Hybrid PET/MRI imaging in healthy unsedated newborn infants with quantitative rCBF measurements using ¹⁵O-water PET

Introduction

The clinical importance of white matter injury or periventricular leukomalacia (PVL) in the premature infant has been recognized in neonatal neurology for more than four decades,¹ as a leading cause of long-term motor and cognitive disability. The etiological understanding has centered on hypoxic-ischemic and infectious/inflammatory damage, but nutritional and hormonal factors may also contribute.² Given that the white matter is a vascular end-zone and a predominant area for injury in the neonatal patient, knowledge of the exact quantitative regional cerebral blood flow (rCBF) value in the healthy newborn can contribute in understanding the underlying pathophysiology of PVL in preterm infants.

Measuring rCBF in newborn infants presents substantial technical and ethical concerns. Regional CBF in newborn infants is known to be relatively low in white matter,^3,4 challenging the accuracy of the applied methods. In addition, the brains of newborn infants are both smaller and vary in anatomical structure and tissue maturity including the developmental stage of myelination when compared to the adult brain.^5,6 Quantitative ¹⁵O-water PET is considered the ‘gold standard’ for rCBF measurements in humans^7,8 but existing PET rCBF studies in the neonatal population are few, and were performed decades ago with a generation of lower resolution PET scanners and on infants with neuropathology or suspected neuropathology.^9–11 Present day state-of-the-art PET imaging methods have improved significantly and may more appropriately address the technical issues found in these pioneering studies. Furthermore, there are no studies in healthy newborns to address what true normal rCBF values are in this age group.

Although there is a far more accessible and non-ionizing alternative imaging technique for rCBF measurements than PET in the form of arterial spin labeling (ASL) using MRI, only a limited number of neonatal publications are available.^12–19 Methodological challenges for ASL, such as variability of arterial transit time and obtaining a sufficient signal-to-noise ratio, are especially challenging in the neonatal population with small organs and generally low rCBF²⁰ and may significantly affect both rCBF distribution and quantitative values.

The purposes of this study was to apply a newly developed method in healthy newborn subjects using PET with ¹⁵O-water and an image-derived input function (IDIF) from the left ventricle previously developed in a piglet model on a hybrid PET/MRI scanner.²¹ This method utilizes the extended axial field of view of the scanner (25.8 cm) to acquire dynamic data from both brain and heart simultaneously and obtains quantitative minimally invasive measures of rCBF with a radioactive dose sufficiently low to be acceptable for medical research in this subject group. PET/MRI allows simultaneous co-registered CBF measurements with ¹⁵O-water PET and ASL(PASL) MRI allowing the best possible scenario to estimate accuracy of CBF values from ASL. This strategy has been used previously in adults,²² but never in the neonatal group. We aimed to investigate if the surprisingly low values of white matter rCBF (∼3 ml/100 g/min) that have been reported in healthy newborn control subjects using single TI PASL MRI^16,19 could be reproduced using ¹⁵O-water PET. By doing this, we aimed to establish a proof-of-concept of the new PET/MRI method in newborn infants. This could be a useful method for future imaging investigations of preterm infants, including better understanding of the underlying pathophysiology and significance of changes in rCBF in neonatal PVL.

Material and methods

Results

Dynamic PET data were obtained in all four subjects without sedation. Three subjects had 7-min acquisitions and one had a 70 s acquisition after tracer injection with early termination due to movements. The fitted delay in the four subjects ranged from −0.97 s to 0.38 s with a mean value of −0.32 s. These values reflect the short distance from the left ventricle to the brain (approximately 10–15 cm in a term infant) and an accordingly short time-shift of the tracer from heart to brain (less than a second). As expected, the fitted delay values vary within a single frame length (2 s).

All subjects had developmentally normal brain T1w MP-RAGE images. This includes normal cortical folding and the expected distribution of increased signal intensity in the myelinated white matter of the posterior third of the internal capsules, thalami, mesencephalon and the central portion of the centrum semiovale.

