Nuclear Medicine Techniques - Essay Example

As far as positives which could be d down for nuclear energy and ionizing radiation, medical imaging can be termed as usefulness which is ed to minimum negative criticism or open disapproval from general civic forums. People advocating in favor of nuclear power point out or symbolize it as the positive aspect of usage of nuclear technology. Having firsthand experience in medical imaging, it became clear that lot of misconception and misunderstanding exists in general public about medical imaging techniques and different types of this medical technique. Given article is summary of main article which is on “Nuclear Imaging in realm of medical imaging” (Nuclear Instruments and Methods in Physics Research A 509 (2003) 213–228), provides a basic overview to the subject. 1. The spectrum of medical imaging techniques Medical Imaging Technique can be categorized in a number of ways. Segmentation could be based on number of criteria’s. Among all these, most common could be appearance for instance tomographic/ non-tomographic images and would assemble CT and MRI. This could be due to presentation of image. On other hand many would also segment them according to fundamental physics. And for given article this scheme would be used. Basis being used are cause and environment of radiation source that would serve as source of transporting information from patient to sensor or detector. 1.1 External sources When source of radiation is exterior, making of body tempers the info with addition of impact of radiations. External point of X-Ray is utilized in X-Ray radiography or CT. When rays pass through the body, body parts tend to absorb certain percentage or amount of rays being generated. Those which are not being absorbed, tend to move in straight line as per law between passage existing between source and indicator. This automatically leads to creation of shadow copy of structures of body. Pulse sound waves from external source are utilized in ultrasound. Direction of pulse is known along with its timings. Reflection between sound waves and connection between different tissues will also reflect. Reconstruction of images can be done by measurement of outgoing and incoming time span. External source of light is used to illuminate organs present inside the body through glass fiber. All this is practiced in endoscopy. Reflected light is observed also organs through ocular or minute cameras. 1.2 Internal sources Due to metabolic activities, body constantly generates heat. All this is generated internally. For capturing images, infrared radiation optic is required which can be done by type of camera known as thermo graphic camera. While conducting Magnetic Resonance Imaging is mostly done by waves which released by Hydrogen nuclei inside the body. Quantity of hydrogen nuclei is sufficient in the body. This could be found in form of water molecules, naturally radio waves aren’t produced or emitted by nuclei. For producing radio waves, they need to be inserted within a defined magnetic field. By just bringing those between magnetic field won’t fulfill the core objective. Activating is the next step which is achieved by selected radio waves pulse at precise and exact frequencies. Response from side of nuclei is in form of radio waves being produced on known frequencies. In every MRI case, nuclei are present internally but just need to be activated to release radiation when activated by similar or known frequencies. Gama rays are the source of carrying nuclear images. Gamma rays are produced by internal sources which could be tracked to internal radioactive traces. Everybody is naturally tracer of radioactivity. Due to environment of radioactive deterioration of radioactive body components; hence it is very problematic to be used. If medical point of view is considered given information cannot be of much use. Hence administration of artificial radioactive traces is very important. External detection and distribution of space/ time reflects clinical details. Hence choosing the appropriate way is very important and holds the key towards radioactive decay. Ultrasound, MRI, Radiography and nuclear imaging hold upmost importance and hence details will be discussed in next sections. 2 Ultrasound Japanese research on sonar devices and systems revealed medical use of ultrasound in new way which previously hadn’t been known to medical world. It was early fifties when the world got its first scanner which was of US based and by seventies the technology was widespread. Reflection of sound waves which have been emitted by external source is information ultrasound. Usually piezoelectric crystal resonance ranging between 1 and 10 MHz. Clinical information is degraded by Refraction, captivation and scattering. Frequency of the wave, speed of sound and its density are basic and core physical parameters which hold upmost importance. Muscle fat interface is about 1% in reflection whereas it becomes 99.9% in skin/ air interface. For decreasing or reducing this non desirable reflection gel has to be used. Among list of other artefacts, the two distinctive artefacts in ultrasound. The first artefact happens because of clear nature, which sound wave has. Sound wave can be termed as having a comprehensible and coherent pulse which leads to restriction with its reflected, refracted and components being transmitted which than gives increase or rise to marking. Quiet comparable to the speckling or marking which is usually observed in laser light. Physics of reflection is the reason for second artefact: linking between tissues that equivalent or parallel to the wave spread. This will not be reflecting the wave and hence in ultrasound can neither be viewed or seen. Applications like obstetrics or cardiology, image reflects high clinical information. to its advantage, the technique is termed by many in industry as safe for health & environment and above all safe. Current trend in the market and research being done is helpful in elimination of artefacts, improvement of contrast in image and data presentation. As we progress, more research and effort now is