Single-shot turbo spin-echo (SSTSE) T2-weighted imaging (T2WI) is frequently used for fast lung magnetic resonance (MR) imaging. However, flow-related artifacts often obscure small lesions in SSTSE-T2WI. The black-blood technique can eliminate the signals arising from pulmonary vessels and improve lesion conspicuity (Figure 1). We previously demonstrated that breath-hold black-blood fat-saturated T2WI (BB-FST2WI) with three-dimensional gradient-echo T1-weighted imaging enables precise detection of both solid nodules and focal ground-glass opacities¹. However, incomplete signal suppression was occasionally observed, which led to false-positive findings. We consider cardiac gating as a remedy for this problem, with peripheral pulse unit (PPU) gating being more convenient than electrocardiogram (ECG) gating.

The aim of this study was to investigate the feasibility of PPU gating during black-blood lung MR imaging, and determine the optimal signal acquisition timing for effective and uniform attenuation of signals within pulmonary vessels.

Seven healthy male volunteers (age, 28–58 years; body mass index, 21.2–23.6) participated in this study. Images were acquired using the 3.0-Tesla MR part of the Ingenuity TF hybrid positron emission tomography (PET)/MR system (Philips Healthcare, Cleveland, OH, USA). Prior to MR imaging, an ECG and the peripheral pulse at rest were simultaneously acquired to record the delay time from the R-wave to the peripheral pulse peak.

Transaxial BB-FST2WI with SSTSE sequences was performed under breath-hold conditions and with PPU gating. The variable refocusing flip-angle (VRFA) technique, which was introduced by Yoneyama et al.², was deployed for obtaining the black-blood effect (Figure 2). The sequence parameters were as follows: repetition time /equivalent echo time, infinity/77 ms; turbo spin echo factor, 77; fat suppression; spectral attenuated inversion recovery; section thickness, 7 mm; section gap, 0 mm; acquisition matrix, 244 × 157; in-plane reconstruction resolution, 0.74 × 0.75 mm2; parallel imaging acceleration factor, 2; number of signal averages, one; number of slices, 21; and scan duration; 21 R-R intervals.

For PPU gating, the trigger point was set at the point showing the maximal upslope gradient in the peripheral waveform. The trigger delay times were set as follows: (1) random (without gating), (2) shortest (fixed to 166 ms), (3) longest (374–616 ms, depending on the participants’ heart rate), and (4) the average of (2) and (3). FS-T2WI SSTSE sequences without black-blood preparation were also obtained and served as a reference. This imaging session to acquire five FS-T2WI image sets was repeated four times per volunteer.

Image analysis was performed using the open-source software package ImageJ version 1.48 (National Institutes of Health, Bethesda, MD, USA). A single slice containing the right middle and lower lobes and left lingular and lower lobes was selected. A 1-cm² circular region of interest (ROI) was marked on the interlobar fissure. The standard deviation of signals within the ROI was considered as the reference value. Each side of the lung field was manually contoured and extracted. An intraluminal signal was defined as ≥5 times the reference value. The ratio of the intraluminal signal area to the lung field area was defined as the vessel area, while the attenuation rate was defined as 100 × [(vessel area of the image of interest)/(vessel area of the reference image); Figure 3)].

The mean attenuation rates from the four sessions were calculated for each sequence. Repeated-measures ANOVA and Tukey’s test were used for comparisons of the attenuation rates among the signal acquisition timings. Pearson’s correlation coefficient (r) was computed to assess the relationship between the attenuation rates for the right and left lung fields.

The average heart rate for the seven volunteers ranged from 59 to 71 bpm. For all volunteers, the longest trigger delay corresponded to the systolic phase, whereas the shortest and average trigger delays corresponded to the diastolic phase. The attenuation rates (%, right/left) for the random, shortest, longest, and average trigger delay groups were 41.18/42.38, 54.73/56.33, 22.91/22.61, and 49.66/45.92, respectively. The strongest black-blood effect was obtained when the trigger delay was set to the longest (p < 0.05; Figure 4). Figure 5 shows the relationship between the signal acquisition timing and image appearance in a typical case. The attenuation rates for the right and left lung fields showed a moderate to strong correlation with statistical significance, correlating particularly well in the longest and random groups (r = 0.6054 and 0.6495, respectively).The results of our study suggest that peripheral pulse gating is a feasible method for breath-hold black-blood lung MR imaging. The longest trigger delay, which corresponds to the systolic phase, should be selected to obtain a strong and uniform black-blood effect.