Principles of Tomographic Methods (CT, PET) PDF

 Principles of tomographic methods. X-ray absorption computed tomography (CT), positron emission tomography (PET) The text under the slides was written by Peter Nagy 2019 This instructi...

 Principles of tomographic methods. X-ray absorption computed tomography (CT), positron emission tomography (PET) The text under the slides was written by Peter Nagy 2019 This instructional material was prepared for the biophysics lectures held by the Department of Biophysics and Cell Biology Faculty of Medicine University of Debrecen Hungary https://biophys.med.unideb.hu 2 The slide summarizes the main aims and topics of the lecture. Tomography is any imaging approach, which provides a three-dimensional image of the human body. The word originates from the Greek words (slice) és (write). X- ray absorption computed tomography (or briefly CT), magnetic resonance imaging (MRI), SPECT (single photon emission computed tomography) and PET (positron emission tomography) are tomographic techniques. Although ultrasound imaging also reveals the third dimension, since it is based on completely different principles, it is typically not regarded as a tomographic approach. Tomographic techniques facilitate the localization of pathological processes considerably. In their absence a medical doctor could rely only on the clinical symptoms or on data from other instrumental investigations. While mainly bones are visible in conventional X-ray images, CT also reveals soft tissues. You are going to get introduced to two tomographic techniques during the lecture, CT and PET. The major aims are to understand the principles of image formation and the medical implications (e.g. risk) of the approaches. 3 Disadvantages of conventional X-ray images: 1. Information about depth is lost. A conventional X-ray image is called a summation image because the summed absorption of superimposed organs and tissues is displayed. The depth of absorbing objects cannot be determined. The position of absorbing bodies cannot be determined in such summation images (e.g. the location of the object marked by the yellow arrows in image A). Taking a lateral image (side view, image B) besides the posteroanterior picture (frontal view, image A) facilitates localization, but does not provide the exact shape and location of the object. 2. Soft tissues are hardly displayed or are not visible at all. In order for an object to be visible in an X-ray image, it must provide contrast relative to its surrounding, i.e. its X-ray absorbing capacity must be different from that of its environment. Tiny differences in the X-ray absorption capacity of soft tissues are averaged out (i.e. cancel each other) due to superimpositions, therefore such tissues and organs are typically not visible in conventional X-ray images. It is for this reason why the brain is not visible inside the skull in a conventional X-ray image, although it is revealed in CT (images C and D). Despite these advantages, CT does not provide better resolution in every respect, since tiny bony structures are better resolved in conventional summation images (image E). The background of this statement is going to be explained later. 4 CT measures the X-ray attenuation potential of tissues. Let us look at the object in upper left part of the slide (A). Black corresponds to no absorption in the picture. The intensity of X-ray penetrating the object is plotted as a function of distance under the object (B). The first equation under the graph shows the intensity of X-ray after going across the first object (I1). The equation is based on the exponential decay of X-ray. For CT imaging the term density was introduced, and it is usually used instead of the attenuation coefficient. Radiographic density is not to be confused with conventional density (volumetric mass density). Using logarithmic identities and the definition of density displayed in yellow in the slide intensity I1 is given by the following equation: 1 x1 D1 I1 I0 e I0 10 2 x2 The second equation under the graph displays intensity I2. Intensity I1 decays further to e -times its initial value. It is important to realize that the initial value in this second step is I1. Using the definition of density again: 2 x2 1 x1 2 x2 1 x1 2 x2 D1 D2 I2 I1e I0 e e I0 e I0 10 We can draw the conclusion that the resultant decay is determined by the sum of densities (D1+D2). This conclusion can be extended and generalized to an arbitrary number of absorbers: n Di I I0 10 i 1 5 Therefore, density is additive, i.e. the sum of the densities of individual volume units ( ) raised to the negative power of 10 determines the final intensity. It is for this advantageous property that density is used in CT investigations. As we have already seen density depends on both the attenuation coefficient and the penetration depth (slice thickness). Since an image consists of pixels of identical size, the relationship between densities is identical to the relationship between the attenuation coefficients of pixels. Therefore, the density information available in images can easily be interpreted as a map of attenuation coefficients. 