CT Scan


Introduction

A computed tomography (CT) scan, commonly referred to as a CT, is a radiological imaging study. The machine was developed by physicist Allan MacLeod Cormack and electrical engineer Godfrey Hounsfield.[1][2][3] Their development awarded them the Nobel prize in Physiology or Medicine in 1979.[4] The first scanners were installed in 1974. Since then, technological advances and math have allowed single images to be computed into two-dimensional informative images.

The CT scan is essentially an X-ray study, where a series of rays are rotated around a specified body part, and computer-generated cross-sectional images are produced. The advantage of these tomographic images compared to conventional X-rays is that they contain detailed information of a specified area in cross-section, eliminating the superimposition of images, which provides a tremendous advantage over plain films. CT scans provide excellent clinicopathological correlation for a suspected illness.[5][6] 

The use of CT scans augments the physician's ability to diagnose a patient's illness accurately. Low-dose CT scans are proving useful in preventative medicine and cancer screening. The study was initially called a CAT scan representing computer axial tomography, where the table moved after each axial image was obtained.[7][8][9] 

In a spiral or helical scan, the table moves continuously as the x-ray source and detectors rotate. This reduces the duration of the study significantly to provide quick results in emergent situations. It rapidly substituted cerebral angiography for detecting head trauma injuries and brain masses in a fast and extremely reliable way.[10][11][12] A radiologic technician acquires CT scans, which are interpreted and reported by a trained radiologist.

Procedures

The CT scanner machine rotates the X-ray tube around the patient's body through a circular structure known as the gantry. Each time the machine rotates, computerized information is acquired. The patient is slowly moved up or down in the table, and different cross-section images are produced. In each rotation, a 2D image slice is constructed. Each subsequent image slice's thickness is decided on the operator and the physician/radiologist's request but usually ranges from 1 to 10 millimeters. The gantry can be moved at the desired angle to accommodate the best cross-sectional image. When the desired number of slices are obtained, a scan is reproduced into the computer image and can easily be reproduced and stored.

The image is created using pixels according to its radiosensitivity and is displayed using the Hounsfield scale units, which are compared to known tissue density. Water is 0, while air is negative 1000, and bone is positive 400 to 2000. Intravenous iodine can be injected into the bloodstream to demarcate blood vessels, tumors and identify infectious processes.[13] 

Intravenous iodine-based or oral barium-based contrast is used to visualize the digestive system. The images can be computer-tacked together to produce a 3D image of the area of interest. The CT scans are obtained in the cranial direction, meaning from feet to head. It is important to note that current CT machines display the image opposite the patient's side as the image is produced as viewed from the patient's foot. Thus the right side of the image is the patient's left side.

Indications

A CT scan is used for numerous clinical indications depending on the organs to evaluate.[14] The CT scan can be utilized in both inpatient and outpatient clinical settings. In an emergent setting, it can rule out serious illness. Indications for obtaining a CT scan are to help the physician diagnose, narrow the differential diagnosis, and confirm the doctor's suspicions. It can also be used for cancer screening, staging, and follow-up. Its use helps to perform biopsies adequately and to assist during a surgical procedure.

  • Brain: tumors, traumatic or spontaneous hematomas, stroke, edema, skull fracture, calcifications, arteriovenous malformations, hydrocephalus, sinusitis, and empyema
  • Neck: tumors, benign masses, thyroid nodules, lymphadenopathy
  • Chest: tumor, pneumonia, metastasis, benign masses, pulmonary edema, pleural edema, tuberculosis, pulmonary embolism, traumatic injury to the lungs, esophageal rupture, ingested foreign body, fibrosis
  • Abdomen: primary tumors, metastases, abscess, ascites, cholecystitis, appendicitis, renal calculi, pancreatitis, obstruction, lymphadenopathy, foreign body
  • Spine: fractures, degenerative changes, stability, osteomyelitis, disc pathology
  • Bone: complex bone fractures, eroded joints, knee, tumors, osteomyelitis
  • Gyn: cyst, fibromas, tumors
  • Screening: colon and lung cancer
    • CT colonography/colonoscopy is used to diagnose colon disease and early-stage cancer with good sensitivity and specificity.[15][16][17][18]
    • Low-emission CT can be used to diagnose lung cancer in smokers and former smokers with a high smoking history aged between 55 and 80 years old using a low radiation dose.[19][20][21][22]
  • Biopsy: CT guided to different organs for adequate tissue extraction
  • CT Angiography: brain, heart, lung, kidney, extremities
  • Intraoperative: CT scan can be used for neuronavigation procedures during brain biopsy or tumor resection.[23]

