Ct Scan Protocol Books Pdf
Germaine Greenweig
Positron emission tomography (PET) is a prime example of molecular imaging, which has contributed immensely to understanding cardiac anatomy and pathophysiology over the last two decades. Similar to other molecular imaging, PET utilizes intrinsic tissue characteristics as the source of image contrast, which leads to a better understanding of integrative biology and provides earlier detection and accurate diagnosis of disease. This article reviews the role of PET scans in the assessment of cardiovascular disease and its protocol and interpretation by the interprofessional team.
Objectives:
- Describe the role of PET scans in cardiovascular diseases.Review the commonly used protocols for a cardiac PET scan.Summarize cardiac PET scan analysis and interpretation.Outline how interprofessional teamwork can improve the diagnostic value of PET scans and drive better patient treatment and outcomes.
Noninvasive imaging plays a pivotal role in assessing epicardial coronary artery anatomy, myocardial perfusion, and ventricular function in patients with known or suspected cardiovascular diseases. The increasing global burden of cardiovascular diseases has led to the introduction of highly sensitive and specific imaging modalities.[1][2] Over the last two decades, the development of new software has contributed tremendously and broadened non-invasive imaging dimensions. Molecular imaging has revolutionized diagnostic imaging by utilizing high spatial and temporal resolution, significantly improving sensitivity and specificity.[3] Because of increasing knowledge of cardiovascular biology and advances in imaging technologies, molecular imaging has become an essential tool in the fields of cardiovascular medicine.[4]
Positron emission tomography (PET) is a prime example of molecular imaging, which has contributed immensely to understanding cardiac anatomy and pathophysiology over the last two decades.[5][6] Similar to other molecular imaging, PET utilizes intrinsic tissue characteristics as the source of image contrast, which leads to a better understanding of integrative biology and provides earlier detection and accurate diagnosis of disease.[7] Stress myocardial positron emission tomography with Rubidium (Rb) provides a powerful estimate of cardiovascular mortality and accurately predicts prognosis in patients with coronary artery disease.[8] Whereas PET with 2-deoxy-2-(F) fluoro-D-glucose (F-FDG) has been utilized as the gold standard for assessing myocardial viability with the help of glucose metabolism.[9]
The introduction of hybrid positron emission tomography with computed tomography (PET/CT) imaging by utilizing 3-dimensional acquisitions has been demonstrated as an important milestone in the field of myocardial perfusion imaging. It has significantly shortened the imaging protocol and reduced radiation exposure.[10] Cardiac PET has also been proven as an effective non-invasive imaging modality for diagnosing myocardial infiltrative diseases, cardiac ischemia, and cardiac infections.[11][12][13]
The human myocardium utilizes fatty acids as a primary substrate for its metabolism and production of adenosine triphosphate (ATP). Glucose and ketone bodies are the secondary sources of ATP production for cardiac myocytes.[14] Although a central mechanism regulates the relative use of these resources, the conditions of stress, disease, metabolic insults, and availability can alter the relative utilization of these resources. It is reported that in patients with diabetes mellitus, the proportion of ATP produced from fatty acids increases.[15] On the other hand, in obstructive coronary artery disease and heart failure, metabolism shifts towards glucose as it requires less oxygen compared to fatty acid metabolism.[16] Cardiac positron emission tomography utilizes this relative substrate to assess pathology and the underlying mechanisms for disease.[17]
Radiotracer used in positron emission tomography undergoes radioactive decay, which releases a positron. A positron is an antiparticle of an electron. It travels a short distance (depending on the positron range) and interacts with an electron. This interaction between the electron and positron results in the destruction of both the particles and conversion into two photons of the same energy but in the opposite direction. PET depends on the detection of these photons simultaneously on the opposite side detectors. By this coincident detection of two photons per decay, PET provides greater spatial resolution and less noise than single-photon emission tomography (SPECT).[22]
Rb is a potassium analog that is produced artificially in the nuclear laboratory via a generator. It has a short half-life of around 76 seconds, and it mimics Thallium-201 in its kinetic properties. Rb is the most commonly used radiotracer for the assessment of myocardial perfusion with PET.[24]
