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Computed Tomography (CT)

Computed tomography (CT) is one of the most commonly used imaging methods because it is widely available, fast, and reliable. CT scans deploy X-rays X-rays X-rays are high-energy particles of electromagnetic radiation used in the medical field for the generation of anatomical images. X-rays are projected through the body of a patient and onto a film, and this technique is called conventional or projectional radiography. X-rays to obtain cross-sectional images of the body. A CT scanner consists of a tube that rotates around the patient and emits an X-ray X-ray Penetrating electromagnetic radiation emitted when the inner orbital electrons of an atom are excited and release radiant energy. X-ray wavelengths range from 1 pm to 10 nm. Hard x-rays are the higher energy, shorter wavelength x-rays. Soft x-rays or grenz rays are less energetic and longer in wavelength. The short wavelength end of the x-ray spectrum overlaps the gamma rays wavelength range. The distinction between gamma rays and x-rays is based on their radiation source. Pulmonary Function Tests beam and a detector that uses specialized software to receive and convert the beam to an image. The ability to create multiple views (axial, sagittal, coronal) and use contrast (intravenous, oral, rectal) allows for enhanced diagnostic yield. Patients Patients Individuals participating in the health care system for the purpose of receiving therapeutic, diagnostic, or preventive procedures. Clinician–Patient Relationship are exposed to radiation Radiation Emission or propagation of acoustic waves (sound), electromagnetic energy waves (such as light; radio waves; gamma rays; or x-rays), or a stream of subatomic particles (such as electrons; neutrons; protons; or alpha particles). Osteosarcoma, and special consideration should be given to patients Patients Individuals participating in the health care system for the purpose of receiving therapeutic, diagnostic, or preventive procedures. Clinician–Patient Relationship with a history of iodine Iodine A nonmetallic element of the halogen group that is represented by the atomic symbol I, atomic number 53, and atomic weight of 126. 90. It is a nutritionally essential element, especially important in thyroid hormone synthesis. In solution, it has anti-infective properties and is used topically. Thyroid Hormones allergy Allergy An abnormal adaptive immune response that may or may not involve antigen-specific IgE Type I Hypersensitivity Reaction, renal disease, or thyroid Thyroid The thyroid gland is one of the largest endocrine glands in the human body. The thyroid gland is a highly vascular, brownish-red gland located in the visceral compartment of the anterior region of the neck. Thyroid Gland: Anatomy disease or patients Patients Individuals participating in the health care system for the purpose of receiving therapeutic, diagnostic, or preventive procedures. Clinician–Patient Relationship who are pregnant.

Last updated: 3 Jun, 2022

Editorial responsibility: Stanley Oiseth, Lindsay Jones, Evelin Maza

Terminology and Technology

General components

  • Rotating X-ray tube X-Ray Tube X-rays
    • Spins around the patient
    • High-energy electrodes Electrodes Electric conductors through which electric currents enter or leave a medium, whether it be an electrolytic solution, solid, molten mass, gas, or vacuum. Electrocardiogram (ECG) in the tube emit radiation Radiation Emission or propagation of acoustic waves (sound), electromagnetic energy waves (such as light; radio waves; gamma rays; or x-rays), or a stream of subatomic particles (such as electrons; neutrons; protons; or alpha particles). Osteosarcoma beams. 
    • Radiation Radiation Emission or propagation of acoustic waves (sound), electromagnetic energy waves (such as light; radio waves; gamma rays; or x-rays), or a stream of subatomic particles (such as electrons; neutrons; protons; or alpha particles). Osteosarcoma passes through the patient’s body.
  • Radiation Radiation Emission or propagation of acoustic waves (sound), electromagnetic energy waves (such as light; radio waves; gamma rays; or x-rays), or a stream of subatomic particles (such as electrons; neutrons; protons; or alpha particles). Osteosarcoma detectors: 
    • On the opposite side of the patient’s body from the X-ray tube X-Ray Tube X-rays
    • Absorbs and measures the remaining radiation Radiation Emission or propagation of acoustic waves (sound), electromagnetic energy waves (such as light; radio waves; gamma rays; or x-rays), or a stream of subatomic particles (such as electrons; neutrons; protons; or alpha particles). Osteosarcoma (in the form of varying density) after it has passed through tissues
    • Computer software detects the tomography generated by the detector and reconstructs an image.
  • Motorized table: advances the patient through the scanner
Ct device components

CT scan components:
Arrangement of the X-ray tube X-Ray Tube X-rays and detectors in the CT scanner

Image by Lecturio.