being directed towards 3D and 3D means of presenting and acquisition of data. FIGURE 1: 3D ultrasound (© 2000 General Electric,www.gemedicalsystems.com) 3 Magnetic Resonance Imaging Gorter, Rabi, Purcell and Bloch can be regarded for MRI technique research which stemmed from research being carried out in field of physics. This also led to substantial developmental work in field of nuclear magnetic concepts and techniques. All this work was done before and after Second World War. In seventies as previously mentioned; Lauterbur, Damadian and Mansfield could be credited for inroads made in medical applications and imaging. Below given points reflect basic level information pertaining to MRI imaging the magnetization of hydrogen nuclei (their magnetic moment is called spin), is denoted by N(H) the energy transfer between the spins and tissue, is termed by a time constant T1 The energy rearrangement among spins with a time constant T2 flow Not having an external magnetic field or we can say absence of external magnetic field, random directions will be pointed out by magnetic moment of hydrogen nuclei. Hydrogen nuclei in large area of external magnetic field will specifically align and set their spin (1/2 or –1/2 due to quantum mechanical laws) along the magnetic field. Additional spins will bring into line their spin in the path of the field (spin-up) than in the conflicting direction (spin-down) because the energy in spin-up direction is inferior to in spin-down direction. Because of this magnetization will increase whereas decrease will reflect in the global energy of the spin system. Transferring of energy from on system to other is implied as magnetization. Exponential relaxation law characterizes this transfer of energy with T1 being kept constant and being referred as spin lattice relaxation time. Normally in field strength in MRI (0.5 to 1.5 T) T1 is normally the order of 0.5 to 2 s, liable on the tissue type. As for the spins can also interrelate among each other: energy being released by flipping of one spin from down to up, another spin can engross the energy being released and flip from up to down. This spin-spin redistribution of energy and internal to the spin system is characterized by a reduction time, known as spin-spin relaxation time and noted as T2. Typical values for T 2 are 10 – 100 ms, again this depends on the type of tissue. For a characteristic MRI field forte of 1.5 T the difference in energy spin up/down relates to radio waves with a booming frequency of 60 MHz. Radio waves being sent on booming frequency result in spins which spin-up and tend to absorb the energy of the wave being produced and flip to spin-down, thereby resulting in increment in the global energy of the spin system. Hence energy being produced by spin system comes into a non-equilibrium state with respect to temperature of the tissue. This automatically results in violation of normal Boltzmann distribution in equilibrium. Extra energy is automatically being re-emitted in form of radio waves at resonant frequency. Bringing in variations in local magnetic fields, tuning the regularity, checking polarization and radio wave pulses duration to excite and activate the spin system, and by moderating the postponement after which the re-emitted waves (the signal) are measured, reconstruction of MRI images can be has been made possible. Image contrast than starts depending on four factors i.e. N(H), T1, T2 and flow (any undertaking of nuclei during the imaging sequences). In large number of pathologies, clinical value of Medical Imaging Techniques can be recognized. Articulations such as the knee and base of skull can serve as the best example in defined case. FIGURE 2: MRI image (1983) with Fourier reconstruction artefact (bottom folded to top). Scope of current research expands when process of gathering information starts. Best example for this is visualizing of vascular structure through magnetic resonance angiography without contrast media injection, for having image through visualizing of explicit brain function and imaging of diffusion. While current efforts in this area are making headways, minimizing the time for acquisition from ten minute duration to couple of minutes or seconds has been achieved. Many have termed MRI technique as safe both for patients undergoing it and for staff operating. Precautionary measures include removal of any sort of metallic item or item that is metallic in nature to avoid high magnetic field strength. In cases such as where patients have metallic items plucked inside body, such patients should avoid procedure of imaging. Similar also applies for patients having pacemakers. Generation of heat because of induced currents has the most potential hazards. 4 Radiography Using external X-Ray source for imaging is known as Radiography. Goodspeed had accidently discovered X-Ray in 1890 at University of Pennsylvania. In 1895 after discovery of Rontgen, radiography came into existence. Illustration in figure 3, medical application was discovered weeks after. In radiography, detection is done by film or any other appropriate source which should be detectors X-Ray transmission being originated in source point i.e. X-Ray Tube. X-Ray goes under photon-matter interactions when it passes through the path which leads from source to detector. Photoelectric effect, Compton scattering, coherent scattering and pair formation are counted among four forms of classical interactions. But only Photoelectric and Compton are applicable because of the range of their energy. In radiography, photoelectric impact or effect is considered by many specialist as the key photon matter interaction which has importance in field of radiography. Shadow image is created when body structure absorbs and detection is allowed by detector of photons. FIGURE 3: First Belgian military radiograph, April 1896. By far the most used detector today in field is still X-Ray film. But it is also widely known that it is not as per standard sensitive to X-Rays as per its features. Phosphor screen hence is used to convert or transmute x-ray in visible light when film is put against it. Hence