6 The figure demonstrates how tomography can restore the third dimension lost in conventional X-ray images. Let us imagine the object containing only a single absorbing volume unit (upper part of the figure, “I”). Volume elements are called voxels (“volume pixels”) by analogy with pixels (picture element). If an X-ray image is recorded from one side (image A), then a one- dimensional projection of this two-dimensional object is obtained. In practice, conventional X-ray images provide two-dimensional projections of the three-dimensional human body. We do not know where the absorber is located based on this projection image (question marks). If another image is taken perpendicularly to the other one (image B), the location of the absorber can be figured out using the information available from the two images (red arrows). Let us view the model (object II) containing two absorbing voxels. If two images of this model are recorded (images A and B), we still cannot determine the location of the two absorbers, since both of the arrangements (III and IV) on the right may generate the two projection images. If a third image is recorded obliquely (image C), the location of the two absorbers can be unequivocally determined (red tick mark). Therefore, it seems that the more absorbers there are, the more images recorded from different angles we need for determining the spatial position of all absorbers. We can therefore conclude that many conventional, summation X-ray images recorded from different angles are required to determine the location of absorbers in the human body, which contains a large number of absorbers. 7 CT images are generated in two steps without the user knowing about it. The imaged object is on the left side of the slide (black – no absorption, gray – weak absorption, white – strong absorption). In the first step the CT device records many conventional X-ray images of the human body from different angles, and these images are stored in a dataset. This dataset is called the sinogram or Radon transform of the original object (explained later). Although the sinogram contains all the information about the spatial location of all voxels of the body, it is not suitable for easy human interpretation. Therefore, a computer generates an easily interpretable picture using a purely computational approach in the second step using inverse Radon transformation, which is also called back-projection. 8 According to the previous slides a CT device prepares many summation X-ray images of the object from different directions. The collection of these images is called the Radon transform of the object to be imaged in honor of Johann Radon, an Austrian mathematician who derived the mathematical formulas forming the foundation of tomography. Let us observe an object in which only a single voxel exhibits X-ray attenuation capacity (bright square in the lower left corner of the black object, A). Let us take a summation image of this object horizontally (0 ). The detector reveals absorption only at a single location (according to the density profile (B) and the one-dimensional projection image (C)). This image is stored in the first column of the dataset or the Radon transform (G, first projection). The CT camera rotates around the body and records image from many different directions. Besides the one recorded at 0 the figure only displays the images taken at 45 (D), 90 (E) and 135 (F). Each image is stored in the corresponding column of the dataset resulting in the Radon transform. Pay attention to the fact that the position of the signal (the white stripe) generated by the absorbing body changes as a function of the detection angle. The Radon transform is also called a sinogram, since a Radon transform of a single point is a sine function (shown in G). 9 The figure demonstrates how a CT device generates an image of an object somewhat more complicated than the one shown in the previous slide, while it rotates around the body and records summation images from many different angles. The object, the X-ray beams (red lines) and the projections are shown on the left, while the Radon transform or sinogram formed from the series of projections is displayed on the right. The animation in the lecture material can be viewer at the following link: https://www.youtube.com/watch?v=gQUOBM5Hon4 10 After recording the X-ray images, i.e. Radon transform, of the body, the computer in the CT device calculates the spatial distribution of the X-ray attenuation capacity of the body, i.e. the CT image. Let us assume that the CT device recorded only four projections (0 , 45 , 90 , 135 ). These projections have been stored in the positions marked by red rectangles in the Radon transform (A). Let us first use the first, 0 -projection. From this single image we can only deduce that the absorber must be located somewhere along the blue arrow (B). The bright spot in the 0 -image is basically projected back on to the body, i.e. we smear the bright spot along the blue arrow. This is why the procedure is also called back-projection. In the second step let us use the 90 -projection besides the one recorded at 0 (C). Let us back-project both images on to the original body, and the image of the X-ray absorbing object is already beginning to emerge at the intersection of the two lines. If we use the other two projection images (D and E), the quality of the reconstruction gradually improves. Since the procedure is basically the reverse of the Radon transform, it is also called the inverse Radon transform. 11 If we increase the number of images used for back-projection, the quality of the reconstruction increases. If we use all the images recorded from 180 directions (0 -179 , lower right corner), we obtain a practically perfect reconstruction, although a bright halo is generated around the object. 