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. As radiation by X-rays can cause adverse effects depending on the absorbed dose, it is necessary to take protective measures to reduce harm. Digital radiography uses the digital data format and allows for the digital manipulation of images. Common uses include evaluation of chest, mediastinal, spinal, and bone/joint conditions. While radiography is still used to visualize head and abdominal structures, more advanced modalities (CT and MRI) are now preferred. Radiography remains an essential component of initial tests in many diseases, given its wide availability, low cost, and ease of operation.

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Overview

Definition

An X-ray is a discrete, high-energy particle of electromagnetic radiation (photon) that propagates through space at the speed of light.

X-ray production

  • X-rays are produced through different processes:
    • Characteristic X-ray radiation: 
      • Result from the movement or transition of electrons from an outer shell (orbit) to vacancies in the inner shell
      • Emission of X-ray photon is material dependent.
    • Braking radiation (Bremsstrahlung): 
      • Electrons move rapidly toward the anode (positively charged electrode) and decelerate when they collide.
      • During deceleration, 99% of the energy dissipates as heat and 1% is released as X-ray photons.
  • X-ray imaging utilizes an X-ray tube consisting of:
    • A heated filament that emits electrons
    • A tungsten target/anode where electrons strike, producing X-rays
  • X-rays penetrate matter and interact with the atomic electrons of the material. During this process, X-rays can be absorbed or scattered.
  • Not all X-rays can penetrate a patient. Most X-rays are scattered and do not contribute to image creation.
A diagram of an X-ray tube

Diagram of an X-ray tube:
In the tube, electrons are accelerated toward a tungsten target (anode), which then decelerate after hitting the target, releasing heat and X-ray photons.

Image by Lecturio.

Effects of X-ray radiation

  • Biological damage by X-rays is attributed to the ionizing radiation that is produced as X-rays interact with matter.
    • The absorbed dose is the energy (from the interaction) deposited in matter.
    • Absorbed radiation: measured in units known as Gray (Gy) or rad (100 rads are equivalent to 1 Gy) 
  • Types of radiation effects:
    • Deterministic effect: 
      • Damage occurs when a threshold of radiation is crossed, such that the ability of a cell to repair itself is overwhelmed. 
      • Results from very high doses of radiation, causing skin erythema, cataracts, and sterility
    • Stochastic effect: Damage is additive and the probability of the effect increases with increased exposure.
      • Damage occurs at the genetic level during cell division and can lead to carcinogenesis.
      • The likelihood of effects increases with radiation dosage.
  • Ultimately, the resulting damage includes:
    • Formation of free radicals
    • Disruption of normal metabolic function and mitosis
  • Cancer induction:
    • Organs with the most rapidly dividing cells are most susceptible, which also explains why children, overall, are most susceptible. 
    • Most susceptible organs:
      • Bone marrow
      • Colon
      • Lungs
      • Stomach
    • Moderately susceptible organs:
      • Bladder
      • Breast 
      • Liver
      • Esophagus
      • Thyroid

Fetal risk of radiation

Table: Fetal risk of radiation
Weeks post conceptionEffects of major exposure
2
  • 10–50 rads: risk of failure to implant
  • > 50 rads: high likelihood of implantation failure
3–5
  • 10–50 rads: growth restriction possible
  • > 50 rads: congenital abnormalities, growth restriction, risk of miscarriage
6–13
  • 10–50 rads: growth restriction possible
  • > 50 rads: growth restriction, risk of miscarriage
14–23
  • 10–50 rads: noncancer health effects unlikely
  • > 50 rads: growth restriction, risk of miscarriage, possible congenital abnormalities
24 weeks to term
  • 10–50 rads: noncancer health effects unlikely
  • > 50 rads: miscarriage, neonatal death (depending on dose)

Radiation protection

  • Minimize the radiation dosage whenever possible (ALARA: as low as reasonably achievable).
  • Measures:
    • Exposed personnel should be monitored using a film badge.
    • Lead shielding and increasing the distance from the source
    • Shielding within rooms
    • Increasing the kilovoltage of the X-ray beam and, thus, increasing its penetration

