1.0 s Ultrafast MRI in non-sedated infants after reduction with spica casting for developmental dysplasia of the hip: a feasibility study

Developmental dysplasia of the hip (DDH) treatment consists of initial concentric reduction followed by stabilization. Several reduction techniques are commonly used, such as the Pavlik harness [1], acute manual closed reduction [2], gradual reduction by traction [3] and open reduction [4]. In all of these procedures, confirmation of concentric reduction and monitoring of the concentricity during the stabilization phase are imperative. Radiography [5], ultrasonography [6], computed tomography (CT) [7] and magnetic resonance imaging (MRI) [8] are the standard imaging modalities used to assess the location of the femoral head.

Radiography has been the preferred diagnostic method to diagnose DDH due to the benefits of less radiation exposure, greater convenience and low cost. However, radiography has the disadvantage that it does not enable easy confirmation of the relation between the cartilaginous femoral head and acetabulum in spica cast application. Using ultrasonography it is possible to image the cartilaginous femoral head, and this modality is reported to be reliable when compared with radiography [5]. However, care is necessary because false-positive findings can occur if the imaging plane is suboptimal. In cases evoking doubt, CT and MRI are used to determine the exact position of these hips [8, 9].

MRI can also provide a high tissue contrast when used for infants with DDH, does not expose the infant to radiation and enables assessment of the femoral hip location in two orthogonal planes (transverse and coronal views) to prevent misdiagnosis [8, 10, 11]. However, because conventional MRI is particularly sensitive to motion artefacts, the infant may need to be sedated [12]. In addition, MRI is an expensive and time-consuming process. Consequently, a useful alternative is of interest.

To solve these problems, we have developed and employed a 1.0 s ultrafast MRI protocol to confirm the location of the femoral head in non-sedated infants with DDH after reduction with spica cast application in the clinical setting. The acquisition was achieved by employing a balanced steady-state free precession sequence (bSSFP) [13] and immobilizing the patient using dedicated sandbags. Thus, the aim of this study was to assess the validity of ultrafast MRI by comparing its efficacy with conventional MRI in the same patient population.

This was a retrospective study and was approved by the institutional review board of the Shiga Medical Center for Children. Written informed consent was obtained from the parents or guardians of the infants received sedation and participated in the MRI examination.

The novel ultrafast MRI procedure was employed to confirm the location of the femoral head in all 13 infants participating in the study (14 DDHs, 12 normal hips) after casting. In one infant with bilateral DDH, the reduction was performed in another hospital, and the infant then transferred to our clinic for confirmation of the location and stability of the femoral head. Transverse and coronal ultrafast MRI demonstrated an incomplete reduction of the right hip and left reduced hip in this infant (Fig. 2), which was confirmed by conventional MRI. Because gradual reduction with spica casting was performed for this infant at our hospital, this study includes two sets of MRI studies for this infant. Another infant was diagnosed with an incomplete reduction of left hip after casting by ultrafast MRI (Fig. 3a, b). Because the re-casting procedure was immediately performed for this patient, conventional MRI was not used. After re-casting, the second ultrafast MRI (Fig. 3c, d) and conventional MRI (Fig. 3e, f) revealed a reduced left hip in this infant. Therefore, in this study, ultrafast MRI and conventional MRI were used for 15 and 14 examinations in 13 infants (14 DDHs and 12 normal hips), respectively.

Our records show that the mean study times (from enter to exit of the MRI scanning room) of ultrafast MRI and the conventional second MRI were 6 ± 1 and 14 ± 3 min, respectively (P < 0.001). No complications due to sedation, such as hypoxia, were reported in this patient population (Table 1). The average sedation waiting time was 1 h 25 min ± 34 min. No sedation was required for three infants because they fell asleep during the ultrafast MRI examination.

Both reviewers stated independently of each other that there were no motion artefacts for non-sedated infants during the ultrafast MRI examination and that the quality of both the ultrafast MRI and conventional MRI images was acceptable to assess the location of the femoral head in infants with spica cast application after the reduction procedure (Table 1). Inter-observer agreement (κ) was 0.782 (substantial agreement).

The results of the reduction procedure show that two hips were categorized as ‘incomplete reduction’, thereby requiring a re-reduction procedure (Table 2; Figs. 2, 3). Inter- and intra-observer agreement (κ) were both 1.0 (almost perfect agreement).

Closed reduction with spica cast application is conventionally performed under general anaesthesia. This procedure has become an important and widely accepted procedure because adductor muscle tenotomy or arthrography can be concurrently performed in the operating room [15]. Location of the femoral head can subsequently be confirmed by three-dimensional (3D) fluoroscopy [16]. Although hip location in spica cast can be confirmed using these techniques during the procedure, MRI may be applied without additional general anaesthesia or sedation to assess femoral head location [17, 18]. However, the major drawback of this procedure is the general anaesthesia used in the operating room.