12 The disadvantage of the simple back-projection algorithm described previously is the generation of a bright halo around the absorbing objects during reconstruction. This is only minimally disturbing in the case of simple objects, e.g. the one shown in the previous slide. If an improved version of the back-projection algorithm (filtered back-projection, not discussed in detail) is used, the bright halo is not generated. This feature is important since the blur surrounding absorbing bodies can be so much disturbing in the case of slightly more complex bodies (images in the bottom part of the slide) that the objects are made undiscernible. The filtered back-projection algorithm works perfectly in such a case as well. It follows from the aforementioned principles that a CT image is the result of calculations. Therefore, resolution of a CT image is determined by the size of voxels, i.e. the size of those volume units whose density is to be determined in the calculations. The voxel size is clinical CT is on the order of 1 mm as a result of a combination of technical factors not discussed here. This limitation leads to the fact that structures smaller than this size cannot be visualized in CT images (e.g. the tiny bony structures mentioned in the introduction). 13 During X-ray imaging X-ray photons with an energy between 120-140 keV are used. In this energy range only the photoeffect and the Compton effect contribute to X-ray attenuation. Therefore, the attenuation coefficient ( ) can be partitioned to two components with one of them ( ) describing the contribution of photoeffect, while the other one ( ) giving the contribution of Compton effect to absorption. The attenuation coefficient of an absorber with unit density is called the mass attenuation coefficient. The attenuation coefficients and not only depend on the density of absorbers, but also on their atomic number. In multicomponent systems we must use the effective atomic number (Zeff) instead of the atomic number, which depends on the atomic number (Z) n of the components and on their mole fraction (w): Z eff 3 wi Z i 3. The probability of i 1 photoeffect is proportional to the third power of the effective atomic number. The constant describing the strength of Compton effect ( ) inversely depends on the mass number, besides a linear relationship with the effective atomic number. 14 According to the principles described previously it is obvious why soft tissues, mainly composed of elements with low atomic index, absorb X-ray weakly. In contrast, calcium attenuates X-ray much more strongly explaining why bones are more visible in X-ray images. Although neither iodine, nor barium can be found in large quantities in the human body, their strong X-ray attenuation potential is relevant, since both elements are used as contrast agents in X-ray imaging. When applying X-ray contrast agents, they are injected into certain organs or tissues (e.g. into the stomach or blood vessels as can be seen in the bottom image), and the contrast agent sharply outlines the organ. In CT angiography shown in the bottom image blood vessels supplying the brain were made visible. 15 Attenuation coefficients determined by a CT device are displayed in Hounsfield units after linear transformation. The unit is named after Godfrey Hounsfield, who shared the Nobel prize in medicine with Allan MacLeod Cormack for developing CT in 1979. Hounsfield units describe the attenuation coefficient of voxels normalized to the attenuation coefficient of water. It follows from the formula that the attenuation coefficient of water expressed in Hounsfield units is zero. The Hounsfield unit of materials absorbing X-ray less strongly than water is negative, while the Hounsfield unit of tissues in which X-ray decays more steeply than in water is positive. The radiodensity of tissues expressed in Hounsfield units is characteristic of the tissues. Therefore, it can be used as a rough evaluation of the intactness of tissues (“CT histology”). 16 The technique of CT has undergone great development since its invention in the 1970s. Although the principles of imaging described in the lecture are still valid, the specific mathematical procedures applied has changed considerably. Currently the most advanced devices are spiral CTs, in which the X-ray source rotates around the patient while the bed, on which the patient lies, is moved along the foot-head axis. As a result, the radiation source is essentially moving along a spiral path around the body. During such a revolution of the radiation source, taking approximately 15-30 seconds, a CT image of a large body section (e.g. chest) can be generated. 17 In high resolution CT (HRCT) parameters of the examination are adjusted in such a way so as to make the size of detected voxels small. This is achieved primarily by decreasing slice thickness, which is approximately 0.6-1.2 mm in HRCT. The algorithm used for reconstructing an image from the acquired data is also optimized to ensure high spatial resolution. HRCT procedures are mainly used for investigating the lung since diffuse changes in the structure of the lung tissue can be detected. Such a diffuse change is e.g. accumulation of connective tissue fibers in lung fibrosis leading to restrictive pulmonary disease (for detailed explanation consult the lecture material about breathing). 