A positron emission tomography, PET, imaging system, comprising: a bore for receiving a subject, the bore comprising an axis; and a plurality of detector modules (130 1..i); wherein each detector module (130 1..i) comprises a scintillator array (130a 1..i) coupled to a photodetector array (130b 1..i), and wherein the detector module is configured to generate event data in response to received gamma quanta (140 a, 140 b); wherein the detector modules (130 1..i) are arranged around the axis of the bore such that the detector modules generate the event data in response to gamma quanta received from within the bore; wherein each detector module (130 1..i) further comprises a cooling unit (130c 1..i), the cooling unit including a cooling fluid inlet, a cooling fluid outlet, a heat transfer region, and a passageway arranged between the cooling fluid inlet and the cooling fluid outlet for transferring heat from the heat transfer region via a cooling fluid inputted into the cooling fluid inlet; wherein the heat transfer region of each cooling unit (130c 1..i) is thermally coupled to the photodetector array (130b 1..i) of the corresponding detector module for cooling the photodetector array; and wherein the cooling units (130c 1..i) are configured to be fluidically coupled in parallel to at least one cooling fluid source; and wherein within the heat transfer region, at least a portion of the passageway extends in a direction parallel to the axis of the bore and returns in an opposing direction such that a cooling fluid inputted to the cooling fluid inlet of the cooling unit (130c 1..i) flows in opposing directions within the heat transfer region before being outputted from the cooling fluid outlet of the cooling unit; and wherein the at least a portion of the passageway comprises a first path and a second path, wherein the first path extends in the direction parallel to the axis of the bore and the second path returns in the opposing direction, and characterised in that the heat transfer region is configured to provide direct thermal contact between the opposing paths of the passageway continuously along their length. a bore for receiving a subject, the bore comprising an axis; and a plurality of detector modules (130 1..i); wherein each detector module (130 1..i) comprises a scintillator array (130a 1..i) coupled to a photodetector array (130b 1..i), and wherein the detector module is configured to generate event data in response to received gamma quanta (140 a, 140 b); wherein the detector modules (130 1..i) are arranged around the axis of the bore such that the detector modules generate the event data in response to gamma quanta received from within the bore; wherein each detector module (130 1..i) further comprises a cooling unit (130c 1..i), the cooling unit including a cooling fluid inlet, a cooling fluid outlet, a heat transfer region, and a passageway arranged between the cooling fluid inlet and the cooling fluid outlet for transferring heat from the heat transfer region via a cooling fluid inputted into the cooling fluid inlet; wherein the heat transfer region of each cooling unit (130c 1..i) is thermally coupled to the photodetector array (130b 1..i) of the corresponding detector module for cooling the photodetector array; and wherein the cooling units (130c 1..i) are configured to be fluidically coupled in parallel to at least one cooling fluid source; and wherein within the heat transfer region, at least a portion of the passageway extends in a direction parallel to the axis of the bore and returns in an opposing direction such that a cooling fluid inputted to the cooling fluid inlet of the cooling unit (130c 1..i) flows in opposing directions within the heat transfer region before being outputted from the cooling fluid outlet of the cooling unit; and wherein the at least a portion of the passageway comprises a first path and a second path, wherein the first path extends in the direction parallel to the axis of the bore and the second path returns in the opposing direction, and characterised in that the heat transfer region is configured to provide direct thermal contact between the opposing paths of the passageway continuously along their length.