Gradual reduction by traction with spica cast application has been reported as an alternative reduction procedure [3]. Because general anaesthesia or sedation is not required to perform this type of procedure, its use in the infant patient population has clear benefits. In this procedure, ultrasonography is conventionally used to confirm concentricity of the femoral head. Although ultrasonography is a convenient and repeatable technique, the acoustic window is limited due to the presence of the spica cast [5] and, consequently, false-positive findings can occur if the imaging plane is suboptimal. In these cases, if general anaesthesia or sedation is not required for the MRI study, MRI is a powerful technique for assessment of the location of the femoral head. Desai et al. reported that spica MRI could be performed without sedation by soothing or swaddling the infant in a blanket [18]. However, spica casting does not eliminate the motion artefact altogether. Many infants become agitated during the MRI examination even with soothing from parents or guardians. As a result, conventional MRI is not a useful diagnostic modality to image the femoral head and acetabulum without motion artefact. Therefore, the most effective strategy is to perform the imaging study when the infant is asleep or our new ultrafast acquisition.

Temporal resolution refers to the time that is needed to acquire one image, making it the most important factor in the imaging procedure when the aim is to reduce the motion artefact. The temporal resolution of CT is considered to be higher than that of MRI, being approximately 0.5 s for an abdominal CT [19, 20]. Although CT enables the image to be acquired without a motion artefact, it may not be practical for imaging infants due to its relatively higher radiation dose and lower tissue contrast [21]. With recent developments in MRI scanning, the bSSFP MRI sequence has been developed and released onto the commercial market [13]. This method is tolerant of patient motion due to the high temporal resolution of the sequence (48–64 ms) [22]. We have customized the bSSFP sequence to have a higher contrast between the cartilage and acetabulum with a temporal resolution of 1.0 s. This temporal resolution is comparable to that of abdominal CT and is 100-fold higher than that of the conventional MRI sequence (1.0 s vs. 1 min 54 s on MEDIC). In our study, the higher temporal resolution definitely contributed to the successful capture of the image of the non-sedated infant without motion artefact (Figs. 2, 3).

MRI is a conventionally time-consuming imaging modality, but one of its major drawbacks for application in the infant population is the length of the total study time. However, our proposed ultrafast MRI offers a short study time of ≤7 min, including patient immobilization, coil setup and scanning. In addition, our method was not only able to confirm concentric reduction after gradual reduction but could also monitor the concentricity during the stabilization phase of DDH.

We believe that knowledge of the ultrafast MRI sequence is useful to paediatric orthopaedic surgeons for the following reasons. First, ultrafast MRI can confirm the location of the femoral head without any motion artefact in non-sedated infants after the reduction with spica cast application, with an acquisition time of only 1.0 s per one axial or coronal image. In other words, no sedation or general anaesthesia is required for confirmation of the concentricity of the femoral head after spica cast application. Second, if the ultrafast MRI result shows pseudo-reduction, the re-casting procedure can be performed immediately after the imaging study (Fig. 3). Furthermore, ultrafast MRI can be performed after the re-casting procedure as needed because no sedation is required for the imaging. Third, the ultrafast MRI study would be beneficial in a busy radiology department because of the total study time of ≤7 min (including time of immobilization, coil setup and scanning). Lastly, the ultrafast bSSFP MRI sequence is commercially available through the recently released MRI system.

This study has two limitations. We are currently employing the ultrafast MRI to confirm location of the femoral head in clinical practice just after reduction with spica casting (fourth step in FACT-R). Although the contrast in ultrafast MRI is sufficiently efficient to confirm the location of the femoral head, it is definitely inferior to that of conventional images (Figs. 2, 3). Consequently, it was impossible to conduct a truly blinded analysis. Moreover, ultrafast MRI cannot detect the inverted labrum due to tissue contrast and spatial resolution. Consequently, it cannot replace conventional MRI in our hospital. Second, our study had retrospective design, and only 13 infants were analysed. Therefore, further research may be required to verify the usefulness of ultrafast MRI in confirming the location of the femoral head. Despite these shortcomings, we believe that ultrafast MRI is an effective imaging technique for non-sedated infants for confirmation of the location of the femoral head because there is no need for general anaesthesia or sedation.

In conclusion, we developed a 1.0 s ultrafast MRI to confirm the location of the femoral head in non-sedated infants with DDH after gradual reduction with spica cast application. The ultrafast MRI sequence provides excellent image quality without motion artefacts within a total study time of ≤7 min.