Types of CT scanners

Helical (“spiral”):

  • Most common due to its speed
  • As the patient is moved through the CT, the rotating beam and X-ray X-ray Penetrating electromagnetic radiation emitted when the inner orbital electrons of an atom are excited and release radiant energy. X-ray wavelengths range from 1 pm to 10 nm. Hard x-rays are the higher energy, shorter wavelength x-rays. Soft x-rays or grenz rays are less energetic and longer in wavelength. The short wavelength end of the x-ray spectrum overlaps the gamma rays wavelength range. The distinction between gamma rays and x-rays is based on their radiation source. Pulmonary Function Tests detector spin. This creates a helical path.
  • Results in a 3-dimensional data set
  • Minimizes errors due to patient movement or breathing

Sequential (step-and-shoot):

  • Was the conventional method before helical CT
  • The patient is moved through the CT with short pauses to capture images at each position.
  • Results in an ↑ radiation Radiation Emission or propagation of acoustic waves (sound), electromagnetic energy waves (such as light; radio waves; gamma rays; or x-rays), or a stream of subatomic particles (such as electrons; neutrons; protons; or alpha particles). Osteosarcoma dose
  • Used for high-resolution scanning of the lungs Lungs Lungs are the main organs of the respiratory system. Lungs are paired viscera located in the thoracic cavity and are composed of spongy tissue. The primary function of the lungs is to oxygenate blood and eliminate CO2. Lungs: Anatomy and coronary arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries: Histology and for coronary CT angiography Angiography Radiography of blood vessels after injection of a contrast medium. Cardiac Surgery
Helical vs sequential ct

Differences between sequential and helical CT scanners:
Sequential CT takes discrete imaging slices as the patient moves through the scanner, whereas the continuous movement of the helical CT results in a spiral path.

Image by Lecturio.

Image processing

Digital images are created with a matrix of voxels (3-dimensional pixels), which are measured in Hounsfield units (HU).

  • An index used to universally quantify the radiodensity of imaged findings on CT
  • Based on the amount of radiation Radiation Emission or propagation of acoustic waves (sound), electromagnetic energy waves (such as light; radio waves; gamma rays; or x-rays), or a stream of subatomic particles (such as electrons; neutrons; protons; or alpha particles). Osteosarcoma that a material absorbs:
    • Dense material will appear bright.
    • Less dense material will appear dark.
  • The index is constructed on assigned values to water (0 HU) and air (–1000 HU).

Postprocessing can be used to accentuate tissues of different densities.

  • Lung window
  • Bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Bones: Structure and Types window
  • Soft tissue Soft Tissue Soft Tissue Abscess window

Images can also be produced in different viewing planes.

  • Axial (looking from the feet up to the head)
  • Sagittal (looking from the side)
  • Coronal (looking from the front)
Table: Density of different material images on CT
Substance Hounsfield units (HU)
Air –1000 The darkest (most hypodense)












The brightest (most hyperdense)
Fat –100 to –50
Water 0
Soft tissue Soft Tissue Soft Tissue Abscess 20–300
Bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Bones: Structure and Types 250‒700
This table shows the density of different materials that would be imaged on CT. As you can see, materials with a higher density than water will have a positive value (and are brighter), while negative values are assigned to those with lower densities (and are darker).
Ct image planes

CT image viewing planes:
The slices through this model demonstrate how axial, coronal, and sagittal images correlate with the patient’s anatomy.

Image by Lecturio.

Contrast Enhancement

Contrast

Contrast agents may be used to enhance visualization of targeted tissues.

  • Oral:
    • Used for defining the bowel on abdominal and pelvic CT scans
    • Agents: 
      • Barium sulfate (most common)
      • Isovue (an iodine-based solution)
      • Gastrografin (water-soluble, used to evaluate for bowel perforation Perforation A pathological hole in an organ, blood vessel or other soft part of the body, occurring in the absence of external force. Esophagitis)
    • Does not affect Affect The feeling-tone accompaniment of an idea or mental representation. It is the most direct psychic derivative of instinct and the psychic representative of the various bodily changes by means of which instincts manifest themselves. Psychiatric Assessment the kidneys Kidneys The kidneys are a pair of bean-shaped organs located retroperitoneally against the posterior wall of the abdomen on either side of the spine. As part of the urinary tract, the kidneys are responsible for blood filtration and excretion of water-soluble waste in the urine. Kidneys: Anatomy
  • IV:
    • Used for enhancement and differentiation of vascular structures and solid organs
    • Agent: iodinated, low ionic, low osmolar solution
    • X-rays X-rays X-rays are high-energy particles of electromagnetic radiation used in the medical field for the generation of anatomical images. X-rays are projected through the body of a patient and onto a film, and this technique is called conventional or projectional radiography. X-rays are absorbed by the contrast → ↑ attenuation
      • Contrast becomes more dilute as it moves from the arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries: Histology → tissues → veins Veins Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. Veins: Histology
      • Image appearance changes over time. 
    • Enhancement is based on:
    • Excreted by kidneys Kidneys The kidneys are a pair of bean-shaped organs located retroperitoneally against the posterior wall of the abdomen on either side of the spine. As part of the urinary tract, the kidneys are responsible for blood filtration and excretion of water-soluble waste in the urine. Kidneys: Anatomy (should be used judiciously in acute or chronic renal failure Renal failure Conditions in which the kidneys perform below the normal level in the ability to remove wastes, concentrate urine, and maintain electrolyte balance; blood pressure; and calcium metabolism. Renal insufficiency can be classified by the degree of kidney damage (as measured by the level of proteinuria) and reduction in glomerular filtration rate. Crush Syndrome)
  • Rectal:
    • Used for suspected penetrating colonic injury
    • Agents: 
      • Isovue
      • Gastrografin
    • Given as an enema

Phases

Multiphase CT can identify structures at various intervals after IV contrast administration.