18 X-ray investigations, including CT, are associated with radiation burden. The table display the radiation exposure, expressed in mSv, associated with a couple of medical imaging approaches involving ionizing radiation. For comparison, a couple of radiation exposures are shown. From among these values the average annual effective dose a human individual is exposed to ( 3 mSv) is worthy of attention. Its value was determined a couple of decades ago, and it became a textbook fact. The issue of the annual exposure of a human individual will be analyzed further later. It is remarkable that the dose delivered by CT examinations is on the order of the annual radiation exposure. It is also very important to realize that a CT examination exposes the patient to a much larger radiation dose than conventional X-ray, e.g. a chest CT is associated with a 50 larger radiation dose than a conventional chest radiogram. 19 In developed countries with a high standard of medical care the number of medical imaging examinations has significantly increased recently. Many of them are associated with radiation exposure. Consequently, the average annual radiation exposure and its composition with regard to the source have also been considerably altered in countries with developed medical systems. The average annual radiation dose has increased from 3.6 mSv to 6.2 mSv questioning the textbook data mentioned previously. This increase was caused almost exclusively by radiation exposure due to medical imaging. Although we are aware of the deleterious consequences of ionizing radiation in general (mutations, cell death, cancer), the fact that medical imaging examinations associated with radiation burden lead to detectable harmful consequences is less well known. The graph in the lower part of the slide displays the consequence of a single abdominal CT. The lifetime probability of death attributable to this single abdominal CT scan is shown on the vertical axis. A CT examination carried out after the age of 35 does not have detectable consequences, but if such a scan is performed at a younger age (e.g. at the age of 1), the probability that the patient will die of the malignant tumor caused by this single CT investigation is not negligible (0.1%). Therefore, medical doctors must be aware of the harmful consequences of radiation exposure, and they should order medical imaging examinations associated with radiation burden only when the benefit outweighs the risk. 20 Realization of the harmful consequences of radiation exposure associated with CT examinations led to the development of low-dose CT. In a low-dose CT the patient is exposed to a considerably reduced radiation dose (e.g. in the case of chest CT the radiation dose is decreased from 7 mSv to 1,5-2,5 mSv). This is achieved by - reducing the intensity of the X-ray beam. The second row of the table displays the X- ray dose as a product of the current flowing in the X-ray tube (mA) and the time required to capture an image of a slice (s). The unit of the product is mAs, i.e. milliampere-second. The current flowing in the X-ray tube is proportional to the intensity of the generated X-ray beam. - increasing the slice thickness, and therefore reducing the number of slices recorded. Although the quality of the image generated by low-dose CT is inferior to that of conventional CT, diagnostic decisions based on these imaging modalities are usually congruent (e.g. in the case of lung cancer there is 90% agreement between them). 21 Images recorded by modern CT devices (top images) and an image generated by a first- generation CT used in the 1970s-1980s (bottom image). The superior image quality of state- of-the-art scanners is obvious. The following major pathologic signs are visible in the CT image of the patient having suffered stroke (not required to learn): - area with increased radiodensity at the arrow (white, most likely blood) - area with reduced radiodensity surrounding the white area (most likely edema surrounding the damaged tissue) - compression of the brain ventricles (dark gray-black areas) and shift of the midline to the right as a result of the space occupying stroke on the left. 22 Positron emission tomography - is a functional imaging modality revealing the functional state (metabolic activity) of organs instead of their morphology + - using a radiopharmaceutical with a -decaying isotope injected into the patient. + The positron emitted by a -decaying isotope collides with an electron in a couple of millimeters leading to their annihilation. This couple of millimeters sets a limit on the + resolving power of the technique. Although the location of the -decaying isotope is to be determined, the decay itself is not detectable, only the gamma photons generated within 1-2 + millimeters from the decay. The distance between the annihilation and the decay depends on the energy of the positron, and hence on the type of the nucleus emitting it (see the table in the slide). The direction of propagation of the two gamma photons generated in the annihilation is almost perfectly opposite to each other. The patient is surrounded by a ring of detectors. Since gamma photons propagate with the speed of light, the two detectors reached by these two gamma photons respond almost at the same time (within 5-10 ns), i.e. in coincidence. The two detectors signaling in coincidence define a line (“line of response”). The intersection of many lines of response reveals the location of annihilation. 