Improved Delineation of Ventricular Shunt Catheters Using Fast Steady-State Gradient Recalled-Echo Sequences in a Rapid Brain MR Imaging Protocol in Nonsedated Pediatric Patients

Discussion

Greater awareness of the risks of childhood radiation^1–3 and campaigns to reduce radiation exposure in pediatrics (ie, Image Gently)¹³ have resulted in increased interest in substituting rapid brain MR imaging for head CT in multiply imaged patients with certain neurologic conditions (ie, hydrocephalus). Frequently, single-shot TSE-T2 sequences are used.^7,8 These sequences are an acceptable substitute for standard multiecho spin-echo T2 sequences due to their similar image contrast resolution with a much shorter imaging time. In these sequences, a number of consecutive 180° radio-frequency pulses are applied per excitation, resulting in the acquisition of multiple spin-echoes per TR.^11,12,14 The drawback to these sequences, however, is that the multiple 180° radio-frequency pulses and the short echo spacing result in a reduced sensitivity to susceptibility effects.^9–12,14

GRE images have been shown to be more sensitive in the detection of blood products than fast single-shot TSE-T2 sequences.^9,10 Our study shows that GRE sequences can additionally provide improved delineation of standard intracranial catheters compared with TSE-T2 sequences. This increased catheter conspicuity is probably related to a combination of increased contrast resolution (catheter versus background) and the intrinsic properties of the catheter. The fast SS-GRE acquisitions, which were used in our study, were performed in significantly less time (30 seconds for fast SS-GRE versus 4 minutes for standard GRE) without significant difference in contrast-to-noise ratio. Generally the axial images alone were sufficient to localize the catheter tip. The additional images in the coronal plane only occasionally provided increased catheter-localization confidence but were often helpful in evaluating other common abnormalities such as extra-axial convexity blood products.

The reduction in scanning time associated with faster GRE sequences over standard GRE sequences is possible due to the decreased TR of fast SS-GRE sequences (for our protocol 480 ms versus 820 ms). Briefly, standard GRE sequences contain an elongated readout gradient duration (and thus a longer TR) to achieve intrinsic transverse magnetization “spoiling.” Fast GRE sequences either actively “spoil” transverse magnetization or refocus it to contribute to a steady-state formation. Both techniques enable application of a shorter TR compared with standard GRE sequences.^{9–12,14,15,16} The SS-GRE sequences can be further subdivided into 2 categories when it is remembered that the resultant steady-state precession signal intensity is composed of both FID and spin-echo components. The fast SS-GRE sequence used in our rapid nonsedated pediatric brain protocol is the variety that samples the FID (hence the increased conspicuity of susceptibility effects).¹⁶ The vendor-specific names for this sequence are fast-field echo (Philips), fast imaging with steady-state precession (Siemens, Erlangen, Germany), and gradient-recalled acquisition in steady-state (GE Healthcare).¹⁵

The increased conspicuity of the intracranial shunts and catheters is likely due to a combination of the susceptibility effects of the intrinsic catheter components and by intravoxel phase dispersion caused by the local magnetic field gradients of the internal tubing structure.^17–19 Most intracranial shunts and catheters are composed of a hardened silicone elastomer and are impregnated with barium. Briefly, the silicones are synthetic polymers composed of a repeating silicone-oxygen (siloxane) backbone with organic groups attached through silicone-carbon bonds. There is a variable degree of cross-linking of the siloxane backbone, which determines whether the polymer exists in the gel form, as present in breast implants (10% cross-linking), or in the elastomer form of medical-grade tubing (95% cross-linking).²⁰

Although, to our knowledge, there is no review in the literature of the MR imaging properties of medical grade elastomer tubing, an analysis of the MR imaging properties of silicone elastomer orthopedic tendon spacers does exist.²⁰ In this study, the signal intensity of the imaged silicone elastomer on GRE sequences was at least 70% less than the signal intensity on spin-echo T2 images. These authors hypothesized that the signal-intensity reduction was secondary to local magnetic field gradients within the polymer backbone, resulting in intravoxel phase dispersion of mobile protons traveling along the polymer. Additionally, it is possible that another source of intrinsic susceptibility may come from the diamagnetic effects of the barium with which most catheters are impregnated to provide radio-opacity.

Despite the benefits of the fast GRE sequences that were used in this study, the acquisition parameters did result in a few evident limitations. The relatively long acquisition window (compared with other fast SS-GRE sequences) and its multishot excitation profile (compared with TSE-T2 sequences) resulted in an increased sensitivity to patient motion artifacts. This is not a trivial problem in a protocol designed to image a nonsedated patient who already has a low baseline level of cooperation. The increased susceptibility effect of the shunt catheter reservoir itself was also episodically problematic. This was especially evident when it was located at the same level as the shunt. In our study, increased susceptibility artifacts from the shunt reservoir completely obscured visualization of the intracranial aspect of the catheter in 2 patients. However when these artifactual limitations were not encountered, the fast GRE sequences enabled catheter tip localization equivalent to the criterion standards of CT and standard multishot T2 MR imaging.

The clinical impact of improving the delineation of intracranial shunt catheters is difficult to assess. The clinician responsible for managing an individual patient's shunt catheter often has valuable clinical information regarding the insertion and prior position of the catheter as well as its drainage status. With this knowledge, the radiologist's description of the location of the shunt catheter may not impact clinical decision-making. Given this limitation, it is, however, possible that when taking into account the possible long-term radiation risks of multiple CT examinations, clinicians may increasingly consider substituting rapid MR imaging for CT. Continued technical improvement in rapid MR imaging sensitivity and scanning time may help facilitate its diagnostic ability and increased use.

In an attempt to assess the clinical impact of our rapid MR imaging protocol modification (addition of SS-GRE sequences), a limited review of the ordering preference (rapid MR imaging versus head CT) of the neurosurgery department for patients with hydrocephalus was performed. The small increase in the percentage of rapid MR imaging studies ordered for hydrocephalus (47% versus 39%) after adding the SS-GRE sequences may indicate that the added capabilities of the protocol favorably impacted the confidence of the ordering clinicians (neurosurgery) in substituting rapid MR imaging for CT. This trend, although encouraging, will nonetheless require additional monitoring to assess its validity.