  • Noncontrast phase: prior to contrast injection
  • Vascular (arterial) or bolus phase:
    • 15–20 seconds after injection
    • Contrast diffuses into the vasculature.
    • Opacifies the aorta Aorta The main trunk of the systemic arteries. Mediastinum and Great Vessels: Anatomy and its branches
    • Allows differentiation of renal cortex and medulla
  • Redistribution (venous) phase:
    • 1–3 minutes after injection
    • Contrast diffuses from intravascular to extravascular compartment.
    • Opacifies the inferior vena cava Inferior vena cava The venous trunk which receives blood from the lower extremities and from the pelvic and abdominal organs. Mediastinum and Great Vessels: Anatomy, large veins Veins Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. Veins: Histology, and solid organ parenchyma
  • Equilibrium Equilibrium Occurs when tumor cells survive the initial elimination attempt These cells are not able to progress, being maintained in a state of dormancy by the adaptive immune system. In this phase, tumor immunogenicity is edited, where T cells keep selectively attacking highly immunogenic tumor cells.This attack leaves other cells with less immunogenicity to potentially develop resistance to the immune response. Cancer Immunotherapy (delayed) phase:
    • 6–10 minutes after injection
    • Contrast reaches dynamic equilibrium Equilibrium Occurs when tumor cells survive the initial elimination attempt These cells are not able to progress, being maintained in a state of dormancy by the adaptive immune system. In this phase, tumor immunogenicity is edited, where T cells keep selectively attacking highly immunogenic tumor cells.This attack leaves other cells with less immunogenicity to potentially develop resistance to the immune response. Cancer Immunotherapy in the intravascular and extravascular compartments.
    • Opacifies the renal collecting system, ureters Ureters One of a pair of thick-walled tubes that transports urine from the kidney pelvis to the urinary bladder. Urinary Tract: Anatomy, and urinary bladder Bladder A musculomembranous sac along the urinary tract. Urine flows from the kidneys into the bladder via the ureters, and is held there until urination. Pyelonephritis and Perinephric Abscess
Hepatic alveolar echinococcosis imaging

Axial multiphase CT images of the abdomen:
a: Unenhanced image shows an infiltrative tumor-like hepatic mass Mass Three-dimensional lesion that occupies a space within the breast Imaging of the Breast.
b: Postcontrast arterial phase shows a nonenhancing, hypoattenuating lesion.
c: Postcontrast portal-venous phase shows a faint enhancement of fibroinflammatory components surrounding the parasitic pseudocyst.

Image: “Hepatic alveolar echinococcosis Echinococcosis Echinococcosis is a parasitic disease caused by Echinococcus tapeworms. Infection most often occurs from the ingestion of Echinococcus eggs in food or water contaminated with dog feces. Signs and symptoms are caused by hydatid cyst development in visceral organs and depend on the species. Echinococcus/Echinococcosis imaging” by Wenya Liu et al AL Amyloidosis. License: CC BY 4.0

Interpretation

The best approach is a systematic approach.

  • Check the patient’s demographics and name and the date.
  • Note the reason for the study.
  • Determine the part of the body that was imaged.
  • Look for prior imaging to allow for comparison.
  • Determine the image viewing plane and orientation Orientation Awareness of oneself in relation to time, place and person. Psychiatric Assessment.
  • Use multiple window levels and scroll through multiple times to ensure that all sections are covered. 
  • Viewing windows can be changed to optimize imaging of the desired organ system.
  • Evaluate one organ at a time.

Indications

Head CT

Noncontrast:

  • Severe head trauma Head trauma Head trauma occurs when external forces are directed to the skull and brain structures, resulting in damage to the skull, brain, and intracranial structures. Head injuries can be classified as open (penetrating) or closed (blunt), and primary (from the initial trauma) or secondary (indirect brain injury), and range from mild to severe and life-threatening. Head Trauma
  • Stroke
  • Intracranial bleeding (appears hyperdense when acute)

Contrast:

  • Enhances neoplasms Neoplasms New abnormal growth of tissue. Malignant neoplasms show a greater degree of anaplasia and have the properties of invasion and metastasis, compared to benign neoplasms. Benign Bone Tumors and infections Infections Invasion of the host organism by microorganisms or their toxins or by parasites that can cause pathological conditions or diseases. Chronic Granulomatous Disease
  • The angiographic phase is used to look for: 
    • Large-vessel occlusion
    • Arteriovenous malformation Arteriovenous malformation Abnormal formation of blood vessels that shunt arterial blood directly into veins without passing through the capillaries. They usually are crooked, dilated, and with thick vessel walls. A common type is the congenital arteriovenous fistula. The lack of blood flow and oxygen in the capillaries can lead to tissue damage in the affected areas. Erysipelas
    • Aneurysm Aneurysm An aneurysm is a bulging, weakened area of a blood vessel that causes an abnormal widening of its diameter > 1.5 times the size of the native vessel. Aneurysms occur more often in arteries than in veins and are at risk of dissection and rupture, which can be life-threatening. Thoracic Aortic Aneurysms