For anatomical correlation to PET imaging of the heart, T1w MP-RAGE imaging covered the thorax. The input curve for the IDIF was obtained from the left ventricle from all subjects. A plot of the original IDIF and whole brain tissue curves of subject no. 1 are shown in Figure 2.

The rCBF PET distribution is shown in Figures 4 and 5, which shows PET with relatively high rCBF in the striatum, thalami, mesencephalon, pons, cerebellar vermis, and medulla, and to a lesser degree in the perirolandic cortex, mesial occipital cortex and hippocampal area.

The average quantified rCBF PET measurements for periventricular unmyelinated white matter were 10.3 ml/100 g/min (range: 9.3–11.5 ml/100 g/min). The whole brain PET CBF values are shown in Table 2. The average ratio between thalamic rCBF and unmyelinated white matter was 0.36 (range: 0.24–0.40). The left hemisphere had a significantly higher mean CBF 10.1 ml/100 g/min than the right hemisphere 9.0 ml/100 g/min (p = 0.01).

The calculations of rCBF ASL in subjects 1 and 2 show areas of high focal CBF related to the position of known arteries in the insula and the mesial and lateral cortices. In subject 4, there is relatively high cortical rCBF in the frontal lobes with a defect in the occipital areas (see supplementary material). In all three subjects, the distribution of the rCBF ASL values deviated significantly from both the simultaneous PET rCBF measurements, and the expected rCBF found in previous ASL studies^16,20 and were, therefore, interpreted as too technically inadequate and unreliable to analyze further.

No ASL data were obtained in subject 3 because the infant woke up before scanning.

Discussion

This study is a human implementation of an experimental set-up for simultaneous rCBF measurements using ¹⁵O-water PET and ASL on a hybrid PET/MRI scanner that we have previously employed in piglets.²¹ In that study, we documented that reliable whole brain CBF values could be obtained even at a very low injected activity of 20 MBq ¹⁵O-water using an IDIF from the heart. We found that whole brain CBF ASL and CBF PET were congruent during baseline but not during hyperperfusion. The results were, however, encouraging enough to translate to human studies. In newborn infants, we expected better quality data because of larger brain size (brain weight in piglets 3 weeks old 50 g and in newborn infants approximately 350–400 g), particularly in the estimation of white matter rCBF that could not be segmented reliably in piglets because of resolution limitations. Furthermore, the input function derived from the heart should be valid since the VOI in the heart is of similar size: the left ventricular long-axis is approximately 2.6 cm in three week old piglets³¹ and was measured to be 2.8 cm in three of the four newborn infants in this study. An important difference between the present study and the animal studies was the lack of sedation in the infants. This circumstance increases the risk of image degradation due to head movements, but allows measurement of the native rCBF distribution uninfluenced by sedatives. Avoiding sedation was safest, but also a condition for the inclusion of healthy volunteers. A very low injected activity of 14–15 MBq ¹⁵O-water allowed imaging of sufficient quality at a radiation dose low enough to be acceptable for medical research. Besides corroborating existing estimates of global CBF in healthy term infants obtained by a range of methods, each having their own assumptions, we present the first normal rCBF values in this population. As the presented method relies on an IDIF,²¹ the new method cannot be referred to as ‘gold standard’. However, since PET is based on relative few assumptions, and since we have explored the sources of error and their magnitude in a newborn piglet model (e.g. the possible overestimation due to the noninvasive determination of the input function), we consider the results reliable. The use of an IDIF would not affect the rCBF distribution that is better defined by ¹⁵O-water PET than with ASL MRI due to the longer half-life of ¹⁵O-water. The method is an adaptation of classical rCBF PET methodology using state-of-the-art hardware, and can serve as a valuable tool for the comparison, evaluation and optimization of more clinically available methods, such as ASL MRI in the neonatal population.