Interfering Factors

CT scans may be deemed inconclusive if artifacts obscure the images. Interfering factors from metal-based objects such as dental implants, shrapnel, bullet fragments, surgical clips, pacemaker, and body piercings will cause a "flare"  known as streak artifacts. These artifacts within images obscure underlying structures, causing difficulty in proper visualization and assessment of active pathology.[24] 

To counteract the flared images, metal artifact reduction algorithms and normalized metal artifact reduction enhance the images and reduce the risk of error.[25][26][27] In some cases, where previous images are available, the Gaussian diffusion sinogram can be applied to reduce the streak artifacts from dental implants; however, this is a limited study since previous images may not be available.[28]

When using intravenous contrast to obtain detailed CT scans, the iodine-radio-labeled dye may interfere with laboratory testing of specific biological markers and chemical compounds such as troponin, angiotensin-converting enzyme, or electrolytes such as zinc or iodine.[29] A significant disadvantage of the CT scan is that it does not adequately show tendons, ligaments, spinal cord, or intervertebral discs. In such cases, magnetic resonance imaging (MRI) is the test of choice.

Complications

The CT scan involves ionizing radiation, which has the potential to cause biological tissue harm.[30][31][32][33][34][35][36] CT scans can have 50 to 1,000 times higher radiation dose than conventional X-rays.[31][37] They account for the largest portion of radiation after natural/environmental sources to the population. CT scans comprise approximately 50% of all medical radiation.[37][38][39] 

It has been estimated that for every 1.0 mSv of exposure, there is a 5% risk of developing fatal cancer. Thus, a radiation dose of 100 mSv will have a 0.5% risk of cancer.[40] Roughly, one fatal cancer is developed for every 1000 CT scans performed in a pediatric patient.[41] Utilizing the A-bomb data, the lifetime risk of leukemia from one pediatric head CT scan is approximately 1 in 10,000, and for brain cancer is approximately 1 in 2,000 to 10,000.[34][35][42][43]

This radiation exposure is especially critical in pediatric patients due to the developing organs' vulnerability when performed under ten years and the cumulative lifelong exposure.[44][45][46] Exposure should be limited following the ALARA (as low as reasonably achievable) principle. Multiple examinations should be avoided. They should be done if the benefit by far outweighs the risk.[30] The radiation dose of a CT scan ranges from 1.0 mSv to 27.0 mSv. 1 mSv=1 mGy. Natural/environmental exposure is approximately 3.0 mSv per year.[44] An adult abdominal CT exposes the patient to 10 mSv; however, during a neonatal abdominal CT, the exposure is 20 mSv.[31]

Contrast agents may cause allergic reactions, usually mild, involving itching rash; however, severe reactions can occur such as bronchospasm and anaphylactic reaction.[47][48][49][50] The probability for a fatal reaction is about 1 in 100,000. If the patient has an iodine allergy, steroids must be given to counteract any potential side effects if contrast must be given.[51] Kidney failure due to the iodine contrast material can occur in 2 to 7%, with greater risk in those with preexisting kidney disease.[52][53][54][55][56] Contrast-induced nephropathy, when severe, can require dialysis to clear the dye. In non-serious conditions, adequate hydration before the post-contrast injection will eliminate contrast from the body.

Patient Safety and Education

When a single scan is performed, the estimated radiation obtained is similar to the person's natural exposure to the environment for several months to a few years. If a patient is pregnant, the radiation dose is small and usually does not affect the fetus. However, the study is done only if it provides a significant benefit. Using spiral or helical CT scan, the radiation dose is reduced compared to sequential CT scan.[57] 

A low-emission CT scan allows minimal radiation exposure with the benefit of early cancer detection in smokers. A CT scan is ideal for trauma cases as it can detect organ injury in a fast and reliable manner. The use of intravenous contrast highlights the vascular structures and can identify the cause of subarachnoid hemorrhages and spontaneous hematomas. For gunshot wounds, the CT scan provides details of the injury, which can not be done with MRI due to the gunshot's metallic nature.[58]

The CT scan has several advantages over magnetic resonance imaging. The CT scan can be used safely in patients with pacemakers and programmable pumps or shunts because it will not affect the device setting. Claustrophobic patients usually can not complete the MRI study; however, they feel comfortable inside the CT scan as the procedure is quicker and noiseless.