Chest CT

  • Lung parenchymal diseases:
    • Interstitial lung disease
    • Pneumonia Pneumonia Pneumonia or pulmonary inflammation is an acute or chronic inflammation of lung tissue. Causes include infection with bacteria, viruses, or fungi. In more rare cases, pneumonia can also be caused through toxic triggers through inhalation of toxic substances, immunological processes, or in the course of radiotherapy. Pneumonia
    • Lung cavitations
    • Abscess Abscess Accumulation of purulent material in tissues, organs, or circumscribed spaces, usually associated with signs of infection. Chronic Granulomatous Disease
    • Lung cancer Lung cancer Lung cancer is the malignant transformation of lung tissue and the leading cause of cancer-related deaths. The majority of cases are associated with long-term smoking. The disease is generally classified histologically as either small cell lung cancer or non-small cell lung cancer. Symptoms include cough, dyspnea, weight loss, and chest discomfort. Lung Cancer
  • Chest trauma:
  • Pleural disease:
    • Loculated effusions
    • Empyema Empyema Presence of pus in a hollow organ or body cavity. Pneumonia
    • Hemothorax Hemothorax A hemothorax is a collection of blood in the pleural cavity. Hemothorax most commonly occurs due to damage to the intercostal arteries or from a lung laceration following chest trauma. Hemothorax can also occur as a complication of disease, or hemothorax may be spontaneous or iatrogenic. Hemothorax
  • Mediastinal pathology:
  • Vascular disease:
    • Pulmonary embolism Pulmonary Embolism Pulmonary embolism (PE) is a potentially fatal condition that occurs as a result of intraluminal obstruction of the main pulmonary artery or its branches. The causative factors include thrombi, air, amniotic fluid, and fat. In PE, gas exchange is impaired due to the decreased return of deoxygenated blood to the lungs. Pulmonary Embolism
    • Aortic aneurysm Aortic aneurysm An abnormal balloon- or sac-like dilatation in the wall of aorta. Thoracic Aortic Aneurysms or dissection

Abdominal and pelvic CT

Evaluation for abdominal and pelvic pathology:

  • Blunt or penetrating abdominal injury Penetrating abdominal injury Penetrating abdominal injuries are created by an object puncturing the abdominal wall. Injuries can be high velocity, like gunshot wounds, or low velocity, like stab wounds. Different structures can be injured, including the duodenum, spleen, liver, kidneys, and pelvic organs. Penetrating Abdominal Injury
  • Appendicitis Appendicitis Appendicitis is the acute inflammation of the vermiform appendix and the most common abdominal surgical emergency globally. The condition has a lifetime risk of 8%. Characteristic features include periumbilical abdominal pain that migrates to the right lower quadrant, fever, anorexia, nausea, and vomiting. Appendicitis
  • Diverticulitis Diverticulitis Inflammation of a diverticulum or diverticula. Diverticular Disease
  • Pancreatitis Pancreatitis Inflammation of the pancreas. Pancreatitis is classified as acute unless there are computed tomographic or endoscopic retrograde cholangiopancreatographic findings of chronic pancreatitis. The two most common forms of acute pancreatitis are alcoholic pancreatitis and gallstone pancreatitis. Acute Pancreatitis
  • Intra-abdominal infections Infections Invasion of the host organism by microorganisms or their toxins or by parasites that can cause pathological conditions or diseases. Chronic Granulomatous Disease
  • Renal, ureteral, and bladder Bladder A musculomembranous sac along the urinary tract. Urine flows from the kidneys into the bladder via the ureters, and is held there until urination. Pyelonephritis and Perinephric Abscess calculi 
  • Pneumoperitoneum Pneumoperitoneum A condition with trapped gas or air in the peritoneal cavity, usually secondary to perforation of the internal organs such as the lung and the gastrointestinal tract, or to recent surgery. Pneumoperitoneum may be purposely introduced to aid radiological examination. Perforated Viscus
  • Bowel obstruction Bowel obstruction Any impairment, arrest, or reversal of the normal flow of intestinal contents toward the anal canal. Ascaris/Ascariasis
  • Aortic aneurysm Aortic aneurysm An abnormal balloon- or sac-like dilatation in the wall of aorta. Thoracic Aortic Aneurysms and dissection
  • Retroperitoneal Retroperitoneal Peritoneum: Anatomy hemorrhage

Evaluation of visceral malignancies:

Ct scan of middle abdomen showing a subcapsular hepatic hematoma

CT scan of the middle abdomen showing a subcapsular hepatic hematoma Hematoma A collection of blood outside the blood vessels. Hematoma can be localized in an organ, space, or tissue. Intussusception on the surface of the right lobe of the liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy (hypodense area indicated by the arrows) with a source of active bleeding (circle)

