Cerenkov luminescence imaging of medical isotopes

Abstract

The development of novel multimodality imaging agents and techniques represents the current frontier of research in the field of medical imaging science. However, the combination of nuclear tomography with optical techniques has yet to be established. Here, we report the use of the inherent optical emissions from the decay of radiopharmaceuticals for Cerenkov luminescence imaging (CLI) of tumors in vivo and correlate the results with those obtained from concordant immuno-PET studies.

Methods: In vitro phantom studies were used to validate the visible light emission observed from a range of radionuclides including the positron emitters (18)F, (64)Cu, (89)Zr, and (124)I; beta-emitter (131)I; and alpha-particle emitter (225)Ac for potential use in CLI. The novel radiolabeled monoclonal antibody (89)Zr-desferrioxamine B [DFO]-J591 for immuno-PET of prostate-specific membrane antigen (PSMA) expression was used to coregister and correlate the CLI signal observed with the immuno-PET images and biodistribution studies.

Results: Phantom studies confirmed that Cerenkov radiation can be observed from a range of positron-, beta-, and alpha-emitting radionuclides using standard optical imaging devices. The change in light emission intensity versus time was concordant with radionuclide decay and was also found to correlate linearly with both the activity concentration and the measured PET signal (percentage injected dose per gram). In vivo studies conducted in male severe combined immune deficient mice bearing PSMA-positive, subcutaneous LNCaP tumors demonstrated that tumor-specific uptake of (89)Zr-DFO-J591 could be visualized by both immuno-PET and CLI. Optical and immuno-PET signal intensities were found to increase over time from 24 to 96 h, and biodistribution studies were found to correlate well with both imaging modalities.

Conclusion: These studies represent the first, to our knowledge, quantitative assessment of CLI for measuring radiotracer uptake in vivo. Many radionuclides common to both nuclear tomographic imaging and radiotherapy have the potential to be used in CLI. The value of CLI lies in its ability to image radionuclides that do not emit either positrons or gamma-rays and are, thus, unsuitable for use with current nuclear imaging modalities. Optical imaging of Cerenkov radiation emission shows excellent promise as a potential new imaging modality for the rapid, high-throughput screening of radiopharmaceuticals.

Figure 1

Phantom images recorded using optical (CLI) imaging (A) and PET (B) of 6 samples of ⁸⁹Zr activity in water. At time 0 h, the Eppendorf tubes labeled 1–6 corresponded to activity concentrations of 40.3, 32.6, 27.4, 20.4, 13.3 and 0.00 kBq/μL. Optical images were recorded by using an integration time of 30 s and f/stop 1.

Figure 2

Quantitative analysis of the phantom studies. Positive correlation observed between measured average radiance (background-corrected in units of p/s/cm²/sr) and ⁸⁹Zr activity concentration (kBq/μL) (A), rate of decay observed in normalized radiance vs. time/h (B), and linear correlation observed between average radiance (background and decay-corrected in units of p/s/cm²/sr) vs. mean PET signal intensity (measured in units of %ID/g, commonly used for quantification of in vivo PET studies) (C). Ave. = average; Exp. = exponential.

Figure 3

Plot of ratio of average radiance (p/s/cm²/sr)/activity concentration (μCi/μL) vs. radionuclide. Positron-emitting radionuclides are arranged in order of increasing average β⁺ kinetic energy (¹⁸F: E_β+=249.8 keV [I_β+=100%]; ⁶⁴Cu: E_β+=278.2 keV [I_β+=17.6%]; ⁸⁹Zr: E_β+=395.5 keV [I_β+=22.7%]; ¹²⁴I: E_β+=820 keV [I_β+=22.7%]). For ¹³¹I, E_β−=181.9 keV [I_β− 100%]. ²²⁵Ac decays by 100% α-particle emission with E_α in the range 5021–5830 keV (1). Ave. = average

Figure 4

Temporal images of ⁸⁹Zr-DFO-J591 uptake (10.9–11.3 MBq, [295–305 μCi], 60–62 μg of mAb, in 200 μL 0.9% sterile saline) recorded in dual subcutaneous LNCaP (PSMA-positive) tumor-bearing Severe combined immune deficient mice between 24 and 96 h after administration. (A) signal observed in the optical spectrum from in vivo CLI of ⁸⁹Zr-DFO-J591 tumor uptake in 3 mice. (B) Corresponding coronal and transverse immuno-PET images recorded for mouse 3. (C) Optical image recorded of the organs after acute ex vivo biodistribution at 96 h. Transverse and coronal planar immuno-PET images intersect center of tumors. Upper and lower thresholds of CLI and immuno-PET images in A-C have been adjusted for visual clarity, as indicated by scale bars. Trans. = transverse; +ve = positive; T(L) = left tumor; T(R) = right tumor; He = heart; Lu = lungs; Li = liver; Sp = spleen; Ki = kidneys; L. Int. = large intestine; Bo = bone; Mu = muscle.

Figure 5

Bar chart showing selected tissue biodistribution data (%ID/g) for uptake of ⁸⁹Zr-DFO-J591 in male severe combined immune deficient mice at the end of optical and immuno-PET experiments (96 h post-injection). T = tumor

Figure 6

Time-activity curves showing ROI and volume-of-interest analysis of CLI and immuno-PET images for ⁸⁹Zr-DFO-J591 uptake in well-established (large) LNCaP tumors. Volume-of-interest analysis of immuno-PET images shows change in ⁸⁹Zr activity in heart-blood pool and muscle tissue.