Motion is the most prevalent of MRI artifacts especially in pediatric populations. Most young children are either sedated or anesthetized in order to have a successful imaging session, which is undesirable due to their potentially adverse health effects. Although a number of works have tried to eliminate or reduce motion artifacts through retrospective [1] or prospective correction [2], to our knowledge there is no study on pediatric populations to show typical motion patterns and their effect on image quality.

We performed a motion measurement study on 82 subjects at our children’s hospital. The study was performed on a 3T Siemens TRIO scanner (Erlangen, Germany). Subjects were selected from patients who require a brain screen study and were cleared to be scanned without sedation or anesthesia. Mean age of the group was 13.4 years, with a minimum of 5 years and a maximum of 24 years. Five scans were removed from the study due to technical problems with the motion measurement system, i.e. the system reported errors or we encountered difficulties with sensor placement.

Motion was measured using an electromagnetic tracker [3] developed by Robin Medical Inc. (Baltimore, USA). The tracker had two sensors that were attached to the foreheads of subjects and whose locations and orientations were reported in real-time. We used a modified T1-weighted 3D GRE sequence as our test sequence. The product sequence was modified to include short and low amplitude bipolar gradients in the x-y-z axes after each RF pulse. The sensors used these gradients to measure the changes in the magnetic field and estimate motion parameters. Sequence parameters were TE=2.93ms, TR=15ms, FA=20 degrees, with a 1mm isotropic resolution.

The data from the sensors were analyzed retrospectively. We used three different criteria to characterize motion: maximum displacement from the reference position, mean distance traveled from the reference position, and motion-free time. The reference position was the position at the time that the center of k-space was acquired. Motion-free time was defined as the percent of the scan duration for which the position was less than 0.1mm from the reference position. The scans were then graded by an expert radiologist according to the severity of their motion artifacts using a Likert scale of 1 to 4; 1) images contain severe motion artifacts that can not be used for diagnosis; 2) images contain motion artifacts, but can be used for gross diagnostic implications; 3) images contain some motion artifacts and can be used for diagnostic purposes; 4) images do not contain visible motion artifacts. We have used these grades to classify what kind of motion patterns might result in non-diagnostic images.

Figure 1 shows examples of images graded by the expert radiologist in terms of motion artifacts (from grade 1 (left) to grade 4 (right)). Figure 2 shows motion patterns corresponding to each of those cases, with the y-axis showing the average sensor displacement from the reference position for the whole scan (~6 minutes). Figure 3 summarizes the relationship between the 3 motion criteria and the motion artifact grades. The average motion-free time for grades 1 to 4 were 0.35, 0.72, 0.91 and 0.98 respectively, while the mean distances traveled from the reference point for the same grades were 1.76mm, 0.84mm, 0.5mm and 0.29mm.In this work we reported results from a large pediatric study that shows the effect of motion. Our results show that both motion-free time and average displacement from the center of k-space had high correlation with image quality, whereas maximum displacement was not a good predictor. Among the 77 subjects where motion was measured successfully, 17 of them had average motion larger than 0.5mm, and this resulted in non-diagnostic images in 11 of them (14.2 percent). Similarly, 14 subjects had less than 90 percent motion-free time, which also led to non-diagnostic images. It should be noted that these study subjects were selected to undergo MRI without sedation because they were cooperative, and the broader pediatric population (especially younger children) is generally sedated because even greater motion is expected from them during scans.