PET/MR in children. Initial clinical experience in paediatric oncology using an integrated PET/MR scanner

Abstract

Use of PET/MR in children has not previously been reported, to the best of our knowledge. Children with systemic malignancies may benefit from the reduced radiation exposure offered by PET/MR. We report our initial experience with PET/MR hybrid imaging and our current established sequence protocol after 21 PET/MR studies in 15 children with multifocal malignant diseases. The effective dose of a PET/MR scan was only about 20% that of the equivalent PET/CT examination. Simultaneous acquisition of PET and MR data combines the advantages of the two previously separate modalities. Furthermore, the technique also enables whole-body diffusion-weighted imaging (DWI) and statements to be made about the biological cellularity and nuclear/cytoplasmic ratio of tumours. Combined PET/MR saves time and resources. One disadvantage of PET/MR is that in order to have an effect, a significantly longer examination time is needed than with PET/CT. In our initial experience, PET/MR has turned out to be an unexpectedly stable and reliable hybrid imaging modality, which generates a complementary diagnostic study of great additional value.

Fig. 1

Two-point volume interpolated breathhold examination (VIBE) Dixon sequence for determination/segmentation of fat, lung tissue, air and water (soft tissue) and assignment of a map of linear attenuation coefficients (μ-map). All four components of the sequence are required for calculating an attenuation correction pattern (μ-map) for the PET data: in-phase and opposed-phase T1 images (a, b), and fat image and water image (c, d). From these images, the μ-map that is utilized for attenuation correction of the PET data is generated/segmented (e). For the subsequent PET/MR fusion, only the attenuation-corrected PET data are used

Fig. 2

A 13-year-old boy with Hodgkin disease stage II. a Involvement of both sides of the submandibular, cervical, and upper and middle mediastinum (arrows). b Involvement of the submandibular lymph nodes (arrows) was not positively identified on MR criteria (size), but the PET image shows significantly enhanced FDG uptake. Arrows indicate positive PET/MR findings

Fig. 2

A 13-year-old boy with Hodgkin disease stage II. a Involvement of both sides of the submandibular, cervical, and upper and middle mediastinum (arrows). b Involvement of the submandibular lymph nodes (arrows) was not positively identified on MR criteria (size), but the PET image shows significantly enhanced FDG uptake. Arrows indicate positive PET/MR findings

Fig. 3

A 2.5-year-old boy with bilateral cervical (arrows) Burkitt lymphoma. Arrows indicate positive PET/MR findings

Fig. 4

An 11-year-old boy with T-cell lymphoma with involvement of the neck, mediastinum, kidneys and bone marrow. a Significantly increased glucose metabolism of the pleural effusion also indicates tumour involvement in this region, while on MR this effusion was interpreted as an expression of congestion due to the large mediastinal tumour mass (arrows). b Note the significant pathological structure, size and metabolic activity (encircled, arrows) of the kidneys and the bone marrow in the left proximal femur. c The additional findings shown by the whole-body diffusion-weighted image (left column) and ADC map (middle column) show, concordant with PET findings, a distinct diffusion disorder in the area of the mediastinal mass (top row, arrow), the left kidney (middle row, arrow), and the proximal left femur (bottom row, arrow). Because of the bilateral similarity of the changes in the kidney, the interpretation of the PET scan was less certain. On the basis of significantly restricted diffusion and scale, the MR scan with diffusion-weighted image provided more conclusive identification of an active tumour tissue. Arrows in Fig. 4a and 4b indicate positive PET/MR findings. Arrows in Fig. 4c indicate positive ADC value typical for tumour tissue

Fig. 4

An 11-year-old boy with T-cell lymphoma with involvement of the neck, mediastinum, kidneys and bone marrow. a Significantly increased glucose metabolism of the pleural effusion also indicates tumour involvement in this region, while on MR this effusion was interpreted as an expression of congestion due to the large mediastinal tumour mass (arrows). b Note the significant pathological structure, size and metabolic activity (encircled, arrows) of the kidneys and the bone marrow in the left proximal femur. c The additional findings shown by the whole-body diffusion-weighted image (left column) and ADC map (middle column) show, concordant with PET findings, a distinct diffusion disorder in the area of the mediastinal mass (top row, arrow), the left kidney (middle row, arrow), and the proximal left femur (bottom row, arrow). Because of the bilateral similarity of the changes in the kidney, the interpretation of the PET scan was less certain. On the basis of significantly restricted diffusion and scale, the MR scan with diffusion-weighted image provided more conclusive identification of an active tumour tissue. Arrows in Fig. 4a and 4b indicate positive PET/MR findings. Arrows in Fig. 4c indicate positive ADC value typical for tumour tissue

