Medical Imaging Techniques in nuclear medicine for Biomedical Engineer

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Nuclear Medicine:

www.biomedikal.in Nuclear Medicine Please note this PPT is a submission of a user of Biomedikal.in

Single Photon Emission Computed Tomography:

www.biomedikal.in Single Photon Emission Computed Tomography Tomography: Imaging of transverse cross section of 3D object Computed: Images are reconstructed from projections Emission: Source of imaged quantity (radioactivity) is within object Single-Photon: Radionuclides that emit isotropically a single gamma ray photon when decaying.

Brief History:

www.biomedikal.in Brief History Investigation started in 1920 1950 – rectilinear scanner that scanned the radionuclide concentration in the body

Brief History:

www.biomedikal.in Brief History This device produced planar images by mechanically scanning a detector in a raster-like pattern over the area of interest. By today's standards, this technique required very long imaging times because of the sequential nature of the scanning.

Contd.:

www.biomedikal.in Contd. In the late 1950's , Anger replaced the film and screen with a single NaI crystal and PMT array. This formed the basis for the "Anger Camera" which is now the standard clinical nuclear imaging device. Modern Anger Cameras use a lead collimator perforated with many parallel, converging or diverging holes instead of the original pin-hole configuration.

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www.biomedikal.in 1970s radon transform was used for reconstructing images after its application to CT 1970 – First PET scanner 1992 – Human studies in PET

First PET scanner:

www.biomedikal.in First PET scanner

Radionuclides:

www.biomedikal.in Radionuclides In nuclear medicine the function or metabolism is measured. A tracer molecule is administered to the patient by intravenous injection Tracer is a particular molecule carrying an unstable isotope- radionuclide Unstable isotope produce gamma rays, which allow us to measure the concentration of the tracer molecule in the body as a function of position and time.

Beta decay:

www.biomedikal.in Beta decay When a nucleus is unstable, there are three ways it can decay. It can emit: An alpha particle A beta particle A gamma ray or

Beta decay:

www.biomedikal.in Beta decay Beta-minus decay (β -) Beta-plus decay (β +)

Electron capture:

www.biomedikal.in Electron capture p + + e - n If a daughter product is in the metastable state, it decays toward a low energy state by emitting a gamma photon. 123 I – half life 13 hrs.

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www.biomedikal.in The most important single photon tracer is 99m Tc .This is a metastable daughter product of 99 MO Half life of 99m Tc is 6 hrs. Energy 140keV. Positron emitter used in imaging is 18 F

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www.biomedikal.in Interaction of  -Rays with matter Energy loss effects for  and X-ray radiation are characterized by Photo electric effect Compton scattering Pair production Photons interact with matter by photo absorption which causes excitation or ionization of atoms. Only photons of well defined energies corresponding to the excitation energies in the atoms are absorbed for excitation processes.

Photoelectric effect:

www.biomedikal.in Photoelectric effect

Compton scattering:

www.biomedikal.in Compton scattering

Pair Production:

www.biomedikal.in Pair Production Schematic representation of a positron-electron annihilation. When a positron comes in the neighborhood of an electron, both particles are converted into a pair of photons of 511 keV each and traveling in opposite directions .

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www.biomedikal.in The range of alpha and beta particles in tissue is too small to allow in vivo imaging using external detectors. X and gamma rays above 100 KeV can be captured by external detectors as photon interactions in tissue are dominated by Compton scattering.

The number of detected photons:

www.biomedikal.in The number of detected photons the number of detected photons is generally much smaller than in X-ray Consequently, noise plays a more important role and the imaging process is often considered to be stochastic. The exact moment at which an atom decays cannot be predicted. All that is known is its decay probability per time unit, which is an isotope dependent constant α .

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www.biomedikal.in the decay per time unit is dN(t)/d(t) = - α N(t) N(t) – number of radioactive isotope at ‘t’ Solving this eqn we get N(t)=N(t 0 )e - α (t-t 0 ) = N(t 0 )e -(t-t 0 )/ τ Τ =1/ α time constant of the exponential decay N(t) is the expected value. Larger the value better the estimate.

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www.biomedikal.in replacing t by the half-life T 1 / 2 and t 0 by 0 yields N(T 1 / 2 ) = N( 0 ) e− T 1 / 2 /τ = ½[ N( 0 )] − T 1 / 2 /τ = ln(1/2) = −ln 2 T 1 / 2 = τ ln 2 = 0.69 τ .

