MRI information


Six sites have collaborated in the recruitment of a representative sample of approximately 550 children, aged at first scan from 10 days to 18 years and 3 months (18:3 yrs), who will be studied using anatomic magnetic resonance imaging (aMRI), diffusion tensor imaging (DTI), magnetic resonance spectroscopy (MRS) and behavioral testing at multiple time points (minimum 3 times, younger children up to 10 times so far) over the course of the project.

The minimal/primary aMRI protocol consists of a T1W and PD/T2W. All subjects are required to complete the aMRI sequences successfully in order to qualify the scan as passed and therefore included in the database.

Objective 1 MRI (in order of priority):

  • 3D T1W (or fallback)
  • PD/T2W (or fallback)
  • MRS
  • DTI
  • MRSI
  • Expanded DTI

Objective 2 MRI (in order of priority):

  • T1W
  • PD/T2W
  • T1 Relaxometry
  • DTI
  • MRS
  • 2nd Dual Contrast
  • Expanded DTI

Objective 1 aMRI: An acquisition time of 30–45 minutes was allocated, with 1 mm in-plane resolution, 1–2 mm slice thickness, whole brain coverage and multiple contrasts (T1W, T2W and PDW).

A 3D T1-weighted (T1W) spoiled gradient recalled (SPGR) echo sequence was used. The protocol provides 1 mm isotropic data from the entire head. As the priority measure for Objective 1, it was acquired immediately following the localizer scan and, if significant motion artifacts were observed, was immediately repeated. On GE scanners, the maximum number of slices was 124, and hence the slice thickness was increased ( 1.5 mm) to give whole head coverage. Sagittal acquisition was chosen, being the most efficient way to obtain complete head coverage.

A dual contrast, proton density- and T2-weighted (PDW and T2W) acquisition provided additional information for automated multi-spectral tissue classification/segmentation. An optimized 2D multislice (2 mm) dual echo fast spin echo (FSE) sequence was used. An oblique axial orientation (parallel to the AC–PC line) was selected, both for potential use of the data within a radiological atlas and for consistency between Objectives 1 and 2.

Not all Objective 1 subjects, particularly the youngest, could tolerate the optimal scanning protocol described above (a 15-min 3D T1W and 10-min PDW/T2W scan). In anticipation of this problem, we implemented a “fall-back” MR protocol which is the one employed for Objective 2 subjects. It consists of shorter 2D acquisitions which provides acceptable structural images and continuity with the Objective 2 MR protocol.

A 2D T1W multislice (MS) spin echo (SE) was substituted when motion degraded the 3D T1W scan. Data were collected parallel to the AC–PC line with a 1 _ 1 _ 3 mm spatial resolution. If the PDW/T2W scan was degraded by motion, slice thickness was increased from 2 mm to 3 mm, reducing scan time and likelihood of motion (refer to Objective 2 MRI Protocol table below).

The frequency of reverting to the fallback sequences is broken down as follows: Data was collected using the fallback sequences for 13% of T1W and 22% of PD/T2W subject scans. In some instances, sites had to revert to using both the T1W and PD/T2W fallbacks (13%).

Objective 2 aMRI: The movement problems which occur when scanning very young children dictate short acquisitions. This fast, robust protocol provides data similar to Objective 1, as well as quantitative relaxation data.

The principal component acquires data similar to Objective 1, for image segmentation. A 3D T1W 1 mm isotropic acquisition is unrealistically long for this age group, so a 2D T1W multislice spin echo was a practical compromise. Data were collected parallel to the AC–PC line with a 1 _ 1 _ 3 mm spatial resolution. The parameters of this sequence are identical to the Objective 1 fall-back T1W scan (refer to table below). The sequence took less than 5 min and was repeated if degraded by motion artifacts.

The final component of the Objective 2 protocol is the acquisition of quantitative relaxometry data. Considering T2 relaxometry first, it was recognized by all that good quality multi-component T2 relaxation measurements could only be performed one slice at a time using 32 or more echoes and would require a scan time of at least 6 minutes per slice. While multi-component T2 data has the potential to provide very exciting information regarding myelination, practical technical limits prevent it from being used to acquire data over the entire brain. On the other hand, it was recognized that the dual (effective) echo FSE data could be used to calculate an estimate of T2 for a single compartment model. Thus, the compromise arrived at was the use of the FSE data for whole brain single-component T2 calculation and the collection of a single slice multi-echo data set later in the protocol (after quantitative T1 measurement, DTI, and MRS).