Fig. 4

An 11-year-old boy with T-cell lymphoma with involvement of the neck, mediastinum, kidneys and bone marrow. a Significantly increased glucose metabolism of the pleural effusion also indicates tumour involvement in this region, while on MR this effusion was interpreted as an expression of congestion due to the large mediastinal tumour mass (arrows). b Note the significant pathological structure, size and metabolic activity (encircled, arrows) of the kidneys and the bone marrow in the left proximal femur. c The additional findings shown by the whole-body diffusion-weighted image (left column) and ADC map (middle column) show, concordant with PET findings, a distinct diffusion disorder in the area of the mediastinal mass (top row, arrow), the left kidney (middle row, arrow), and the proximal left femur (bottom row, arrow). Because of the bilateral similarity of the changes in the kidney, the interpretation of the PET scan was less certain. On the basis of significantly restricted diffusion and scale, the MR scan with diffusion-weighted image provided more conclusive identification of an active tumour tissue. Arrows in Fig. 4a and 4b indicate positive PET/MR findings. Arrows in Fig. 4c indicate positive ADC value typical for tumour tissue

Fig. 5

A 15-year-old boy with left testicular tumour with left retroperitoneal, supraclavicular and hepatic metastases (arrows). a The primary tumour in the left testicle and the retroperitoneal metastasis exhibit a very inhomogeneous enhancement of the glucose metabolism with larger metabolically inactive areas showing no enhancement. This can only be explained in part by tumour necrosis. The liver metastasis shows significantly enhanced glucose uptake. b A small metastasis (3 mm, arrow) in the left lung was diagnosed with good respiratory triggering by MR and by PET. This indicates that the system operates with high performance and yields superior images even of very small lesions. Arrows indicate positive PET/MR findings

Fig. 6

A 2-year-old boy with a large abdominal neuroblastoma that emanates from the sympathetic chain. Top row: the tumour (arrows) is growing into the spinal canal and involves the fourth and fifth lumbar vertebrae. The highly differentiated tumour shows only slightly intense inhomogeneous FDG uptake. Lower row: transaxial slices show multiple (arrows) lung metastases and pleural metastases. The metastases are seen on the concordant respiratory-triggered T2-weighted TIRM images (a) and on the PET images (b, c); smaller metastases of 3–5 mm are clearly shown on the MR images. Arrows indicate positive PET/MR findings

Fig. 7

Anaplastic lymphoma with involvement of the ilium. Top row: initial, decisive MR image (a) and PET/CT image (arrow) (b). Bottom row: after treatment. In the whole-body MRI, residual bone lesions (arrows) can still be seen (a), which are not positive on the PET image (arrow) (b), and thus would be considered as inactive residual tissue

Fig. 8

Large right-sided thoracic primitive neuroectodermal tumour residuum (arrow) in a 13-year-old boy at the end of neoadjuvant chemotherapy. The dark tumour on the T2-weighted MR image and mostly unenhanced FDG uptake on the PET image argue for predominant fibrous components. This was confirmed later histologically (Salzer-Kuntschik grade 2). However, in the cranial portion of the tumour there is a narrow zone of increased FDG uptake. On MRI, the still active residual tumour tissue was only ambiguously identified. Arrow indicates positive PET/MR finding

Fig. 9

A 13-year-old boy with a history of extraosseous metastatic Ewing sarcoma of the pelvis now showing new imaging findings (arrows) suspicious for soft tissue metastasis in the adductor longus muscle. On MR imaging, size regression was seen within 7 weeks. Because of an intensive increase in glucose utilization, a biopsy was performed, which revealed inflammatory changes. Arrows indicate the positive PET/MR finding