Even though we had a low participation rate of families, we don’t believe this affected the result’s representativeness, but rather highlighted the difficulties in recruiting infants to research projects that do not directly benefit the parents nor infant.

Since PET inherently involves ionizing radiation, a pediatric PET research method should be reliable, quantitative, and ethically and clinically acceptable in terms of minimal invasiveness and radiation dose, by complying with the principles of ALARA (“as low as reasonably achievable”).³² The level of what is applied in our study is well within what EU legislation for radiation dose in healthy children. This tracer dose (0.33 mSv per injection in this study) could also be applied in scientific protocols in a patient group of infants with prematurity or neuropathology such as PVL, aiming directly at the diagnosis, cure, or prevention of disease.^24,25 It should be noted that most data regarding the absorbed dose estimates of ¹⁵O-water in pediatric patients from ICRP80 are based on adult data.^{33 15}O-water kinetics is simple, and because of the short half-life of the isotope (122 sec), the radiation dose is limited. However, when following the present precautionary principle model, one should always carefully consider the clinical value of a study using ionizing radiation in pediatric patients.

In the present study, the left ventricle of the heart could comfortably be contained in the extended axial FOV of the scanner enabling dynamic sampling of the IDIF in a large blood volume. The IDIF alleviated the need for invasive arterial blood sampling while still enabling a valid quantification of rCBF. The IDIF method for quantitative PET measurements has been thoroughly investigated in the literature using different approaches to optimize the input curve.^34–36 In newborn infants, only one previous study applied the left ventricle for IDIF extraction and quantitative glucose uptake values.³⁷ In our study on piglets, we observed an overestimation of approximately 20% when compared to gold standard arterial blood sampling which may in part be explained by the 3-s time framing used as opposed to 1 - and 2-s framing giving a better time resolution and minimizing the underestimation of the peak of the input curve. We applied 2-s time framing in the PET data reconstruction of the newborn infants to minimize this effect. However, since the structures in question are small (the left ventricle of the heart is approximately 2 mL in a full term neonate³⁸ and the applied VOI in the left ventricle was 0.26 mL), partial volume effects and resolution recovery may influence the values obtained from the dynamic time framing leading to an underestimation of the values in the IDIF. Consequently, there is a possible overestimation of rCBF values in this first human study, but considering that the values in question are relatively small especially in white matter, the implications for the conclusions of this study are modest. With present ASL MRI techniques, quantified rCBF in white matter is neither reliable in adults nor neonates owing to the prolonged arrival time of magnetized blood to the periventricular watershed areas.^39,40 However, use of multidelay (TI) ASL approach to quantify both arrival time and CBF can improve quality of the ASL data,^41,42 presumably also in the neonatal patient group.

The normal metabolic development of the brain of newborn patients with suspected pathology has been investigated in a limited number of FDG PET studies.^6,43,44 The studies showed increased activity in the sensorimotor area, basal ganglia and the brainstem. At birth, myelination of white matter has reached the cortical tracts passing through the centrum semiovale in the internal capsule and cortical spinal tracts relating to the sensorimotor cortex. Hence, the myelin is visible on MRI imaging.^5,45 In our PET/MRI study on healthy subjects, this known myelination pattern was replicated as high signal intensity changes on T1w MP RAGE images. Noticeably, increased rCBF could be seen in the corresponding areas on the PET image (Figure 4) and in the quantified rCBF values (Table 2), which correlates to the myelination process in cortical and subcortical areas. This result can be interpreted as an expression of the known neuro-vascular coupling between metabolism and tissue perfusion. In line with these findings, the frontal and occipital periventricular unmyelinated white matter, showing low intensity signal on T1w MPRAGE images, had substantially lower PET rCBF values (Table 2) with a mean of 10.3 ml/100 g/min. This result correlated to the myelination process in these areas occurring much later between one and two years of age.⁵

