**2.4 Future perspectives**

*Medical Isotopes*

*2.2.1 Current status*

of treatment after radionuclide therapies.

delayed-event subtraction [16].

losses are corrected regularly.

*2.2.2 Advances*

**2.3 PET/MRI**

correction.

MRI data.

tion of the SUV [17].

However, PET has application in planning of treatment, dosimetry, and assessment

Similar to SPECT quantitative PET is also used for correction techniques. Correction of attenuation for PET can be done through determination of the sonogram associated with attenuation correction, which works on the basis of coregistered CT data. Scatter correction is often done with single-scatter simulation method in clinical practice [15]. Correction for random counts is often done using

The difference in time between annihilation photons gives information regarding location of the annihilation and also about the line of response. Now time-offlight information in the reconstruction at the time of back projection step enhances image quality. The availability of time-of-flight estimation has opened the opportu-

As intrinsic resolution of PET detectors are not freely available, so shape of the point-spread function is used to improve the quality of images by incorporating it

When there are high count rate radiation detection systems does not work properly due to dead-time effect caused by pulse pile-up. Because of these Dead-time

There are better quality of PET images with enhanced resolutions and sensitivity due to regular improvement in the instrument which provides precise determina-

The advantages of PET/MRI over PET/CT are higher soft-tissue contrast that is essential for planning of treatment, dosimetry, and assessment post radionuclide therapies. Additionally, for accurate dosimetry it is beneficial as it provides the simultaneous coregisteration of MR images. Also, MRI can be employed for determining the tolerable dose with least organ damaging activity of radionuclide. Along with it anatomic and molecular images acquisition provides better motion

Integrating of PET and MRI modalities is challenging as there will be interference between both the modalities. For instance, photomultiplier tubes that are present in PET detectors malfunction in magnetic fields exerted by MRI. In addition to this, PET module affects the radiofrequency signal associated with MRI [18]. Due to this, the first generation of PET/MRI systems modalities were separated. Integration of PET detectors and MR scanner has been done to obtain PET and MR images simultaneously. Detector systems is avalanche photodiodes types or SiPMs types which are not sensitive to magnetic field. The simultaneous measurement provides better 4-dimensional acquisitions because of spatial agreement of PET and

Disadvantages associated with PET/MRI are high costs and the ferromagnetic metallic implants which are used is contradictory to MRI. In addition to this it's challenging to correct attenuation of PET/MRI. For dosimetry it is essential to have accurate attenuation correction. As CT images are electron-density images and MR images are proton density image, CT image are better suited for attenuation

nities for low positron abundance imaging isotopes like 90Y.

during reconstruction method. This is called as resolution recovery.

**76**