Image: “CT scan of middle abdomen showing a subcapsular hepatic hematoma Hematoma A collection of blood outside the blood vessels. Hematoma can be localized in an organ, space, or tissue. Intussusception” by From the Department of General Surgery, C.S. General and Emergency Surgery, Azienda Ospedaliera – IRCCS Arcispedale Santa Maria Nuova, Reggio Emilia, Italy. License: CC BY 4.0

Special Considerations and Contraindications

Radiation Radiation Emission or propagation of acoustic waves (sound), electromagnetic energy waves (such as light; radio waves; gamma rays; or x-rays), or a stream of subatomic particles (such as electrons; neutrons; protons; or alpha particles). Osteosarcoma exposure

Like other imaging methods, CT scans expose patients Patients Individuals participating in the health care system for the purpose of receiving therapeutic, diagnostic, or preventive procedures. Clinician–Patient Relationship to radiation Radiation Emission or propagation of acoustic waves (sound), electromagnetic energy waves (such as light; radio waves; gamma rays; or x-rays), or a stream of subatomic particles (such as electrons; neutrons; protons; or alpha particles). Osteosarcoma.

  • Ionizing radiation Radiation Emission or propagation of acoustic waves (sound), electromagnetic energy waves (such as light; radio waves; gamma rays; or x-rays), or a stream of subatomic particles (such as electrons; neutrons; protons; or alpha particles). Osteosarcoma is additive.
  • Number of scans should be limited whenever possible.
  • Radiation Radiation Emission or propagation of acoustic waves (sound), electromagnetic energy waves (such as light; radio waves; gamma rays; or x-rays), or a stream of subatomic particles (such as electrons; neutrons; protons; or alpha particles). Osteosarcoma dose:
    • Chest and abdominal CT: 10 mSv
    • Pelvic CT: 7 mSv
    • Head CT: 2 mSv
  • In context:
    • Chest X-ray Chest X-ray X-ray visualization of the chest and organs of the thoracic cavity. It is not restricted to visualization of the lungs. Pulmonary Function Tests: 0.013 mSv
    • Low risk of fetal malformation: < 50 mSv
    • Substantial fetal damage: > 500 mSv
  • Effects of radiation Radiation Emission or propagation of acoustic waves (sound), electromagnetic energy waves (such as light; radio waves; gamma rays; or x-rays), or a stream of subatomic particles (such as electrons; neutrons; protons; or alpha particles). Osteosarcoma:
    • Molecular damage
    • Free radical Free Radical Highly reactive molecules with an unsatisfied electron valence pair. Free radicals are produced in both normal and pathological processes. They are proven or suspected agents of tissue damage in a wide variety of circumstances including radiation, damage from environment chemicals, and aging. Natural and pharmacological prevention of free radical damage is being actively investigated. Nitroimidazoles formation
    • Disruption of cellular metabolic function
    • Cell death Cell death Injurious stimuli trigger the process of cellular adaptation, whereby cells respond to withstand the harmful changes in their environment. Overwhelmed adaptive mechanisms lead to cell injury. Mild stimuli produce reversible injury. If the stimulus is severe or persistent, injury becomes irreversible. Apoptosis is programmed cell death, a mechanism with both physiologic and pathologic effects. Cell Injury and Death after a certain threshold Threshold Minimum voltage necessary to generate an action potential (an all-or-none response) Skeletal Muscle Contraction
    • Carcinogenic risk increases with exposure.

Contraindications Contraindications A condition or factor associated with a recipient that makes the use of a drug, procedure, or physical agent improper or inadvisable. Contraindications may be absolute (life threatening) or relative (higher risk of complications in which benefits may outweigh risks). Noninvasive Ventilation

