential alternatives to CT for patients with malignancies requiring longitudinal follow-up.

Acknowledgments

We would like to express our sincere gratitude to Chuan-Sheng Zheng and Zi-Qiao Lei for their invaluable assistance and consistent support.

Footnote

Reporting Checklist: The authors have completed the STARD reporting checklist. Available at https://qims.amegroups.com/article/view/10.21037/qims-2025-1871/rc

Data Sharing Statement: Available at https://qims.amegroups.com/article/view/10.21037/qims-2025-1871/dss

Funding: This study was supported by Science and Foundation of Union Hospital, Tongji Medical College, Huazhong University of Science and Technology, Wuhan, China (No. 2024XHYN063).

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://qims.amegroups.com/article/view/10.21037/qims-2025-1871/coif). T.Y. and T.B. work in Siemens Healthineers Ltd. P.S. works in Neusoft Medical Systems Co. Ltd. Q.F. reports that this study was supported by Science and Foundation of Union Hospital, Tongji Medical College, Huazhong University of Science and Technology, Wuhan, China (No. 2024XHYN063). The other authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. This study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. This current prospective study was approved by Medical Ethics Committee of Tongji Medical College, Huazhong University of Science and Technology (approval No. 2023122). All the enrolled participants agreed to be enrolled into this study and written informed consent was obtained from all the patients.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

