The use of monolithic scintillator-based photon detectors in positron emission tomography (PET) has emerged as an attractive alternative to traditional pixelated array designs. Monolithic-based detector designs employ the scintillation light distribution (LD) shape to provide a single 3D photon interaction position per event, enabling high spatial resolution throughout the crystal volume. Since there are no intercrystal gaps, monolithic designs provide higher intrinsic detection efficiency compared to pixelated designs. However, in order to make the monolithic detector design practical for clinical PET systems, some major drawbacks need to be addressed such as the timeconsuming and complex calibration procedures to obtain precise spatial and timing information. This paper gives a historical review of monolithic-based PET detectors, a description of their main advantages and challenges, describes the state-of-the-art, including their use in current commercial system, and ends with a future prospective.

In this review, we will cover both clinical and technical aspects of the advantages and disadvantages of organ specific (dedicated) molecular imaging (MI) systems, namely positron emission tomography (PET) and single photon emission computed tomography, including gamma cameras. This review will start with the introduction to the organ-dedicated MI systems. Thereafter, we will describe the differences and their advantages/disadvantages when compared with the standard large size scanners. We will review time evolution of dedicated systems, from first attempts to current scanners, and the ones that ended in clinical use. We will review later the state of the art of these systems for different organs, namely: breast, brain, heart, and prostate. We will also present the advantages offered by these systems as a function of the special application or field, such as in surgery, therapy assistance and assessment, etc. Their technological evolution will be introduced for each organ-based imager. Some of the advantages of dedicated devices are: higher sensitivity by placing the detectors closer to the organ, improved spatial resolution, better image contrast recovery (by reducing the noise from other organs), and also lower cost. Designing a complete ring-shaped dedicated PET scanner is sometimes difficult and limited angle tomography systems are preferable as they have more flexibility in placing the detectors around the body/organ. Examples of these geometries will be presented for breast, prostate and heart imaging. Recently achievable excellent time of flight capabilities below 300-ps full width at half of the maximum reduce significantly the impact of missing angles on the reconstructed images.

The relativistic recoil effect has been the object of experimental investigations using highly charged ions at the Heidelberg electron beam ion trap. Its scaling with the nuclear charge $Z$ boosts its contribution to a measurable level in the magnetic-dipole ($M1$) transitions of B- and Be-like Ar ions. The isotope shifts of ${}^{36}\mathrm{A}\mathrm{r}$ versus ${}^{40}\mathrm{A}\mathrm{r}$ have been detected with sub-ppm accuracy, and the recoil effect contribution was extracted from the $1s^{2}2s^{2}2p$ ${}^{2}P_{1/2}-{}^{2}P_{3/2}$ transition in ${\mathrm{Ar}}^{13+}$ and the $1s^{2}2s2p$ ${}^{3}P_1-{}^{3}P_2$ transition in ${\mathrm{Ar}}^{14+}$. The experimental isotope shifts of 0.00123(6) nm (${\mathrm{Ar}}^{13+}$) and 0.00120(10) nm (${\mathrm{Ar}}^{14+}$) are in agreement with our present predictions of 0.00123(5) nm (${\mathrm{Ar}}^{13+}$) and 0.00122(5) nm (${\mathrm{Ar}}^{14+}$) based on the total relativistic recoil operator, confirming that a thorough understanding of correlated relativistic electron dynamics is necessary even in a region of intermediate nuclear charges.

The lifetime of the ${\mathrm{Ar}}^{13+}$ $1s^{2}2s^{2}2p{}^{2}P_{3/2}^o$ metastable level was determined at the Heidelberg Electron Beam Ion Trap to be $9.573(4)(5)\mathrm{ms}(\mathrm{stat})(\mathrm{syst})$. The accuracy level of one per thousand makes this measurement sensitive to quantum electrodynamic effects like the electron anomalous magnetic moment (EAMM) and to relativistic electron-electron correlation effects like the frequency-dependent Breit interaction. Theoretical predictions, adjusted for the EAMM, cluster about a lifetime that is approximately $3\sigma$ shorter than our experimental result.