  • Allergic reactions Allergic Reactions Type I hypersensitivity reaction against plasma proteins in donor blood Transfusion Reactions to contrast agents:
    • May manifest as hives Hives Urticaria is raised, well-circumscribed areas (wheals) of edema (swelling) and erythema (redness) involving the dermis and epidermis with associated pruritus (itch). Urticaria is not a single disease but rather is a reaction pattern representing cutaneous mast cell degranulation. Urticaria (Hives) or anaphylaxis Anaphylaxis An acute hypersensitivity reaction due to exposure to a previously encountered antigen. The reaction may include rapidly progressing urticaria, respiratory distress, vascular collapse, systemic shock, and death. Type I Hypersensitivity Reaction
    • Patients Patients Individuals participating in the health care system for the purpose of receiving therapeutic, diagnostic, or preventive procedures. Clinician–Patient Relationship can be premedicated with steroids Steroids A group of polycyclic compounds closely related biochemically to terpenes. They include cholesterol, numerous hormones, precursors of certain vitamins, bile acids, alcohols (sterols), and certain natural drugs and poisons. Steroids have a common nucleus, a fused, reduced 17-carbon atom ring system, cyclopentanoperhydrophenanthrene. Most steroids also have two methyl groups and an aliphatic side-chain attached to the nucleus. Benign Liver Tumors and antihistamines Antihistamines Antihistamines are drugs that target histamine receptors, particularly H1 and H2 receptors. H1 antagonists are competitive and reversible inhibitors of H1 receptors. First-generation antihistamines cross the blood-brain barrier and can cause sedation. Antihistamines if a contrast CT is necessary.
  • Pregnancy Pregnancy The status during which female mammals carry their developing young (embryos or fetuses) in utero before birth, beginning from fertilization to birth. Pregnancy: Diagnosis, Physiology, and Care: Iodinated contrast can cross the placenta Placenta A highly vascularized mammalian fetal-maternal organ and major site of transport of oxygen, nutrients, and fetal waste products. It includes a fetal portion (chorionic villi) derived from trophoblasts and a maternal portion (decidua) derived from the uterine endometrium. The placenta produces an array of steroid, protein and peptide hormones (placental hormones). Placenta, Umbilical Cord, and Amniotic Cavity.
  • Thyroid Thyroid The thyroid gland is one of the largest endocrine glands in the human body. The thyroid gland is a highly vascular, brownish-red gland located in the visceral compartment of the anterior region of the neck. Thyroid Gland: Anatomy disease: 
    • Iodinated contrast will reduce uptake of radioactive iodine Iodine A nonmetallic element of the halogen group that is represented by the atomic symbol I, atomic number 53, and atomic weight of 126. 90. It is a nutritionally essential element, especially important in thyroid hormone synthesis. In solution, it has anti-infective properties and is used topically. Thyroid Hormones → treatment is less effective
    • Hyperthyroidism Hyperthyroidism Hypersecretion of thyroid hormones from the thyroid gland. Elevated levels of thyroid hormones increase basal metabolic rate. Thyrotoxicosis and Hyperthyroidism patients Patients Individuals participating in the health care system for the purpose of receiving therapeutic, diagnostic, or preventive procedures. Clinician–Patient Relationship may develop thyroid storm Thyroid Storm Thyrotoxicosis and Hyperthyroidism from contrast.
  • Chronic or acutely worsening renal disease: Iodinated contrast can cause acute tubular necrosis Necrosis The death of cells in an organ or tissue due to disease, injury or failure of the blood supply. Ischemic Cell Damage.

Other Imaging Methods

Comparison of imaging methods

Table: Comparison of imaging methods
Radiography CT Ultrasound MRI
Mechanism of acquisition Ionizing radiation Radiation Emission or propagation of acoustic waves (sound), electromagnetic energy waves (such as light; radio waves; gamma rays; or x-rays), or a stream of subatomic particles (such as electrons; neutrons; protons; or alpha particles). Osteosarcoma Ionizing radiation Radiation Emission or propagation of acoustic waves (sound), electromagnetic energy waves (such as light; radio waves; gamma rays; or x-rays), or a stream of subatomic particles (such as electrons; neutrons; protons; or alpha particles). Osteosarcoma Acoustic energy Ferromagnetic pulses
Relative cost Inexpensive Expensive Inexpensive Very expensive
Portable Yes No Yes No
Length of exam Seconds < 1 minute Seconds Approximately 1 hour
Contrast No May be needed May be needed May be needed

