**2.1 SPECT and scintigraphy**

This type of decay of radionuclides determines about the modality for imaging. Planar scintigraphy or SPECT is used for imaging of 177Lu, 90Y, and 131I-which are used for radionuclide therapy. These emit γ-photons (or bremsstrahlung photons), which can be imaged by a γ-camera.

#### *2.1.1 Current status*

SPECT/CT systems which are used nowadays are used for both planar and tomographic imaging. Planar imaging is for acquiring whole-body images in when there is limitation of time. SPECT is meant for acquiring 3-dimensional data of structures which would otherwise overlap on each another on planar images.

Quantitative analysis of SPECT images is determined by converting the acquired counts in terms of distribution of absorbed dose (in Gy), which is beneficial for planning and dosimetry of therapy involving radionuclide. In clinical practice scatter correction is also implemented and is generally performed employing the tripleenergy window method [13]. Quality of image can be enhanced by using resolution recovery. It is performed by characterizing the shape of the point-spread function accurately, that depends its distance from the camera and there is rotational variation due to the hexagonal pattern of the collimator septa. Reconstruction algorithm can be incorporated with point-spread function model subsequently [14].

Effects like scatter, blurring and attenuation which degrades image can be corrected to some extent, Although SPECT images can be degraded by partial-volume effects and quantification errors.

### **2.2 PET**

18F-FDG PET is used for many PET studies that are in the field of clinical practice and is employed for staging and follow-up post radionuclide therapy. However, PET has application in planning of treatment, dosimetry, and assessment of treatment after radionuclide therapies.

## *2.2.1 Current status*

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 delayed-event subtraction [16].

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 opportunities for low positron abundance imaging isotopes like 90Y.

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 during reconstruction method. This is called as resolution recovery.

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 losses are corrected regularly.

### *2.2.2 Advances*

There are better quality of PET images with enhanced resolutions and sensitivity due to regular improvement in the instrument which provides precise determination of the SUV [17].

## **2.3 PET/MRI**

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 correction.

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 MRI data.

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

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*Theranostics: New Era in Nuclear Medicine and Radiopharmaceuticals*

correction. But MR images can be used for attenuation correction by using techniques such as segmentation-based or template- or atlas-based which derives electron density information from MR images [19]. Alternatively, estimation of the attenuation maps can be done by employing algorithms which uses the time-of-

Till today there are no real-time hybrid imaging modalities that can merge nuclear and anatomic for interventional purposes. Fluoroscopic imaging in combination with real-time nuclear imaging gives physicians with valuable information during procedures like as 90Y liver radio embolization by image distribution of the radionuclide in association with the anatomy and the interventional instruments that enhances therapeutic efficiency. Image of same field can be seen by arranging

**3. Different types and applications of radioisotopes for imaging and** 

1. Calcium-47 Important aid to biomedical researchers studying cellular functions and bone formation in mammals 2. Caesuim-137 Used to treat cancerous tumors and to measure correct dosages of radioactive pharmaceuticals

5. Cobalt-60 Used to sterilize surgical instruments and used in cancer treatment, food irradiation and radiography 6. Copper-67 When injected to monoclonal antibodies into a cancer patient, helps the antibodies bind to and destroy the tumor

8. Iodine-123 Widely used to diagnose thyroid disorders and other metabolic disorders including brain functions 9. Iodine-125 Major diagnostic tool used in clinical test and to diagnose thyroid

10. Iodine-129 Used to check some radioactivity counters in in-vitro diagnostic testing

14. Technetium-99m Most widely used radioactive pharmaceutical for diagnostic studies in

liver, spleen and kidney imaging 15. Uranium-234 Used in dental fixtures like crowns and dentures to provide a natural color and brightness

16. Xenon-133 Used in nuclear medicine for lung ventilation and blood flow studies

disorders. Also used in biomedical research

Used in molecular biology and genetics research

nuclear medicine. Different chemical forms are used for brain, bone,

X-ray tube, an X-ray detector, and a γ-camera in a single line [21].

3. Chromium-51 Used in research in red blood cells survival studies 4. Cobalt-57 Used as a tracer to diagnose pernicious anemia

laboratories 11. Iodine-131 Used to treat thyroid disorders (Graves' disease)

12. Iridium-192 In brachytherapy/tumor irradiation

13. Phosphorous-32 and Phosphorous-33

7. Gallium-67 Used in medical diagnosis

*DOI: http://dx.doi.org/10.5772/intechopen.91868*

flight emission or transmission data [20].

*2.4.1 Simultaneous X-ray and nuclear imaging*

**S. no Radioisotopes Uses**

**2.4 Future perspectives**

**therapy**

correction. But MR images can be used for attenuation correction by using techniques such as segmentation-based or template- or atlas-based which derives electron density information from MR images [19]. Alternatively, estimation of the attenuation maps can be done by employing algorithms which uses the time-offlight emission or transmission data [20].