Dose :

www.biomedikal.in Dose the presence of radioactivity in the body depends not only on the radioactive decay but also on biological excretion. Assuming a biological half-life T B , the effective half-life T E can be calculated as 1/ T E = 1/ T B + 1/ T 1 / 2 Currently the preferred unit of radioactivity is the becquerel (Bq). The curie (Ci) is the older unit. One Bq means one expected event per second and 1 mCi = 37 MBq. Typical doses in imaging are on the order of 102 MBq

Image acquisition:

www.biomedikal.in Image acquisition Image is acquired using Gamma Camera

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The Gamma Camera What is about ? :

www.biomedikal.in The Gamma Camera What is about ? The modern gamma camera consists of: - multihole collimator - large area (e.g 5 cm ) NaI ( Tl ) (Sodium Iodide - Thallium activated) scintillation crystal - light guide for optical coupling array (commonly hexagonal) of photo-multiplier tubes - lead shield to minimize background radiation

A crucial component of the modern gamma camera is the collimator. The collimator selects the direction of incident photons. For instance a parallel hole collimator selects photons incident OS the normal. Other types of collimators include pinhole collimator often used in the imaging of small superficial organs and structures (e.g thyroid,skeletal joints) as it provides image magnification. . :

www.biomedikal.in A crucial component of the modern gamma camera is the collimator . The collimator selects the direction of incident photons. For instance a parallel hole collimator selects photons incident OS the normal. Other types of collimators include pinhole collimator often used in the imaging of small superficial organs and structures (e.g thyroid,skeletal joints) as it provides image magnification . . The action of a parallel hole collimator

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www.biomedikal.in Detail of the pin-hole collimator

Collimator:

www.biomedikal.in Collimator

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www.biomedikal.in In SPECT collimation is done with mechanical collimators, while in PET photon pairs are detected by electronic coincidence circuits connecting pairs of detectors SPECT PET

Detector:

www.biomedikal.in Detector Scintillation detector The basic function of a scintillation detector, is to transform the energy of an incoming particle to a measurable electronic signal. A scintillation detector consists of two components: Scintillator Photomultiplier or PIN diode The scintillator converts a fraction of the energy of the incoming particle to light The photomultiplier converts the light to a current signal, which can be manipulated further in an electronics system.

Scintillation:

www.biomedikal.in Scintillation

Photo Multiplier Tube :

www.biomedikal.in Photo Multiplier Tube

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Gamma Camera:

www.biomedikal.in Gamma Camera

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www.biomedikal.in DST-XLi & DSXi Digital Long Axis Nuclear Medicine Systems http://www.smvnet.com/

What object is reconstructed?:

www.biomedikal.in What object is reconstructed? In emission imaging the aim is to image the radio tracer distribution At time t=0 the patient is injected with some radio tracer which contains large number of metastable atoms of some radionuclide such as Tc-99m, I-123, and In-111. The radioisotope decays resulting in the emission of gamma rays. These gamma rays give us a picture of what's happening inside the patient's body.

Radioisotopes Used in Nuclear Medicine :

www.biomedikal.in Radioisotopes Used in Nuclear Medicine For imaging Technetium is used extensively, as it has a short physical half life of 6 hours. Technetium is a gamma emitter. This is important as the rays need to penetrate the body so the camera can detect them. Because it has such a short half life, it cannot be stored for very long because it will have decayed. It is generated by a molybdenum source (parent host) which has a much greater half life and the Tc extracted on the day it is required. The molybdenum is obtained from a nuclear reactor and imported. For treatment of therapy, beta emitters are often used because they are absorbed locally.

How Gamma rays are collected?:

www.biomedikal.in How Gamma rays are collected? The gamma camera can be used in SPECT imaging to acquire 3-dimensional images. The Gamma camera collects gamma rays that are emitted from within the patient, enabling us to reconstruct a picture of where the gamma rays originated. From this, we can determine how a particular organ or system is functioning.

Data Projection:

www.biomedikal.in Data Projection As a SPECT camera rotates around a patient, it creates a series of planar images called projections. At each stop, only photons moving perpendicular to the camera face pass through the collimator. Many of these photons originate from various depths in the patient, the result is an overlapping of all tracer emitting organs along the specific path superposition of all anatomical structures from three dimensions into two dimensions.

Projection Image:

www.biomedikal.in Projection Image Projections Sinogram

Reconstruction:

www.biomedikal.in Reconstruction Filtered Back Projection Fourier transform of the data is taken. Filter the data which is in frequency domain. Take inverse transform Backproject.

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3D reconstruction Image:

www.biomedikal.in 3D reconstruction Image The aim of the reconstruction process is to retrieve the radiotracer spacial distribution from the projection data . This surface rendered image was reconstructed using a fully 3D OSEM algorithm.

SPECT Applications:

www.biomedikal.in SPECT Applications Heart Imaging Brain: Transverse SPECT Image

SPECT Applications :

www.biomedikal.in SPECT Applications Bone Scan:Bone scans are typically performed in order to assess bone growth and to look for bone tumuors. The tumuors are the dark areas seen in the picture

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www.biomedikal.in Lung Cancer Lymphoma Colon Cancer WHAT CAN WE VISUALIZE ?

