Simultaneous ECG-gated PET imaging of multiple mice

Intrinsic Respiratory Gating for Simultaneous Multi-Mouse μCT Imaging to Assess Liver Tumors

Small animal micro computed tomography (μCT) is an important tool in cancer research and is used to quantify liver and lung tumors. A type of cancer that is intensively investigated with μCT is hepatocellular carcinoma (HCC). μCT scans acquire projections from different angles of the gantry which rotates X-ray source and detector around the animal. Motion of the animal causes inconsistencies between the projections which lead to artifacts in the resulting image. This is problematic in HCC research, where respiratory motion affects the image quality by causing hypodense intensity at the liver edge and smearing out small structures such as tumors. Dealing with respiratory motion is particularly difficult in a high throughput setting when multiple mice are scanned together and projection removal by retrospective respiratory gating may compromise image quality and dose efficiency. In mice, inhalation anesthesia leads to a regular respiration with short gasps and long phases of negligible motion. Using this effect and an iterative reconstruction which can cope with missing angles, we discard the relatively few projections in which the gasping motion occurs. Moreover, since gated acquisition, i.e., acquiring multiple projections from a single gantry angle is not a requirement, this method can be applied to existing scans. We applied our method in a high throughput setting in which four mice with HCC tumors were scanned simultaneously in a multi-mouse bed. To establish a ground truth, we manually selected projections with visible respiratory motion. Our automated intrinsic breathing projection selection achieved an accordance of 97% with manual selection. We reconstructed volumetric images and demonstrated that our intrinsic gating method significantly reduces the hypodense depiction at the cranial liver edge and improves the detectability of small tumors. Furthermore, we show that projection removal in a four mice scan discards only 7.5% more projections than in a single-mouse setting, i.e., four mouse scanning does not substantially compromise dose efficiency or image quality. To the best of our knowledge, no comparable method that combines multi-mouse scans for high throughput, intrinsic respiratory gating, and an available iterative reconstruction has been described for liver tumor imaging before.

Comparison of planar, PET and well-counter measurements of total tumor radioactivity in a mouse xenograft model

INTRODUCTION

Quantitative small animal radionuclide imaging studies are often carried out with the intention of estimating the total radioactivity content of various tissues such as the radioactivity content of mouse xenograft tumors exposed to putative diagnostic or therapeutic agents. We show that for at least one specific application, positron projection imaging (PPI) and PET yield comparable estimates of absolute total tumor activity and that both of these estimates are highly correlated with direct well-counting of these same tumors. These findings further suggest that in this particular application, PPI is a far more efficient data acquisition and processing methodology than PET.

METHODS

Forty-one athymic mice were implanted with PC3 human prostate cancer cells transfected with prostate-specific membrane antigen (PSMA (+)) and one additional animal (for a total of 42) with a control blank vector (PSMA (-)). All animals were injected with [18F] DCFPyl, a ligand for PSMA, and imaged for total tumor radioactivity with PET and PPI. The tumors were then removed, assayed by well counting for total radioactivity and the values between these methods intercompared.

RESULTS

PET, PPI and well-counter estimates of total tumor radioactivity were highly correlated (R2>0.98) with regression line slopes near unity (0.95

CONCLUSION

Total mouse xenograft tumor radioactivity can be measured with PET or PPI with an accuracy comparable to well counting if certain experimental and pharmacokinetic conditions are met. In this particular application, PPI is significantly more efficient than PET in making these measurements.

Collapse

Key Words

• Cancer drug bio-distribution studies
• Mouse tumor xenografts models
• Quantitative PET
• Quantitative planar imaging

Collapse

MESH Headings

• Animals
• Cell Transformation, Neoplastic
• Humans
• Male
• Mice
• Positron-Emission Tomography/methods
• Prostatic Neoplasms/diagnostic imaging
• Prostatic Neoplasms/pathology
• Radioactive Tracers
• Radioactivity

Collapse

Grants

• HHSN261200800001C NCI NIH HHS
• HHSN261200800001E NCI NIH HHS
• ZIA BC010668-12 Intramural NIH HHS

Collapse

Affiliation(s)

Michael V Green Molecular Imaging Program, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, MD; Contractor to Frederick National Laboratory for Cancer Research, Leidos Biomedical Research, Inc., Frederick, MD.
Jurgen Seidel Molecular Imaging Program, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, MD; Contractor to Frederick National Laboratory for Cancer Research, Leidos Biomedical Research, Inc., Frederick, MD
Mark R Williams Molecular Imaging Program, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, MD; Contractor to Frederick National Laboratory for Cancer Research, Leidos Biomedical Research, Inc., Frederick, MD
Karen J Wong Molecular Imaging Program, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, MD
Anita Ton Molecular Imaging Program, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, MD
Falguni Basuli Imaging Probe Development Center, National Heart, Lung and Blood Institute, National Institutes of Health, Bethesda, MD
Peter L Choyke Molecular Imaging Program, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, MD
Elaine M Jagoda Molecular Imaging Program, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, MD

Collapse

A simple device to convert a small-animal PET scanner into a multi-sample tissue and injection syringe counter

INTRODUCTION

We describe a simple fixture that can be added to the imaging bed of a small-animal PET scanner that allows for automated counting of multiple organ or tissue samples from mouse-sized animals and counting of injection syringes prior to administration of the radiotracer. The combination of imaging and counting capabilities in the same machine offers advantages in certain experimental settings.

METHODS

A polyethylene block of plastic, sculpted to mate with the animal imaging bed of a small-animal PET scanner, is machined to receive twelve 5-ml containers, each capable of holding an entire organ from a mouse-sized animal. In addition, a triangular cross-section slot is machined down the centerline of the block to secure injection syringes from 1-ml to 3-ml in size. The sample holder is scanned in PET whole-body mode to image all samples or in one bed position to image a filled injection syringe. Total radioactivity in each sample or syringe is determined from the reconstructed images of these objects using volume re-projection of the coronal images and a single region-of-interest for each. We tested the accuracy of this method by comparing PET estimates of sample and syringe activity with well counter and dose calibrator estimates of these same activities.

RESULTS

PET and well counting of the same samples gave near identical results (in MBq, R2=0.99, slope=0.99, intercept=0.00-MBq). PET syringe and dose calibrator measurements of syringe activity in MBq were also similar (R2=0.99, slope=0.99, intercept=- 0.22-MBq).

CONCLUSION

A small-animal PET scanner can be easily converted into a multi-sample and syringe counting device by the addition of a sample block constructed for that purpose. This capability, combined with live animal imaging, can improve efficiency and flexibility in certain experimental settings.