Clinical Significance

CT scans provide information as close to real-time that allows for proper management of multiple diseases. Before the invention of the CT scan, patients with abdominal pain, fever, and elevated white blood cell count would have been taken to the operating room for an exploratory laparotomy to find the cause. CT's invention has provided physicians and surgeons information to avoid unnecessary laparotomies and saved millions in healthcare costs. Ultimately, the patient benefit outweighs the risk of radiation and has remained a mainstay in the clinical diagnosis of disease. Technological advancements to the CT scan significantly augmented patient care by making it more efficient and cost-effective. 

The CT scan was the mainstay instrument to detect and follow-up tumors until the advent of the MRI and PET scans. Tumors avidly enhance with the use of iodine contrast. The combined PET CT scan can detect and stage cancer patients.[59][60][61]

Brain: for patients with hydrocephalus, the CT scan is the principal study to evaluate ventricle size and compare the size in cases of shunt malfunction.[62] For pediatric patients with headaches, loss of consciousness, or seizures, the CT scan is usually normal; thus, judicious use is encouraged to avoid radiation exposure.[62] For a patient involved in a traumatic scenario, the CT scan can show skull fractures, traumatic hematomas (epidural, subdural, and intracerebral), and edema in a fast way to obtain rapid management. CT scan can be used to detect spontaneous subarachnoid hemorrhage, hematomas, and stroke. Cerebral vascular lesions like arteriovenous malformations and aneurysms can be suggested due to the presence of calcifications. CT-perfusion is used to determine the cerebral blood flow to specific brain areas using color-coded maps.[63] CT angiography identifies whether there is adequate blood flow within respective organs such as the brain, heart, lung, kidney, or extremities.[64][65]  

Intraoperative CT scan has been used for neurosurgery, breast cancer, and lung cancer, to provide real-time images during a procedure or immediately follow it.[63][66] During spine surgery, its use permits correct screw placement. For intracranial pathology, the intraoperative CT scan helps for the proper placement of a ventricular catheter or drainage of a cyst. MRI has a clear advantage over a CT scan for the intraoperative residual tumor that can still be resected.[67][68] However, the CT can help rectify the amount of resection, which sometimes is underestimated by the surgeon.[67]

CT-myelography can be used for those patients in whom MRI can not be performed to evaluate the spinal cord for thecal sac compression and leaks.[69][70][71][72]

Abdomen: used for tumors, ascites, effusion, cholecystitis, and obstruction. For abdominal pain, a CT scan can address the cause for pain in most patients; however, its use is more limited in oncologic pain.[73]

Neck: used for tumors, benign masses, thyroid nodules, and lymphadenopathy. For head and neck pathology, the CT scan is the first-line imaging examination. If an adult patient presents with a neck mass, the study localizes and characterizes it and shows if adenopathy is present.[74][75]

Spine: to evaluate fractures, degenerative changes, stability, and osteomyelitis. At the emergency department, the entire cervical spine can be assessed with a cervical CT scan.[76] In severe trauma, the whole spine can be adequately visualized.[77]

Lung: to detect pulmonary embolisms, hemothorax, pneumothorax, excess fluid, emphysema, fibrosis, and pneumonia that may be missed on traditional X-ray. With the surge in Covid-19 cases, the use of chest CT scans has increased for diagnostic purposes.[78][79][80][81][82] An abdominal CT rarely adds more information in Covid-19 cases.[83]

Bone: to identify complex bone fractures, eroded joints, knee, tumors, and osteomyelitis. The CT scan has a higher sensitivity over X-ray imaging in identifying elbow fractures, especially those involving the growth plate.[84] Intraoperative CT is used for complicated surgeries such as resection of a talocalcaneal coalition.[85]

Gyn: to identify cysts, fibromas, and tumors. Ultrasound is the primary diagnostic tool in gynecological pathology; however, CT plays an essential role in those cases where the sonogram is inconclusive.[86]

Biopsy: CT guided biopsy to different organs for tumor diagnosis and pathogen identification.[87][88][89][90][91][92]

Abscess: CT guided aspiration of deeper abscesses that would previously require surgical exploration and removal.[93][94]

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Paula R. Patel

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