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www.biomedikal.in Typical dynamic image of a heart

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Noise reduction:

www.biomedikal.in Noise reduction To keep the amount of radioactivity low, the dose is kept low. So data obtained is also less. The data itself is of poisson nature. If the projection date is filtered, the data is lost along with the noise and the poisson nature of the data is also lost The reconstructed image can be smoothened Interrupt iterations before convergence Define some prior probability function that encourages smooth solutions

Nuclear Imaging Scans:

www.biomedikal.in Nuclear Imaging Scans Brain Scans These investigate blood circulation and diseases of the brain such as infection, stroke or tumor. Technetium is injected into the blood so the image is that of blood patterns. Thyroid Uptakes and Scans These are used to diagnose disorders of the thyroid gland. Iodine 131 is given orally , usually as sodium iodide solution. It is absorbed into the blood through the digestive system and collected in the thyroid. Lung Scans These are used to detect blood clots in the lungs. Albumen, which is part of human plasma, can be coagulated, suspended in saline and tagged with technetium.

Nuclear Imaging Scans:

www.biomedikal.in Nuclear Imaging Scans Cardiac Scans These are used to study blood flow to the heart and can indicate conditions that could lead to a heart attack. Imaging of the heart can be synchronised with the patient's ECG allowing assessment of wall motion and cardiac function. Bone Scans These are used to detect areas of bone growth, fractures, tumors, infection of the bone etc. A complex phosphate molecule is labeled with technetium. If cancer has produced secondary deposits in the bone, these show up as increased uptake or hot spots .

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www.biomedikal.in Positron Emission Tomography P E T

Positron Emission Tomography:

www.biomedikal.in Positron Emission Tomography A short-lived positron emitting radioactive tracer isotope incorporated into a metabolically active molecule, is injected into the living subject. The positron travel a few mm and annihilates with an electron, producing a pair of annihilation photons (similar to gamma rays) moving in opposite directions. These are detected by the detectors in the scanning device. Photons which do not arrive in pairs are ignored

Annihilation:

www.biomedikal.in Annihilation

Typical PET facility:

www.biomedikal.in Typical PET facility

PET scanner:

www.biomedikal.in PET scanner

Detectors:

www.biomedikal.in Detectors When septa are in the field of view, the camera can be regarded as a series of separate 2D systems. Coincidences along oblique projection lines between neighboring rings can be treated as parallel projection lines from an intermediate plane. This doubles the axial sampling, i.e. 15 planes can be reconstructed from 8 rings.

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Randoms :

www.biomedikal.in Randoms

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www.biomedikal.in What organs appear dark in this scan? heart kidney brain bladder

Myocardial perfusion SPECT scan. Rows 1, 3 and 5 show the myocardial perfusionduring a typical stress test. Rows 2, 4 and 6 show the rest images acquired three hours later.The first two rows are horizontal long axis slices, the middle two rows are vertical long axis slices and the bottom two rows are short axis slices. This study shows a typical example of transienthypoperfusion of the anterior wall. On the stress images there is a clear perfusion defect on the anterior wall (horizontal axis slice 9, vertical long axis 16 to 18, short axis slice13 to 18). Theperfusion normalizes on the corresponding rest images:

www.biomedikal.in Myocardial perfusion SPECT scan. Rows 1, 3 and 5 show the myocardial perfusionduring a typical stress test. Rows 2, 4 and 6 show the rest images acquired three hours later.The first two rows are horizontal long axis slices, the middle two rows are vertical long axis slices and the bottom two rows are short axis slices. This study shows a typical example of transienthypoperfusion of the anterior wall. On the stress images there is a clear perfusion defect on the anterior wall (horizontal axis slice 9, vertical long axis 16 to 18, short axis slice13 to 18). Theperfusion normalizes on the corresponding rest images

Lung perfusion (Q) and ventilation (V) scan.:

www.biomedikal.in Lung perfusion (Q) and ventilation (V) scan.

The PET scan shows the uptake of the glucose analogue fluoro-deoxy-glucose in thebody of the patient. Dark areas correspond to a high FDG uptake. This patient suffers from Hodgkindisease with involvement of multiple lymph nodes in the neck, axilla, retroperitoneal, groins, etc. Pathological lymph nodes are characterized by a high FDG uptake:

www.biomedikal.in The PET scan shows the uptake of the glucose analogue fluoro-deoxy-glucose in thebody of the patient. Dark areas correspond to a high FDG uptake. This patient suffers from Hodgkindisease with involvement of multiple lymph nodes in the neck, axilla, retroperitoneal, groins, etc. Pathological lymph nodes are characterized by a high FDG uptake

Image quality:

www.biomedikal.in Image quality Contrast- det by the characteristics of the tracer and the amount of scatter Spatial resolution - positron range- distance travelled depends on the isotope - deviation from 180 deg. - detector resolution - collimator resolution Noise – compton scatter

Image artifacts:

www.biomedikal.in Image artifacts Attenuation Compton scatter Poisson noise Patient motion

Applications :

www.biomedikal.in Applications Clinical oncology :Medical imaging of tumors and the search for metastases. PET neuro imaging: The flow of blood to different parts of the brain is measured using tracer oxygen( 15 O) Clinical cardiology:FDG-PET can identify hibernating myocardium" Neuropsychology : To examine links between specific psychological processes or disorders and brain activity. Pharmacology:In pre-clinical trials,it is possible to radio-label a new drug and inject it into animals and scan them to study the effect

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