23 Besides coincidence detection another approach, time-of-flight (TOF) detection, based on the measurement of the response time of detectors has also been developed (A). In this method the time difference between the response of the two detectors excited almost at the same time by the gamma photons is measured. If the radiation source is at a distance of s from halfway between the detectors, the time required for reaching the first (t1) and second (t2) detector, and the difference between these times ( t) are given by the equations below: d s d s 2 2 2 s t1 , t2 t t 2 t1 c c c Thus, location ( s) of the source can be determined from the time difference ( t). The time measurement must be extremely accurate, since a time difference of 0.1 ns corresponds to a distance of 1.5 cm. The relationship given by the equation above also applies to the uncertainty of distance determination, i.e. an uncertainty of 0.1 ns leads to a localization uncertainty of 1.5 cm. But this is sufficient to reduce the probability of wrong localization due to random coincidences. A random coincidence arises when two gamma photons not generated in the same annihilation collide into detectors within 5-10 ns. Two coincidence pairs define two LORs in panel B according to which the source must be at the red circle. But according to TOF measurement these two coincidence pairs could not be generated from the same source. TOF detection significantly improves the diagnostic sensitivity of PET, which is demonstrated by panel C in which the pathological change is only visible with TOF detection. 24  + The half-life of -decaying isotopes used for PET is usually very short making their diagnostic application challenging. The two most commonly used such isotopes are 18F and 11 C. Due their short half-life these isotopes are usually generated in cyclotrons located relatively close to the PET device. In cyclotrons accelerated particles, e.g. protons, are collided with a certain kind of nucleus. E.g. 11C is generated by the collision of accelerated protons into a 14N target. The proton is transiently incorporated into the target nucleus generating 15O (8 11 protons, 7 neutrons), from which an particle is emitted generating C (6 protons, 5 neutrons). 25 The process of a PET examination begins with the generation of the radioactive isotope (previous slide) followed by the incorporation of this radioactive isotope into a radiopharmaceutical, which is injected into a patient. Data acquisition is followed by the calculation of the image (using coincidence detection or the TOF principle). 26 PET is a functional imaging method used for mapping the metabolic activity of cells in the human body. The two most widely used radiopharmaceuticals are fluorodeoxyglucose (FDG) 11 measuring the glycolytic activity of cells and C-methionine reporting on the amino acid consumption of cells. FDG is taken up by cells in the same way as glucose, and it also enters glycolysis. However, after the first phosphorylation step its phosphorylated derivative, 18FDG- 6-phosphate, cannot proceed along the glycolytic pathway. 18FDG-6-phosphate is charged due to the presence of phosphate and therefore it cannot leave cells (“trapping”). Its accumulation in cells is proportional to the glycolytic activity, which is higher in cancer cells than in their healthy counterparts. This is the basis for detecting tumors and metastases by FDG. Since cancers are also characterized by high amino acid uptake and DNA synthesis, 11C-methionine and 11C-thymidine can be used for their detection. 27 A PET camera (A) and images from three representative PET examinations (B,C,D) can be seen in the slide. FDG is used for measuring glucose consumption in the cerebral cortex (B), which decreases in Alzheimer’s disease facilitating the diagnosis of the condition using PET. Images 11 of a brain tumor can be seen in the lower left corner (C). By comparing the FDG and C- methionine images of the brain tumor it can be concluded that accumulation of the latter is more sensitive for diagnosing brain tumors. In the lower right corner (D) imaging of a liver metastasis by CT and PET (or combined PET-CT) is demonstrated. Comparison of the images before and after therapy reveals the difference between morphological and functional imaging approaches. There is no significant change in the size of the liver metastasis in the CT images, while the FDG uptake of the liver metastasis is considerably lower after therapy. 28 The advantages of morphological and functional imaging approaches can be combined using image fusion. During the process images generated by two different imaging modalities are superimposed on each other making anatomical localization of the functional information possible. In the past, the functional and the morphological images were acquired in two different devices. Nowadays it is becoming widespread to build instruments equipped with two imaging modalities (e.g. CT + PET, MRI + PET). 29 30 PRINCIPLES OF TOMOGRAPHIC METHODS TRUE OR FALSE 1. Computed tomography uses ionizing radiation for imaging. 2. Computer Tomography is a functional imaging device 3. In Computer Tomography the resolution is linearly proportional to the radiation applied 4. Imaging with positron emission tomography is primarily based on the measurement of the attenuation of gamma rays in the tissues. 