References

1. Brenner DJ, Hall EJ. Cancer risks from CT scans: now we have data, what next? Radiology 2012;265:330-1. [Crossref] [PubMed]
2. Pearce MS, Salotti JA, Little MP, McHugh K, Lee C, Kim KP, Howe NL, Ronckers CM, Rajaraman P, Sir Craft AW, Parker L, Berrington de González A. Radiation exposure from CT scans in childhood and subsequent risk of leukaemia and brain tumours: a retrospective cohort study. Lancet 2012;380:499-505. [Crossref] [PubMed]
3. Dournes G, Macey J, Blanchard E, Berger P, Laurent F. Rev Pneumol Clin 2017;73:40-9. [MRI of the pulmonary parenchyma: Towards clinical applicability?].
4. Mayo JR, MacKay A, Müller NL. MR imaging of the lungs: value of short TE spin-echo pulse sequences. AJR Am J Roentgenol 1992;159:951-6. [Crossref] [PubMed]
5. Robson MD, Gatehouse PD, Bydder M, Bydder GM. Magnetic resonance: an introduction to ultrashort TE (UTE) imaging. J Comput Assist Tomogr 2003;27:825-46. [Crossref] [PubMed]
6. Dournes G, Grodzki D, Macey J, Girodet PO, Fayon M, Chateil JF, Montaudon M, Berger P, Laurent F. Quiet Submillimeter MR Imaging of the Lung Is Feasible with a PETRA Sequence at 1.5 T. Radiology 2015;276:258-65. [Crossref] [PubMed]
7. Gai ND, Malayeri A, Agarwal H, Evers R, Bluemke D. Evaluation of optimized breath-hold and free-breathing 3D ultrashort echo time contrast agent-free MRI of the human lung. J Magn Reson Imaging 2016;43:1230-8. [Crossref] [PubMed]
8. Burris NS, Johnson KM, Larson PE, Hope MD, Nagle SK, Behr SC, Hope TA. Detection of Small Pulmonary Nodules with Ultrashort Echo Time Sequences in Oncology Patients by Using a PET/MR System. Radiology 2016;278:239-46. [Crossref] [PubMed]
9. Heidenreich JF, Weng AM, Metz C, Benkert T, Pfeuffer J, Hebestreit H, Bley TA, Köstler H, Veldhoen S. Three-dimensional Ultrashort Echo Time MRI for Functional Lung Imaging in Cystic Fibrosis. Radiology 2020;296:191-9. [Crossref] [PubMed]
10. Roach DJ, Crémillieux Y, Serai SD, Thomen RP, Wang H, Zou Y, Szczesniak RD, Benzaquen S, Woods JC. Morphological and quantitative evaluation of emphysema in chronic obstructive pulmonary disease patients: A comparative study of MRI with CT. J Magn Reson Imaging 2016;44:1656-63. [Crossref] [PubMed]
11. Ohno Y, Koyama H, Yoshikawa T, Kishida Y, Seki S, Takenaka D, Yui M, Miyazaki M, Sugimura K. Standard-, Reduced-, and No-Dose Thin-Section Radiologic Examinations: Comparison of Capability for Nodule Detection and Nodule Type Assessment in Patients Suspected of Having Pulmonary Nodules. Radiology 2017;284:562-73. [Crossref] [PubMed]
12. Bannas P, Bell LC, Johnson KM, Schiebler ML, François CJ, Motosugi U, Consigny D, Reeder SB, Nagle SK. Pulmonary Embolism Detection with Three-dimensional Ultrashort Echo Time MR Imaging: Experimental Study in Canines. Radiology 2016;278:413-21. [Crossref] [PubMed]
13. Landini N, Orlandi M, Occhipinti M, Nardi C, Tofani L, Bellando-Randone S, Ciet P, Wielopolski P, Benkert T, Bruni C, Bertolo S, Moggi-Pignone A, Matucci-Cerinic M, Morana G, Colagrande S. Ultrashort Echo-Time Magnetic Resonance Imaging Sequence in the Assessment of Systemic Sclerosis-Interstitial Lung Disease. J Thorac Imaging 2023;38:97-103. [Crossref] [PubMed]
14. Niwa T, Nozawa K, Aida N. Visualization of the airway in infants with MRI using pointwise encoding time reduction with radial acquisition (PETRA). J Magn Reson Imaging 2017;45:839-44. [Crossref] [PubMed]
15. Grodzki DM, Jakob PM, Heismann B. Ultrashort echo time imaging using pointwise encoding time reduction with radial acquisition (PETRA). Magn Reson Med 2012;67:510-8. [Crossref] [PubMed]
16. Johnson KM, Fain SB, Schiebler ML, Nagle S. Optimized 3D ultrashort echo time pulmonary MRI. Magn Reson Med 2013;70:1241-50. [Crossref] [PubMed]
17. Mugler JP III, Fielden SW, Meyer CH, Altes TA, Miller GW, Stemmer A, Pfeuffer J, Kiefer B. Breath-hold UTE Lung Imaging using a Stack-of-Spirals Acquisition. In: Proceedings of Annual Meeting of ISMRM, Toronto, 2015. (abstract 1476). Available online: https://archive.ismrm.org/2015/1476.html
18. Mugler JP III, Meyer CH, Pfeuffer J, Stemmer A, Kiefer B. Accelerated Stack-of-Spirals Breath-hold UTE Lung Imaging. In: Proceedings of Annual Meeting of ISMRM, Honolulu, 2017. (Abstract 4904). Available online: https://archive.ismrm.org/2017/4904.html
19. Heidenreich JF, Veldhoen S, Metz C, Mendes Pereira L, Benkert T, Pfeuffer J, Bley TA, Köstler H, Weng AM. Functional MRI of the Lungs Using Single Breath-Hold and Self-Navigated Ultrashort Echo Time Sequences. Radiol Cardiothorac Imaging 2020;2:e190162. [Crossref] [PubMed]
20. Cha MJ, Ahn HS, Choi H, Park HJ, Benkert T, Pfeuffer J, Paek MY. Accelerated Stack-of-Spirals Free-Breathing Three-Dimensional Ultrashort Echo Time Lung Magnetic Resonance Imaging: A Feasibility Study in Patients With Breast Cancer. Front Oncol 2021;11:746059. [Crossref] [PubMed]
21. Huang YS, Niisato E, Su MM, Benkert T, Hsu HH, Shih JY, Chen JS, Chang YC. Detecting small pulmonary nodules with spiral ultrashort echo time sequences in 1.5 T MRI. MAGMA 2021;34:399-409. [Crossref] [PubMed]
22. Qian Y, Boada FE. Acquisition-weighted stack of spirals for fast high-resolution three-dimensional ultra-short echo time MR imaging. Magn Reson Med 2008;60:135-45. [Crossref] [PubMed]