Imaging method options by system

  • Imaging of the CNS ( brain Brain The part of central nervous system that is contained within the skull (cranium). Arising from the neural tube, the embryonic brain is comprised of three major parts including prosencephalon (the forebrain); mesencephalon (the midbrain); and rhombencephalon (the hindbrain). The developed brain consists of cerebrum; cerebellum; and other structures in the brain stem. Nervous System: Anatomy, Structure, and Classification, spinal cord Spinal cord The spinal cord is the major conduction pathway connecting the brain to the body; it is part of the CNS. In cross section, the spinal cord is divided into an H-shaped area of gray matter (consisting of synapsing neuronal cell bodies) and a surrounding area of white matter (consisting of ascending and descending tracts of myelinated axons). Spinal Cord: Anatomy, and vertebral column Vertebral column The human spine, or vertebral column, is the most important anatomical and functional axis of the human body. It consists of 7 cervical vertebrae, 12 thoracic vertebrae, and 5 lumbar vertebrae and is limited cranially by the skull and caudally by the sacrum. Vertebral Column: Anatomy): 
    • Radiography is often used to evaluate for fractures of the vertebral column Vertebral column The human spine, or vertebral column, is the most important anatomical and functional axis of the human body. It consists of 7 cervical vertebrae, 12 thoracic vertebrae, and 5 lumbar vertebrae and is limited cranially by the skull and caudally by the sacrum. Vertebral Column: Anatomy
    • CT is a good choice for head trauma Head trauma Head trauma occurs when external forces are directed to the skull and brain structures, resulting in damage to the skull, brain, and intracranial structures. Head injuries can be classified as open (penetrating) or closed (blunt), and primary (from the initial trauma) or secondary (indirect brain injury), and range from mild to severe and life-threatening. Head Trauma and to exclude intracranial hemorrhage Intracranial hemorrhage Subarachnoid hemorrhage (SAH) is a type of cerebrovascular accident (stroke) resulting from intracranial hemorrhage into the subarachnoid space between the arachnoid and the pia mater layers of the meninges surrounding the brain. Most sahs originate from a saccular aneurysm in the circle of willis but may also occur as a result of trauma, uncontrolled hypertension, vasculitis, anticoagulant use, or stimulant use. Subarachnoid Hemorrhage
    • MRI provides more detailed images of the brain Brain The part of central nervous system that is contained within the skull (cranium). Arising from the neural tube, the embryonic brain is comprised of three major parts including prosencephalon (the forebrain); mesencephalon (the midbrain); and rhombencephalon (the hindbrain). The developed brain consists of cerebrum; cerebellum; and other structures in the brain stem. Nervous System: Anatomy, Structure, and Classification and spinal cord Spinal cord The spinal cord is the major conduction pathway connecting the brain to the body; it is part of the CNS. In cross section, the spinal cord is divided into an H-shaped area of gray matter (consisting of synapsing neuronal cell bodies) and a surrounding area of white matter (consisting of ascending and descending tracts of myelinated axons). Spinal Cord: Anatomy, allowing identification Identification Defense Mechanisms of infarction, tumors, disc herniation Herniation Omphalocele, and demyelinating disease.
  • Pulmonary radiology Pulmonary Radiology Pulmonary, or chest, imaging includes imaging of the lungs and surrounding structures in the thorax. Imaging of the chest represents a substantial portion of the imaging tests that are routinely performed. Common imaging methods include X-ray, CT, MRI, and ultrasonography (US). Imaging of the Lungs and Pleura and imaging of the mediastinum Mediastinum The mediastinum is the thoracic area between the 2 pleural cavities. The mediastinum contains vital structures of the circulatory, respiratory, digestive, and nervous systems including the heart and esophagus, and major thoracic vessels. Mediastinum and Great Vessels: Anatomy
    • Radiography is the preferred initial imaging study for viewing lung pathology. 
    • CT scan provides more detailed views of the lung parenchyma, mediastinal structures, and vasculature. 
    • MRI is not often used, but may be employed for evaluating malignancies and cardiac disease. 
    • Ultrasonography can be used for a rapid bedside trauma assessment and for guiding procedures ( thoracentesis Thoracentesis Aspiration of fluid or air from the thoracic cavity. It is coupled sometimes with the administration of drugs into the pleural cavity. Thoracic Surgery).
  • Breast imaging Breast Imaging Female breasts, made of glandular, adipose, and connective tissue, are hormone-sensitive organs that undergo changes along with the menstrual cycle and during pregnancy. Breasts may be affected by various diseases, in which different imaging methods are important to arrive at the correct diagnosis and management. Mammography is used for breast cancer screening and diagnostic evaluation of various breast-related symptoms. Imaging of the Breast
    • Mammography Mammography Radiographic examination of the breast. Breast Cancer Screening is often the initial choice for breast cancer Breast cancer Breast cancer is a disease characterized by malignant transformation of the epithelial cells of the breast. Breast cancer is the most common form of cancer and 2nd most common cause of cancer-related death among women. Breast Cancer screening Screening Preoperative Care
    • MRI may be used to further evaluate and stage breast cancer Breast cancer Breast cancer is a disease characterized by malignant transformation of the epithelial cells of the breast. Breast cancer is the most common form of cancer and 2nd most common cause of cancer-related death among women. Breast Cancer
    • Ultrasonography is helpful for evaluating lymph Lymph The interstitial fluid that is in the lymphatic system. Secondary Lymphatic Organs nodes and to guide biopsy Biopsy Removal and pathologic examination of specimens from the living body. Ewing Sarcoma.
  • Imaging of the abdomen and renal imaging Renal imaging The renal system is composed of 2 kidneys, 2 ureters, a bladder, and a urethra. Varying conditions such as infections, cysts, solid masses, ischemia, and mechanical obstruction can affect the renal system. Evaluation of diseases rely on imaging methods such as radiography, ultrasonography, CT, and MRI. Some of these are also used to guide tissue sampling (e.g., renal biopsy). Imaging of the Urinary System
    • Radiography is often used to evaluate for kidney stones Kidney stones Nephrolithiasis is the formation of a stone, or calculus, anywhere along the urinary tract caused by precipitations of solutes in the urine. The most common type of kidney stone is the calcium oxalate stone, but other types include calcium phosphate, struvite (ammonium magnesium phosphate), uric acid, and cystine stones. Nephrolithiasis, bowel obstruction Bowel obstruction Any impairment, arrest, or reversal of the normal flow of intestinal contents toward the anal canal. Ascaris/Ascariasis, and pneumoperitoneum Pneumoperitoneum A condition with trapped gas or air in the peritoneal cavity, usually secondary to perforation of the internal organs such as the lung and the gastrointestinal tract, or to recent surgery. Pneumoperitoneum may be purposely introduced to aid radiological examination. Perforated Viscus. In addition, barium may be used to assess swallowing Swallowing The act of taking solids and liquids into the gastrointestinal tract through the mouth and throat. Gastrointestinal Motility and bowel function. 
    • CT and MRI provide more detailed assessments of the abdominal viscera and vasculature. 
    • Nuclear medicine Nuclear medicine A specialty field of radiology concerned with diagnostic, therapeutic, and investigative use of radioactive compounds. Nuclear Imaging can be used to assess gallbladder Gallbladder The gallbladder is a pear-shaped sac, located directly beneath the liver, that sits on top of the superior part of the duodenum. The primary functions of the gallbladder include concentrating and storing up to 50 mL of bile. Gallbladder and Biliary Tract: Anatomy function and gastric emptying Gastric emptying The evacuation of food from the stomach into the duodenum. Gastrointestinal Motility and for GI bleeding.
  • Imaging of the uterus Uterus The uterus, cervix, and fallopian tubes are part of the internal female reproductive system. The uterus has a thick wall made of smooth muscle (the myometrium) and an inner mucosal layer (the endometrium). The most inferior portion of the uterus is the cervix, which connects the uterine cavity to the vagina. Uterus, Cervix, and Fallopian Tubes: Anatomy and ovaries Ovaries Ovaries are the paired gonads of the female reproductive system that contain haploid gametes known as oocytes. The ovaries are located intraperitoneally in the pelvis, just posterior to the broad ligament, and are connected to the pelvic sidewall and to the uterus by ligaments. These organs function to secrete hormones (estrogen and progesterone) and to produce the female germ cells (oocytes). Ovaries: Anatomy
    • Ultrasonography is the most commonly used method to evaluate the ovaries Ovaries Ovaries are the paired gonads of the female reproductive system that contain haploid gametes known as oocytes. The ovaries are located intraperitoneally in the pelvis, just posterior to the broad ligament, and are connected to the pelvic sidewall and to the uterus by ligaments. These organs function to secrete hormones (estrogen and progesterone) and to produce the female germ cells (oocytes). Ovaries: Anatomy and uterus Uterus The uterus, cervix, and fallopian tubes are part of the internal female reproductive system. The uterus has a thick wall made of smooth muscle (the myometrium) and an inner mucosal layer (the endometrium). The most inferior portion of the uterus is the cervix, which connects the uterine cavity to the vagina. Uterus, Cervix, and Fallopian Tubes: Anatomy, including assessing pregnancies and the causes of abnormal uterine bleeding Abnormal Uterine Bleeding Abnormal uterine bleeding is the medical term for abnormalities in the frequency, volume, duration, and regularity of the menstrual cycle. Abnormal uterine bleeding is classified using the acronym PALM-COEIN, with PALM representing the structural causes and COEIN indicating the non-structural causes. Abnormal Uterine Bleeding
    • CT and MRI provide more detailed views and are often useful in assessing cysts Cysts Any fluid-filled closed cavity or sac that is lined by an epithelium. Cysts can be of normal, abnormal, non-neoplastic, or neoplastic tissues. Fibrocystic Change, malignancies, and benign Benign Fibroadenoma masses.
  • Imaging of the musculoskeletal system: 
    • Radiography is often used to exclude fractures. 
    • CT is more sensitive to bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Bones: Structure and Types pathology, including osteomyelitis Osteomyelitis Osteomyelitis is an infection of the bone that results from the spread of microorganisms from the blood (hematogenous), nearby infected tissue, or open wounds (non-hematogenous). Infections are most commonly caused by Staphylococcus aureus. Osteomyelitis
    • MRI is preferred for a soft tissue Soft Tissue Soft Tissue Abscess evaluation, such as assessing for malignancy Malignancy Hemothorax and myositis. 
    • Bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Bones: Structure and Types scanning can be useful in finding occult fractures, osteomyelitis Osteomyelitis Osteomyelitis is an infection of the bone that results from the spread of microorganisms from the blood (hematogenous), nearby infected tissue, or open wounds (non-hematogenous). Infections are most commonly caused by Staphylococcus aureus. Osteomyelitis, and metabolic bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Bones: Structure and Types disease.

