SPECT images were acquired using the eXplore speCZT Vision 120 preclinical imaging system (GE Healthcare, Waukesha, USA) illustrated in Fig. 1. The SPECT part consists in a full ring of semiconductors, CZT detectors (10 heads of 8 cm × 8 cm, pixel size 2.46 mm, energy range 20–200 keV, and energy resolution <7% at 140 keV), and rotating multi-pinhole dedicated tungsten collimators for mouse or rat. For mouse imaging, radial field of view (FOV) is 34 mm with a cylindrical multi-pinhole collimator of seven pinholes of 1 mm diameter each (Note 3). For rat imaging, radial FOV is 78 mm using a cylindrical multi-pinhole collimator of five pinholes of 1.5 mm diameter each. Using the helical acquisition mode, the axial FOV can be adjusted to the size of the head and neck of the small rodent but at the expend of longer acquisition times.

Small animals (mice and rats) are anesthetized with a gas mixture (air and 1–2% isoflurane) in an air warmed-up multimodality imaging chamber (Minerve, Esternay, France) to keep constant at 37°C their temperature. Heart rate is obtained through three paws carbon tube electrodes (Note 4) fitted with conductive gel and insensitive to both X-rays or magnetic field as previously described (39) and connected with an electrocardiograph (ODAM Physiogard RSM 784, Wissembourg, France) delivering a physiological trigger corresponding to the QRS of the ECG signal. Monitoring of breathing rate is obtained via a small differential pressure transducer (TSD 110-MRI; Biopac Systems Inc., CA, USA) placed under the thorax of the animal lying in prone position (Note 5).

Micro-SPECT functional images can be fused with anatomic images issued from other imaging techniques as micro-CT (Note 6) or micro-MRI (Note 7). The image fusion allows a correct anatomic location of the radioactivity uptake, which is of particular importance in brain imaging. Translation of the animal from one modality to the other is achieved using the imaging cell and without moving the animal, avoiding then the use of fiducial markers. A close proximity of both imaging systems or better a co-axially dual modality device configuration is therefore recommended.

The CT part of the eXplore speCZT Vision 120 illustrated in Fig. 1 and which we used for multimodal imaging is placed on the same axis and back-to-back with the SPECT and has a high-power rotating anode X-ray tube (focal spot of 50 μ, 120 kV maximum potential, and 50 mA maximum current). X-rays attenuation projections are obtained with a large area CCD detector bonded to a CsI-coated fiber optic taper.

When high-field MRI is considered for morphological co-registration with SPECT, there could be complex physical interactions between the high magnetic field generally used and the nuclear instrumentation preventing then their close proximity (Note 8). For that reason, we developed a low-field MR imager dedicated to preclinical multimodality imaging, Fig. 2 (40). The main magnetic field B ₀ of 0.1 T is oriented vertically and delivered by a small water-cooled open electromagnet (Bouhnik SAS, Velizy, France). The field homogeneity ΔB/B ₀ = 5 × 10⁻⁶ is obtained in an imaging volume of 10 × 10 × 6 cm³ with maximum encoding gradients’ strength of 20 mT/m that are reached in 0.5 ms. Radio frequency (RF) transmit-and-receive solenoid coils are made by adjacent loops of copper wire tuned at 4.26 MHz corresponding to the Larmor frequency of the ¹H at that field.

RF pulses are transmitted to the solenoid coil by a pick-up copper coil rounded outside the imaging cell (as illustrated in Fig. 2), which also receive the encoded MR signal. For anatomical images, co-registration a T1-weighted 3-dimensional (3D) sequence was developed on a SMIS (Guilford, UK) console to achieve typical isotropic (Note 9) spatial resolutions of, respectively, 1 × 1 × 1 mm³ for rat and 0.5 × 0.5 × 0.5 mm³ for mouse. The low-field MR imaging system was used back to back with a small animal SPECT gamma camera (Gaede Medizinsyteme, Freiburg, Germany) equipped with one pinhole of 1.5 mm in diameter and a focal distance of 12 cm (41).

The Microview software driving the eXplore speCZT Vision 120 permitted the SPECT/CT images reconstruction. A 3D OSEM (11 averages and 5 subsets) algorithm, without Compton and attenuation correction, was applied for micro-SPECT images reconstruction leading to reconstructed voxels of 300 × 300 × 300 μ³. A cone beam filtered back-projection Feldkamp algorithm is used for micro-CT images reconstruction leading to isotropic reconstructed voxels of 100 × 100 × 100 μ³. For the SPECT/low-field MRI experiment, the SPECT images were reconstructed using an algebraic reconstruction technique (ART) enabling an isotropic 1 × 1 × 1 mm³ spatial resolution and a voxel size of 470 × 470 × 470 μ³ (42).

Hot lab equipment needs to meet each national nuclear regulatory standards for single photon emitters biological applications including:

Detailed data on numerous SPECT radiopharmaceuticals chemistry and labeling preparation steps can be found in the MICAD database (Note 10). In the following, as typical examples, we will only focus on the principal preparation and quality control steps of two commercial radiopharmaceuticals (respectively, ^99mTc-HMPAO and ¹²³I-FP-CIT) that will be illustrated in small animal SPECT for imaging brain perfusion and dopamine transporters.