Previous ¹⁵O-water PET studies performed in infants with neuropathology^9–11 were performed in 2D acquisition mode with low spatial PET scanner resolution of 11.7 and 18 mm and gave an effective radiation dose of 5.8 mSv for each injection. Reliable quantification of rCBF was challenged by the collection of only a few arterial blood samples to estimate the input function, which was not corrected for delay nor dispersion. An alternative method to avoid direct arterial sampling in older children has been to use a standard arterial input function to calculate the rCBF distribution that is scaled by quantitative global CBF calculated from the clearance of ¹⁵O-water, k₂.^46,47 However, our subject group requires hand held injection for safety reasons to provide the required control over injected activity and volume, and the arterial input function shape, delay and dispersion in infants have not yet been adequately described. These assumptions may be avoided using an IDIF method based on actual measurements. As the proposed method is dependent on the inclusion of both brain and heart in the extended axial field of view of the PET/MRI scanner, this option will only be feasible until the age of nine months to one year, depending on the height of the child and with the field of view of the current clinically available PET/MRI scanner.

None of the aforementioned PET imaging studies^9–11 could give reliable estimates of white matter rCBF, because of the poor resolution of the scanners, but all have indicated that the white matter rCBF is very low in newborn infants and that the ratio to basal ganglia CBF may be as low as 0.23 when measured with SPECT.³ We found a mean CBF ratio of white matter to the thalami of 0.36 which is in the range of values measured in the adult brain (0.30–0.48).^48–51 Regional CBF values from previous studies on term infants with varying degree of neuropathology, ranged from 9 to 76 ml/100 g/min.^9,10,52 The large variation may be caused by the above-mentioned methodological inadequacies, the effects of pathology, and the developmental span.⁵³ Global CBF in premature infants measured with ¹³³Xenon was 20 ml/100 g/ml in non-distressed infants.⁵⁴

Regional CBF in white matter has not been reported previously with modern PET methods in healthy newborn infants. Regional CBF using ASL MRI in periventricular unmyelinated white matter in healthy infants^12,16 has been measured to be between 2.3 and 3.6 mL/100 ml/min or 1/10 of basal ganglia rCBF, which is outside the 95% CIs found in this PET study of 10.3 mL/100 ml/min (95% CI 8.8;11.7 mL/100 ml/min) and 1/3 of thalami rCBF (ratio, 0.36, 95% CI 0.23;0.48). The ASL rCBF values are critically dependent on arrival time of magnetically labeled inflowing arterial blood to the tissue, and delayed arrival can be the cause of falsely reduce rCBF.^55,56 Unfortunately, our own ASL data could not be used to further illuminate the matter. At the time of our studies, the ASL MRI sequence available to us was a single TI pulsed ASL (pASL) sequence. Our assumption was that we could obtain better ASL image quality and data in the larger brains of the infants than in the piglet study by our group.²¹ Motion artefacts are however a problem for ASL in this age group: 25% of scans were discarded because of poor image quality in the study by Boudes et al.²⁰ and 40% in the study by Miranda et al.¹⁵ Unfortunately, the ASL scans of all our subjects were substantially affected by motion artefacts as documented with the on-line marker-less tracking system (Figure 1) and must be considered unreliable. In a clinical setting where sedation is permissible for diagnostic imaging, motion artefacts could be less significant. It would, however, be preferable to avoid sedation and correct for movement.²⁷ For subject no. 1 and 2, there was a pronounced vascular signal that could be attributed to insufficiencies in sequence parameters or the sequence itself. Comparative studies in newborns have recommended pseudo-continuous arterial spin labeling (pCASL) as the superior technique that allowed improved image quality with a more detailed identification of the different brain structures²⁰ in accordance with the ASL white paper recommendations.⁵⁷ Furthermore, as mentioned previously multidelay (TI), ASL could improve quality of the data^41,42 especially in low CBF areas (see supplementary material for ASL rCBF images of the subjects).