it is stated by field experts and is known after in-depth research for sensitivity and allowing relative or proportionate decrease in exposure of radiation to the patient, sensitivity should be drastically decreased. As research is done in given field, probable replacement of film has been semiconductor as per view of many researchers and has been proved also. FIGURE 4: Coronarography of patient with LAD Stenosis Field of clinical applicability of radiography is vast, but from projection point of view it is limited in scope. Along the path of X-Ray which is integrated, information which is present changes in form of absorption and hence path is lost within the image. Key reason behind development of computed X-Ray tomography was losing of image in the path as stated in detail above. Major risk to health exists when characters of X-Ray are ionized. Radiographers had to pay heavily in early stages of development as leukemia became common illness being caused in victims having high exposure of radiation. 5 Computed tomography Projection of a shadow results in loss of information which happens in classical radiography hence putting limits on clinical value. However where there are certain limits, certain methods have been researched and developed in order to minimize the impact being caused by this loss. Few of these well researched techniques are movement of X-Ray source in order to blurring of structures being out of focus and position of film in contradictory direction. On emergence of dominant data sets automatically led to handling through new set of approaches and in year 1972, renowned researcher and scientist bought into light computed tomography. Revolutionary work in this regard was done by Oldendorf and Cormack. FIGURE 5: CT image with beam hardening artefacts For having substantial amount of data set for reconstruction in mathematical form of slices in vitality, projection needs to be done from different angles. Square form of reconstruction is allowed from two different angles. This form of reconstruction hence cannot be termed as satisfactory. It was hence researched that number of projections plays key role in quality of reconstructions and hence resolution also is enhanced. Between resolution and noise, optimization needs to be achieved. While today, CT scanners, fixed solid states in thousands of quantity whereas rotation of X-Ray tube is on very high speed around a patient in circle. Than by analytical algorithm approach, reconstruction of tomographic images is done. From projection radiography point of view, negative burden of radiations on patient specifically children is a serious drawback. Major minimization in dose being given to patient for improvement in quality of image and hence reduction in burden is being made possible but only by having shifting from integrating to detectors. 6 Nuclear Imaging Knowledge about usage of radioactive tracers that are introduced in system of living being with major aim of conducting research on metabolism dates from 1923 when de Hevesy and Paneth researched different forms of transport of radioactive lead inside plants. In year 1935, de Hevesy and Chiewitz became the first ever scientist who successfully were able to apply the means for studying delivery of a radiotracer (P-32) in rats. The key progress in nuclear imaging (also called scintigraphic imaging) took place with the discovery or development of the gamma camera by Anger in year 1956. In same passage of time, development of positron imaging also took place. But today imaging modalities are now proved standards in the field of nuclear medicine. Basis of nuclear imaging is formed by principle of tracer, which is as following, substance which is biologically active substance in radioactive is selected in such a manner that it’s spatial and chronological dissemination in body of any object clearly reflects towards a specific function or metabolic activity. Administration of substances in the body of patient is only done in order to clearly study distribution without creating any form of disturbance in functionality of body. Radiotracers mover towards deadness due to emission of gamma rays or gamma rays annihilation. Detection and mapping of gamma ray function is done only by tracing of radioactive tracers being inferred. Radio nuclides are TC-99M which are mostly utilized in imaging of single photon and photon imaging which is F-18. MO-99 decay daughter could be termed as TC-99M which is a product of fission itself of U. 6 hours is the half- life of TC-99 which according to metabolic studies is optimal but on other side is considered very short for shell storage. MO-99 life is 65 hours which is termed as half-life. This whole process allows milking of TC-99m and storage of MO-99 generator. Hence when decaying takes place, TC-99m converts to TC-99 by gamma rays emission with 140 Kev energy output. Scintillator detectors consider it has optimal energy level. However 211100 years I half – life of TC-99 and hence burden of this is considered as negligible for patients. F-18 is cyclotron produced with half-life of 110 minutes. For stabilization, 0-18 emission of positron is done and hence it decays. Loss of positron leads to loss of kinetic energy through interaction of coulomb with nuclei surrounding it. When at rest, which happens in tissues after having an average range of less than 1 mm, the likelihood of an impact with an electron significantly surges and hence becomes one. Throughout the collision matter-antimatter obliteration follows in which remaining electron mass and the positron is converted into two gamma rays having 511 Kev. Then the two gamma rays instigate at precisely the same time (they are “coincident”) and consent the point of impact in almost contradictory directions. 