References

  1. Kocak, M. (2019). Computed tomography. MSD Manual Professional Version. Retrieved December 2, 2020, from https://www.msdmanuals.com/professional/special-subjects/principles-of-radiologic-imaging/computed-tomography
  2. Stark, P. (2020). Principles of computed tomography of the chest. In Finlay, G. (Ed.), UpToDate. Retrieved December 2, 2020, from https://www.uptodate.com/contents/principles-of-computed-tomography-of-the-chest
  3. Rawson, J. V., Pelletier, A. L. (2013). When to order a contrast-enhanced CT. American Family Physician 88(5):312–316.
  4. Knipe, H., and Nadrljanksi, M.M. (2019). Computed tomography. Radiopedia. Retrieved December 2, 2020, from https://radiopaedia.org/articles/computed-tomography
  5. Fertikh, D. (2015). Head computed tomography scanning. In Taylor, C.R. (Ed.), Medscape. Retrieved December 2, 2020, from https://emedicine.medscape.com/article/2110836-overview
  6. Taylor, C. R., Abramovici, G. (2017). Abdominal computed tomography scanning. In Mathur, M. (Ed.), Medscape. Retrieved December 2, 2020, from https://emedicine.medscape.com/article/2114236-overview
  7. Rogers, D. C., and Tadi, P. (2020). Intravenous contrast. StatPearls. Retrieved January 19, 2021, from https://www.ncbi.nlm.nih.gov/books/NBK557794/

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