5. Molecules with high carbon content are used as contrast material for x-ray imaging 6. Essay 1. What radiation is used by CT for image formation? Is the source of radiation inside or outside the body? How is this radiation produced? This note was created by Clifford Collins. Follow us on Instagram @TheDebrecenTutor  2. What elementary particle is mostly responsible for the attenuation of the radiation inside the body? Name and very briefly describe the mechanisms leading to the attenuation of the radiation was generated with a voltage of 120kV This note was created by Clifford Collins. Follow us on Instagram @TheDebrecenTutor  3. a. In the figure below an object is examined by CT and is irradiated from above and then from the left. Detectors (D1…D6) measure the intensity of the exiting radiations. The attenuation coefficient of the white squares is  = 0 mm-1, for the gray ones  = 0.2 mm-1, and for the black ones  = 0.4mm-1. The side of each square is 1mm. calculate the density for each row (R1, R2, R3) and column (C1, C2 and C3) and write it next to the corresponding label. a. Which detector (D1…D6) registers the greatest intensity? b. Which two detectors register the same intensity. c. If the intensity of the radiation before entering the body is 1000 units, what intensity values would D5 and D6 register? d. In the coordinate system below plot the intensity after passing through the body as a function of density. To create the graph use the values obtained in part (f) and label the appropriate values on the axes. This note was created by Clifford Collins. Follow us on Instagram @TheDebrecenTutor Essays 1. Describe the principles of positron emission tomography (nuclear event, detection)! Compare the principle of image generation and information content of a PET with that of a CT examination. 2.Describe the advantages of computer tomography compared to conventional X ray imaging. 3. Define radioisotope use in PET This note was created by Clifford Collins. Follow us on Instagram @TheDebrecenTutor  4. What is radiopharmacon? 5. What use of PET in medicine? 6. PET a. Which isotopes are in use b. Usage in medicine c. How is the image constructed 7. a) Describe the principles of positron emission tomography (nuclear event, detection)! This note was created by Clifford Collins. Follow us on Instagram @TheDebrecenTutor  b) Compare the principle of image generation and information content of a PET with that of a CT examination (radiation source internal/external, morphological/functional, etc.) c) Specify what 18 FDG and 11C-methionine are used for in PET examinations? d) Why is it advantageous to have an accelerator in the vicinity of the PET system? e) What does image fusion (overlay) visualization mean? What is it good for? 8. Describe the advantages of computer tomography compared to conventional X ray imaging. This note was created by Clifford Collins. Follow us on Instagram @TheDebrecenTutor  9. What kind of isotopes can be used in PET? 10. What is the principle of determination of the location of a radioactive isotope in PET? 11. What is the principle of computer tomography? 12. In which respect does a CT image provide more information than a conventional X- ray image? 13. Answer the specific questions below about positron emission tomography! ・What kind of radioactive isotopes can be used for PET? ・Circle from the list below the isotope that may be used in a PET examination. 11C, 12C, 13C, 14C, 14N, 15N, 16O This note was created by Clifford Collins. Follow us on Instagram @TheDebrecenTutor  ・What kind of elementary particle is emitted by the radioactive isotope and what happens with this elementary particle very soon after radioactive decay? How is the place of the radioactive decay determined? ・What is a radiopharmacon? ・18FDG is used as an analog of what biological molecule in PET imaging? ・What is PET used for in medicine? Relation Analysis 1. ______ In general, computer tomography is more dangerous to the patient than magnetic resonance imaging (MRI) because computer tomography uses ionizing X- ray radiation. 2. The lower the Hounsfield unit for a tissue in CT imaging the less it will absorb X-rays because the Hounsfield unit is the product of the attenuation coefficient of the given tissue and the attenuation coefficient of water. 3. The higher the Hounsfield unit for a tissue in CT imaging the less it will absorb X- rays because the Hounsfield unit is the product of the attenuation coefficient of the given tissue and the attenuation coefficient of water. 4. CT does not give high spatial resolution anatomical image of the various tissues because image formation in CT is based on the differential absorption of gamma radiation in various tissues. 5. MULTIPLE CHOICE This note was created by Clifford Collins. Follow us on Instagram @TheDebrecenTutor  1. The resolution power of positron emission tomography (PET) is inversely proportional to the wavelength of the illuminating light. A. Only positive β decaying isotopes can be used in SPECT. B. PET is a functional imaging method. C. SPECT gives the same type of information as a γ camera, but with 3D (three dimensional) resolution. D. Radioactive isotopes are not used in SPECT. Minimals 1. List the most important types of applica9ons of radioac9ve isotopes in medical diagnosis! 2. What is the principle of direct radioimmunoassay (RIA)? 3. What is the principle of par9cle accelera9on in linear and cyclic accelerators? 4. What is the opera9on principle of a γ-camera? This note was created by Clifford Collins. Follow us on Instagram @TheDebrecenTutor  5. What is the principle of SPECT? 6. What kind of isotopes can be used in PET? 7. What is the principle of determina9on of the loca9on of a radioac9ve isotope in PET? 8. What is the principle of computer tomography? 9. In which respect does a CT image provide more informa9on than a conven9onal X-ray image? This note was created by Clifford Collins. Follow us on Instagram @TheDebrecenTutor