1.  Field of the Invention
The present invention generally relates to an interface device and, more particularly, to a photomultiplier tube (PMT) interface device. 
2.  Description of the Prior Art
The positron emission tomography (PET) scan has been developed since Browell's work on the positron scintillation camera in 1953.  In 1975, Ter-Pogossian developed positron emission transaxial tomography (PETT), using a scintillation detector comprising scintillation crystals at the front end to convert the γ-ray photons into scintillation signals.  On the back end, there is a photomultiplier tube comprising a photocathode to convert the scintillation signals into photoelectrons by the photoelectric effect.  The photoelectrons bombard the dynode, which is applied with 200-400 volts, to generate secondary electrons.  The secondary electrons multiply after bombarding a plurality of dynodes and will be collected by the anode to cause a pulse signal.  The magnitude of the pulse signal is proportional to the number of scintillation photoelectrons received by the photocathode.  A weak optical signal can be amplified by a photomultiplier tube and the amplification depends on the voltage applied on the dynode. 
In early PET systems, the sodium iodide NaI(Tl) crystal is used.  However, the NaI(Tl) crystal is not suitable for use in converting the γ-ray photons into scintillation signals when the photon energy reaches 511 keV, unless the thickness of the NaI(Tl) crystal with lowered imaging resolution.  The BGO (bismuth germanate, Bi3Ge4O12) crystal is widely used in modern PET systems because the BGO crystal exhibits higher efficiency.  The LSO (lutetium oxyorthosilicate) crystal exhibits higher density, higher stopping power and higher conversion efficiency than the BGO crystal.  However, the LSO crystal is not as widely used as the BGO because of its cost and bottleneck issues in manufacture. 
Please refer to FIG.  1, which is 3-D schematic diagram of a conventional scintillation crystal detecting module.  The conventional scintillation crystal detecting module 1 comprises a scintillation crystal unit 10, a PMT module 11 and a circuit substrate 13.  The scintillation crystal unit 10 comprises a plurality of scintillation crystals formed of NaI, LSO or BGO arranged in an array.  One end of the PMT module 11 is coupled to the scintillation crystal unit 10 to receive an optical signal from the scintillation crystal unit 10 and then convert the optical signal into an electrical signal.  At another end, the PMT module 11 comprises a plurality of pins 12 to be electrically connected to the circuit substrate 13.  The circuit substrate 13 is a circuit board.  There are contacts 14 on the circuit board to be connected to the pins 12.  At the center of the circuit substrate 13, a connecting base 15 is provided.  The connecting base 15 is connected to the contacts 14 and provides a terminal 150 for connection with an imaging system (for example, a PET imaging system or a SPECT imaging system). 
However, the conventional scintillation crystal detecting module 1 has some problems such as:
1.  Since there is no protection between the pins 12 and the circuit substrate 13, the pins 12 are possibly damaged and exposed to cause high-frequency noise.  Moreover, it is possible that the user touches the pins 12 to cause hazards. 
2.  The pins 12 of the PMT module 11 are welded to the circuit substrate 13.  Therefore, it is inconvenient to assemble and replace a mal-functional PMT when any PMT in the PMT module 11 is damaged or aged. 
3.  The circuit substrate 13 can be further connected to another circuit substrate 16 using the connecting base 15 with a pin header.  Therefore, the connection between the circuit substrate 13 and the circuit substrate 16 is unfixed and requires additional screws for fixation, which increases the substrate area. 
4.  In order to prevent the PMT pins 12 from contacting the circuitry on the circuit substrate 13, the PMT pins 12 have to be concave inwards, which results in unfixed connection between the PMT pins 12 and the circuit substrate 13. 
Therefore, there is need in providing a PMT interface device and a scintillation crystal detecting module using the PMT interface device to overcome the aforementioned problems.