Interruption of the supply of nutrients and oxygenation can cause PVL in vulnerable areas of the periventricular white matter. Assessing changes in perfusion in white matter is of value in the clinical work-up of PVL patients and cannot be easily assessed with clinically available methods (near infrared spectroscopy (NIRS) and Doppler ultrasound). For this reason, the implementation of a PET imaging technique with high spatial resolution could possibly be of importance in evaluating the severity of PVL. As a one-stop scan, PET alongside advanced MRI in a PET/MRI could potentially add to the value of imaging for predicting severe neurodevelopmental deficits such as debilitating cerebral palsy.⁵⁸ Logistics and safety issues when considering PET/MR scans of premature infants can be addressed by performing the scan using an MR-compatible infant transport incubator which is available for the PET/MR scanner.

PET image quality and resolution in a premature infant of, i.e. 30 weeks’ gestational age should be adequate since the unmyelinated white matter is the most prominent tissue class of the brain and the gyral pattern is relatively simple.⁵⁹ The anatomical structures are sufficiently developed to enable adequate VOIs to determine white and gray matter rCBF, even though the brain volume is small and increases dramatically from 150 mL at 29 weeks to 400 ml at term.⁵⁹ The volume of the left ventricle in premature infant of 1 kg is approximately 1 mL corresponding to a diameter of 14 mm if assuming a spherical shape,³⁸ which would still allow the measurement of central voxels that are not affected by the partial volume effect.

Even though we were unsuccessful in acquiring adequate ASL data, we believe that we have presented a clinical neuropediatric tool for future PET/MRI in the healthy or diseased child during the first year of infancy. In neonatal patients, particularly, PVL is a target diagnosis, where current diagnostic methods are challenged to adequately quantify CBF and characterize the extent of the white matter damage and relate it to anatomy. This should now be possible.

Furthermore, it would be a method of choice for documenting hemodynamic insufficiency in the preoperative planning of infants suffering from early onset Moya-Moya’s disease, a progressive obliterating angiopathy of major supratentorial vascular territories of the brain characterized by repeated transitory ischemic attacks (TIAs) and ischemic stroke.⁴⁷

Also, PET/MRI may be used for independent simultaneous evaluation of new MRI perfusion techniques using PET as the ‘golden’ standard. There are initiatives to render improved ASL sequences more widely available. Furthermore, our method allows advanced combinations of standard and modern MRI techniques with molecular PET imaging, as have been shown in, e.g. pediatric neurooncology.^60,61

No previous studies of ¹⁵O-water PET rCBF in healthy newborn infants have been done. We obtained good quality images in four healthy newborns using a new PET/MRI method with an IDIF. We used improved diagnostic methods with a substantially lower radiation dose than the few existing previous studies performed in infants with pathology. The white/grey matter CBF ratio was close to that measured in the healthy human adult brain. The method may be useful as a ‘golden’ standard for evaluating new MR-based methods as well as for clinical research in specific disease states.

Supplementary Material

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was funded by The Ludvig and Sara Elsass Foundation. The hybrid PET/MRI system at Rigshospitalet was donated by the John and Birthe Meyer Foundation.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Authors’ contributions

JBA contributed with design, recruitment of subjects, acquisition, analysis and interpretation of data, drafting and revising the manuscript and final approval of manuscript. UL contributed with design and analysis of data, revising and approving the manuscript, OO contributed with acquisition and analysis of data, revising and approving the manuscript, DB contributed in analysis and interpretation of data and approving the manuscript, CL contributed with design and analysis of data, revising and approving the manuscript, HL, contributed in analysis of data and revising and approving the manuscript, LH contributed with design and interpretation of data, revising and approving the manuscript, GG contributed with design, analysis and interpretation of data, revising and approving the manuscript, IL contributed with design, analysis and interpretation of data, revising and approving the manuscript.

Supplementary material

Supplementary material for this paper can be found at the journal website: http://journals.sagepub.com/home/jcb