23. Kumar S, Rai R, Stemmer A, Josan S, Holloway L, Vinod S, Moses D, Liney G. Feasibility of free breathing Lung MRI for Radiotherapy using non-Cartesian k-space acquisition schemes. Br J Radiol 2017;90:20170037. [Crossref] [PubMed]
24. Cha MJ, Park HJ, Paek MY, Stemmer A, Lee ES, Park SB, Kim YS. Free-breathing ultrashort echo time lung magnetic resonance imaging using stack-of-spirals acquisition: A feasibility study in oncology patients. Magn Reson Imaging 2018;51:137-43. [Crossref] [PubMed]
25. Kasper L, Engel M, Barmet C, Haeberlin M, Wilm BJ, Dietrich BE, Schmid T, Gross S, Brunner DO, Stephan KE, Pruessmann KP. Rapid anatomical brain imaging using spiral acquisition and an expanded signal model. Neuroimage 2018;168:88-100. [Crossref] [PubMed]
26. Dournes G, Yazbek J, Benhassen W, Benlala I, Blanchard E, Truchetet ME, Macey J, Berger P, Laurent F. 3D ultrashort echo time MRI of the lung using stack-of-spirals and spherical k-Space coverages: Evaluation in healthy volunteers and parenchymal diseases. J Magn Reson Imaging 2018;48:1489-97. [Crossref] [PubMed]
27. Jiang Y, Pu D, Zhang X, Ren Z, Yu N. Comparison of diagnostic performance for pulmonary nodule detection between free-breathing spiral ultrashort echo time and free-breathing radial volumetric interpolated breath-hold examination. BMC Med Imaging 2025;25:15. [Crossref] [PubMed]
28. Rofsky NM, Lee VS, Laub G, Pollack MA, Krinsky GA, Thomasson D, Ambrosino MM, Weinreb JC. Abdominal MR imaging with a volumetric interpolated breath-hold examination. Radiology 1999;212:876-84. [Crossref] [PubMed]
29. Ma W, Sheikh K, Svenningsen S, Pike D, Guo F, Etemad-Rezai R, Leipsic J, Coxson HO, McCormack DG, Parraga G. Ultra-short echo-time pulmonary MRI: evaluation and reproducibility in COPD subjects with and without bronchiectasis. J Magn Reson Imaging 2015;41:1465-74. [Crossref] [PubMed]
30. Bhalla M, Turcios N, Aponte V, Jenkins M, Leitman BS, McCauley DI, Naidich DP. Cystic fibrosis: scoring system with thin-section CT. Radiology 1991;179:783-8. [Crossref] [PubMed]
31. Helbich TH, Heinz-Peer G, Eichler I, Wunderbaldinger P, Götz M, Wojnarowski C, Brasch RC, Herold CJ. Cystic fibrosis: CT assessment of lung involvement in children and adults. Radiology 1999;213:537-44. [Crossref] [PubMed]
32. Wielpütz MO, Lee HY, Koyama H, Yoshikawa T, Seki S, Kishida Y, Sakai Y, Kauczor HU, Sugimura K, Ohno Y. Morphologic Characterization of Pulmonary Nodules With Ultrashort TE MRI at 3T. AJR Am J Roentgenol 2018;210:1216-25. [Crossref] [PubMed]
33. Sommer G, Tremper J, Koenigkam-Santos M, Delorme S, Becker N, Biederer J, Kauczor HU, Heussel CP, Schlemmer HP, Puderbach M. Lung nodule detection in a high-risk population: comparison of magnetic resonance imaging and low-dose computed tomography. Eur J Radiol 2014;83:600-5. [Crossref] [PubMed]
34. MacMahon H, Naidich DP, Goo JM, Lee KS, Leung ANC, Mayo JR, Mehta AC, Ohno Y, Powell CA, Prokop M, Rubin GD, Schaefer-Prokop CM, Travis WD, Van Schil PE, Bankier AA. Guidelines for Management of Incidental Pulmonary Nodules Detected on CT Images: From the Fleischner Society 2017. Radiology 2017;284:228-43. [Crossref] [PubMed]
35. Bland JM, Altman DG. Measuring agreement in method comparison studies. Stat Methods Med Res 1999;8:135-60. [Crossref] [PubMed]
36. Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet 1986;1:307-10.
37. Olthof SC, Reinert C, Nikolaou K, Pfannenberg C, Gatidis S, Benkert T, Küstner T, Krumm P. Detection of lung lesions in breath-hold VIBE and free-breathing Spiral VIBE MRI compared to CT. Insights Imaging 2021;12:175. [Crossref] [PubMed]
38. Huang YS, Niisato E, Su MM, Benkert T, Chien N, Chiang PY, Lee WJ, Chen JS, Chang YC. Applying Compressed Sensing Volumetric Interpolated Breath-Hold Examination and Spiral Ultrashort Echo Time Sequences for Lung Nodule Detection in MRI. Diagnostics (Basel) 2021;12:93. [Crossref] [PubMed]
39. Wielpütz M, Kauczor HU. MRI of the lung: state of the art. Diagn Interv Radiol 2012;18:344-53. [Crossref] [PubMed]
40. Bell LC, Johnson KM, Fain SB, Wentland A, Drees R, Johnson RA, Bauman G, Francois CJ, Nagle SK. Simultaneous MRI of lung structure and perfusion in a single breathhold. J Magn Reson Imaging 2015;41:52-9. [Crossref] [PubMed]
41. Lustig M, Pauly JM. SPIRiT: Iterative self-consistent parallel imaging reconstruction from arbitrary k-space. Magn Reson Med 2010;64:457-71. [Crossref] [PubMed]
42. Bae K, Jeon KN, Hwang MJ, Lee JS, Ha JY, Ryu KH, Kim HC. Comparison of lung imaging using three-dimensional ultrashort echo time and zero echo time sequences: preliminary study. Eur Radiol 2019;29:2253-62. [Crossref] [PubMed]
43. Chandarana H, Feng L, Block TK, Rosenkrantz AB, Lim RP, Babb JS, Sodickson DK, Otazo R. Free-breathing contrast-enhanced multiphase MRI of the liver using a combination of compressed sensing, parallel imaging, and golden-angle radial sampling. Invest Radiol 2013;48:10-6. [Crossref] [PubMed]
44. Stolzmann P, Veit-Haibach P, Chuck N, Rossi C, Frauenfelder T, Alkadhi H, von Schulthess G, Boss A. Detection rate, location, and size of pulmonary nodules in trimodality PET/CT-MR: comparison of low-dose CT and Dixon-based MR imaging. Invest Radiol 2013;48:241-6. [Crossref] [PubMed]
45. Schroeder T, Ruehm SG, Debatin JF, Ladd ME, Barkhausen J, Goehde SC. Detection of pulmonary nodules using a 2D HASTE MR sequence: comparison with MDCT. AJR Am J Roentgenol 2005;185:979-84. [Crossref] [PubMed]