A cyclotron produces N-ammonia. It has a half-life of around 10 min. After injection, N-ammonia is converted to glutamine, and it disappears rapidly from the circulation. Its uptake in the lungs increases significantly in patients with left ventricular systolic dysfunction and chronic lung diseases, which may affect the images' quality adversely by increasing background activity. This effect can be minimized by increasing the time between injection and image acquisition to optimize the contrast between myocardial and background activity. Although N-ammonia allows very high-resolution imaging, its use is limited due to its production by an on-site cyclotron.[25]
Although oxygen-15-water (O-water) radiotracer diffuses freely across the membrane and its retention in the cell is not affected by the metabolic factors, it produces noisy and low count images, so its use is not recommended for clinical imaging.[26]
Fluorine-18-flurpiridaz (F-flurpiridaz) is another radiotracer with relatively smaller kinetic positron energy, leading to a short positron range. F-flurpiridaz has a very long half-life of 110 min as compared to other radiotracers. It does not require cyclotron on-site, which makes it a favorable radiotracer for clinical use. Recent trial shows an added advantage of myocardial blood flow measurement with F-flurpiridaz cardiac PET, making it an excellent agent for coronary artery disease diagnosis.[27]
Viability Tracers: 18F-FDG (fluorodeoxyglucose) is a glucose analog globally used to assess myocardial viability. FDA also approves it for metabolic scanning in clinical oncology. It utilizes myocardial glucose use as an indicator of myocardial viability. Its increased uptake could be found in ischemic tissue, whereas reduced or absent uptake signifies a myocardial scar.[28]
In a typical Rb protocol, a tomographic acquisition used for the patient positioning is followed by a CT transmission scan. Then the radiotracer is injected into the patient at rest. After injecting a radiotracer, six minutes rest imaging acquisition is initiated. After completion of rest imaging, a second CT transmission scan is performed, followed by pharmacologic stress. Pharmacologic stress is commonly produced by using dipyridamole, adenosine, or regadenoson infusion. At two minutes after the infusion of pharmacologic stressor, Rb is administered using a separate intravenous line. A 6-min stress imaging acquisition protocol is initiated with the start of the Rb infusion. Both the rest and stress CT transmission scans are acquired by holding the breath at the end of expiration, and patients are advised to breathe normally during the PET image acquisition.[29]
PET imaging with N-ammonia protocol usually requires a longer time due to the longer decay life. Immediately after the acquisition of a tomogram for patient positioning, a CT transmission scan is acquired. Then N-ammonia is injected into the patients at rest after starting a 10-min rest imaging acquisition protocol. After completion of the rest images, a stress CT transmission scan is usually acquired. Similar to the Rb PET protocol, pharmacologic stress is produced by dipyridamole, adenosine, or regadenoson. Three minutes later, N-ammonia is administered using a separate intravenous. Just like rest imaging, stress image acquisition starts a few seconds before the stress N-ammonia injection.[31]
Currently, available PET scanners operate in three-dimension detection mode without an interplane septum. This mode enables the scanners to detect all coincident photon pairs. The three-dimension model provides an advantage of up to six times higher photon sensitivity than the traditional two-dimensional model. The quantitative three-dimensional PET imaging still has substantial technical limitations due to scanner saturation with the radiotracers' standard bolus dose. Therefore, quantitative PET imaging may require a slow infusion of radiotracers to avoid scanner saturation during first-pass arterial activity.[32]
Respiratory and cardiac motions are the major source of attenuation artifacts in PET scans. Easy availability of attenuation correction is a key component of PET scan, which is routinely not available in SPECT. It is usually performed with transmission sources on standalone PET scanners. Whereas in hybrid PET/CT, the attenuation correction map is formed with the help of non-contrast CT imaging, which is acquired separately for both the rest and stress studies to ensure perfect alignment with PET scan.[33] All new PET scanner models are available in the hybrid PET/CT configuration. Significant differences have been reported between traditional transmission CT-based and CT-based attenuation correction. Although the problem related to the attenuation artifacts has not yet been fully resolved, careful visual verification of alignment is required to minimize these artifacts. New methods, including automatic registration of CT and PET maps, are also proposed to reduce the attenuation artifacts.[34]
Myocardial perfusion imaging with positron emission tomography (PET) is performed with the flow tracers, including Rubidium 82 and Nitrogen 13 ammonia. Images are taken at rest as well as during pharmacologic or physiologic stress. Metabolism imaging using positron emission tomography identifies metabolic activity in the myocardium. It is performed with fluorine 18 2-fluoro-2-deoxyglucose (FDG) as a metabolic radiotracer.
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