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MR imaging of the neonatal brain at 3 Tesla

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Summary

3 Telsa MR scanners are now becoming more widely available and 3 Telsa is likely to become the filed strength of choice for clinical imaging of the brain. The neonatal brain can be safely and successfully imaged at 3 Telsa. The improved signal to noise afforded by a higher field strength may be used to improve image quality or shorten acquisition times. This may be exploited for conventional T1 and T2 weighted imaging and also for advanced techniques such as diffusion tensor imaging, angiography and functional magnetic resonance studies.

Introduction

Magnetic resonance (MR) imaging has revolutionised neuropediatrics. The worldwide availability of MR facilities has resulted in detailed images of the brain in a wide range of neurological disorders. These more specific phenotypes have resulted in the identification of new syndromes and the realisation that a specific genetic defect may manifest in a variety of phenotypes. Most clinical MR imaging is performed at 0.5–1.5 T. More recently a generation of 3 T MR scanners has become available and current opinion is that 3 T MR imaging will become the clinical standard, initially in neuroimaging, and eventually throughout the body.

The greater signal-to-noise (SNR) afforded with higher field strengths may be exploited to improve image quality or to shorten acquisition times. There are now several reviews and studies comparing 1.5–3 T imaging of the brain in adults.1, 2, 3 Systems with 3 T have been exploited to increase detection of multiple sclerosis lesions,4 to improve the blood oxygenation level dependent (BOLD) effect for functional magnetic resonance imaging (fMRI) and to improve enhancement following contrast administration.5 There is as yet little information about the role and use of 3 T imaging in the paediatric population.

Increased field strength provides not only increased SNR, but increased susceptibility caused by paramagnetic effects due to local heterogeneity in magnetic field from, for instance, the frontal sinuses. Highfield imaging also results in increased chemical shift and increased heat deposition [specific absorption rates (SARs)]. Radiofrequency (RF) power varies with the square of the field strength, therefore imaging at 3 T, produces four times more RF power and sequences may have to be adjusted to operate within radiological safety guidelines. These limits are usually preset into the scanner software. The SAR at 3 T is not prohibitive for neonatal and infant scanning, as sequences can be adjusted and still produce good quality images, but it is a problem for fetal imaging, making it unlikely that fetal imaging will be performed at 3 T for the foreseeable future. Studies modelling heat deposition within the pregnant uterus during imaging at 3 T are required. Increased susceptibility is not a major problem in neonatal imaging, as neonatal and infant sinuses are not formed and aerated until later in childhood.

The increased SNR afforded by imaging at 3 T can be used to obtain information in a shorter time. This is very valuable when imaging unsedated children who may have difficulties lying still or for sedated sick neonates: in each circumstance time is of the essence. The combination of parallel imaging with phased array coils at 3 T may improve SNR further but perhaps at the expense of signal homogeneity, which is a potential disadvantage for quantification studies.

Section snippets

Conventional imaging

Imaging at 3 T provides superb detail of the immature brain. Figure 1, Figure 2 show examples of images obtained at 3 T and a comparison of images obtained at 1.5 and 3 T in the same examination. The increased SNR allows high-resolution images to be obtained in acceptable acquisition times such as a multisliced T2 weighted sequences suitable for reformatting (Fig. 3). Increased acquisition times are acceptable when imaging postmortem and excellent quality images can be produced (Fig. 4). The field

Contrast administration

Administration of a gadolinium-based contrast agent produces higher contrast between tumour and normal brain at 3 T than at 1.5 T, helps to detect more cerebral metastases at 3 versus 1.5 T in single and cumulative triple dose, improves the evaluation of macroadenomas of the hypophysis, and makes MR venography at 3 T clinically attractive with increase in spatial resolution within the same measurement time, thus providing more detailed information.2 (Fig. 8). It's role in studying more specific

Spectroscopy at 3 T

With high magnetic fields improved SNR can allow greater accuracy in quantitative measurements and can be used to reduce voxel size, and thus minimise partial-volume effects in heterogenous structures such as the brain. Increased chemical shift dispersion reduces the overlap of resonances obtained in spectroscopy and thus increases the number of metabolites that can be identified and accurately quantified.2 This would allow depiction of resonances from for instance glutamate.15

Functional magnetic resonance imaging (fMRI)

Functional magnetic resonance imaging (fMRI) uses BOLD contrast. BOLD fMRI at 1.5 T can achieve a spatial resolution of up to 3–5 mm. Studies at 3 T will increase SNR and thereby enhance spatial resolution and specificity of fMRI.16, 17 This will provide improved resolution of cortical anatomy.18 In addition as field strength increases, the field gradient around the capillaries becomes larger and extends further into the parenchyma, thus increasing the participation of the brain tissue in the

Summary

The neonatal brain can be imaged safely at 3 T. Problems with increased relaxation times and increased heat deposition can be overcome but altering sequence parameters. Increased susceptibility is not a problem because of the immature sinuses. The increased SNR afforded at higher field strength allows fast or more detailed images. Specific improvements in anatomical definition and in lesion detection and conspicuity may also be obtained in more advanced techniques such as diffusion weighted

Acknowledgements

Staff at the Robert Steiner MR Unit and the Department of Paediatrics, Hammersmith Hospital, The Medical Research Council, The Academy of Medical Sciences, The Health Foundation, Philips Medical Systems are acknowledged.

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