6.1 Single photon imaging Foundation is not point of source for rays, however object is the point of circulation, for the purpose of formation of image optics has to be adapted. Up till now there has been no recognized source which could detour gamma rays as is done by lenses with visible form of light. Hence reliance on careful absorption of rays which is heavily based on geometrical standards. Camera obscure principle is method being historically followed for specific applications: placement of lead cone above the detector and at top pin hole opening is done on the cone, which should be vertical and abrupt to the detector surface center. Only few rays are allowed passing through the pin hole from detector of the image. Image is thus capsized and expanded in line with the object, dependent on space between pin hole, object and detector. Another method is established on hole collimator i.e. tungsten sheet or profuse lead in which number of holes parallel are pierced. Usual hole size are very minute and measured in cm length wise and mm in diameter. Residual limitation to having ultimate resolution of camera is collimator structure. Also to be noted that it has a very low geometric efficiency example of this could be 10-3. Rays which are hitting the detector through the holes are contributing to the image, which automatically links to mapping it one to one for radioactive distribution. FIGURE 6: Bone scan, depicting bone metabolism in young patient. NaI mono-crystal is utilized as the scintillation sensor in a large anger gamma camera. Array of around 100 photomultipliers are used for detecting scintillations. With resolution of few millimeters, detected photomultipliers of scintillation photons are allowed to be placed for detection on crystal. With a precision of between 10% to 15% total number of photons being detected are allowed to determine the energy resolution. Standardized nuclear medicine exercise, acquiring of images is performed during time duration of seconds to minutes. Resolution of spatial of given image ranges between 0.5 to 1.5 cm whereas contrast of resolution is low compared to previous one. Reason behind this could be due to images are more of projections. Photons per pixel may become very minute and small, it can be said that it is bought in use for studying dynamics and functionality of images being formed in large areas. In large number of pixels, region of interest compensates for acquisition in short time. Example for this could be utilization of nuclear imaging for purpose of study of functioning of heart; and in this 8 to 16 images in series as temporal Fourier filtering, hence key clinical information could be accessed and recovered. Rotation of gamma camera across in different directions of the patient and obtaining getting big set of projections makes sufficient amount of data set for reconstruction of tomographic emission images. It was in 1964 that Kihl had developed emission tomography. In given tomographic imaging, image is similar to planar imaging in spatial resolution. In lesion contrast detectability is improved and hence image reflects of better quality. FIGURE 7: Tomographic image of myocardial perfusion defect at exercise 6.2 Positron Emission Tomography (PET) During Positron Emission Tomography, decay of administered radio nuclide which happens because of emission of positron which leads to thrashed after collision with electron. Pair of 511 Kev gamma rays is produced concurrently leaving in opposite direction of annihilation site. Brownell in 1951 is credited for positron imaging. Ring of current PET cameras have positive aspect of having characteristics of annihilation. Patient is surrounded by scintillation detectors. It is considered that annihilation had occurred on some imaginary line when two events are being detected on same time in two detectors opposed. By obtaining a large number of lines, e.g. 109, tomographic reform methods can be utilized to rebuild images of the tracer dissemination. Type of detectors being utilized are scintillating detectors. Stopping power should be up to level of 511 keV photons. In order to achieve this thing, making of detectors should be of high Z material and volume should be high enough. Precision however is lowered by small detectors utilized in spatial localization. Reflection of location of annihilation is not achiever due to scattered rays, which according to many should be rejected. For replacement, good resolution of energy is required for which large size crystals are required. Accurate timing of events is required for coincident detection. In end and most important, precise timing is important for coincident detection. Timing being used by scintillators dependency is upon temporal features of generation of light in detector. Hence setting of accurate and finite coincidence time window is important to set and accommodate response of detector. This without any doubt would lead to random coincidences where attribution of two falsely events is made to same annihilation. Hence degraded data set would reflect due to blurring and scattered trail of events. For this purpose, core objective of this research is improvement of features of detector, geometrical setting and algorithms reconstruction under improvement of image quality which is considered as final. FIGURE 8: PET image of 18-FDG (DeoxyGlucose) metabolism Today spatial resolution of millimeters is part of state of the art clinical cameras, which is able to capture even minute movements in patient during the time of acquisition in time span of few minutes. Small animal scanners reach the fundamental limit due to the positron range. Key role is played by PET for biological processes at molecular level. 7 How do you choose the optimal imaging modality? Diverse imaging modalities produce images that resemble to different features of the body or to different geometrical maps. They posture different short or long-term dangers or anxieties to the patient, the personnel and the operational situation. The investment and consecutively costs of the modalities differ, as do their accessibility. The choice of an imaging technique is based on a balanced assessment of the above factors which have been stated. More than anything else, however, the following query should first be raised and answered: If the result of the inspection is optimistic or negative, will it change the analytics or therapeutic pathway for the patient? If not, the examination should not be done. Read More