Terminology and Technical Aspects

Radiography

  • Radiography: the use of X-rays to generate images
  • Types:
    • Projectional radiography: generation of a radiographic image by projecting a beam of X-ray particles through a subject and onto a film:
      • The X-ray image is a shadow picture obtained using a single “light” source.
      • Fluoroscopy: the use of projectional radiography to observe internal structures in real-time (e.g., GI imaging)
    • CT: generation of a multilayered image by a beam projected by a rotatory X-ray tube onto radiation detectors

Image generation by X-rays

Order of producing an image with X-rays:

  1. X-ray tube: X-rays generated after electrons collide with the anode.
  2. Patient: X-ray beam traverses the patient and is attenuated depending on the tissues in their path.
  3. Antiscatter grid: lead strips that improve image contrast by reducing scattered photons
  4. Image capture is by using an imaging plate in a cassette.

Technologies producing radiographic image:

  • Conventional radiography
    • Screen film is used and the film is developed.
    • High sensitivity, low cost, and easy handling
  • Digital radiography (utilizes digital data format, allowing digital manipulation of images):
    • Computed radiography: Cassette is inserted into a scanner and the image is shown on a monitor.
    • Direct radiography: no cassette used. X-rays are converted into electrical charges by a photoconductor.
Wilhelm Röntgen hand

Image of an early X-ray: X-ray of a left hand taken at a public lecture by Wilhelm Röntgen

Image: “An early X-ray” by Wilhelm Röntgen; current version created by Old Moonraker. License: Public Domain

Interpretation

Basic radiological densities

  • Air
  • Fat
  • Fluid
  • Calcium (bone)
  • Metal

Terminology according to the density of the object

  • Radiolucent: an object of low density that is permeable to X-rays (looks black)
  • Radiopaque: an object of high density that blocks X-rays (looks white)

Principles of radiography

  • Summation of shadows: Images appear more radiopaque due to overlapping densities.
  • Silhouette sign: 
    • Edges of an object are indistinguishable when densities are adjacent to one another.
    • Think of pneumonia in the right middle lobe, which obscures the right heart border.
  • Orthogonal imaging: taking 2 projections of the same structure to better document its 3-dimensionality

Elements that reduce the diagnostic yield of an X-ray

  • Excessive or insufficient penetration
  • Rotation of the patient
  • Image magnification
  • Movement of the patient
  • Artifacts, such as dust particles

Chest

Projections

X-ray images of the chest can be produced in the following projections:

  • Posteroanterior (PA):
    • The X-ray beam initially penetrates the posterior aspect of the body while the cassette is placed in direct contact with the anterior aspect.
    • Preferred method for evaluating cardiac silhouette size
  • Anteroposterior (AP):
    • The X-ray beam initially penetrates the anterior aspect of the body while the cassette is placed in direct contact with the posterior aspect.
    • Used in portable radiography (very common in hospitalized patients who are unable to move)
    • Structures further away from the cassette appear magnified, creating false positives for cardiomegaly.
  • Lateral:
    • The X-ray beam is incident on a lateral aspect of the body and the cassette is placed in contact with the other lateral aspect.
  • Lordotic and semi-upright positioning:
    • The X-ray beam penetrates the patient at an angle to display 2 different structures at different levels.
  • Decubitus:
    • The patient lays on their right or left side. 
    • A replacement for lateral projection used for patients who cannot stand up

Technical aspects

To obtain an optimal anatomical image:

  • Inspiration: The patient is asked to take a deep breath while an X-ray is being obtained.
  • 8–9 posterior ribs should be made visible for the inspiration to be optimal.

The following aspects reduce the quality of the anatomical image:

  • Penetration: Excessive or deficient penetration of X-rays through the anatomical structures affects results.
    • Overpenetrated regions can simulate collections of air (pneumothorax).
    • Underpenetrated regions can simulate consolidations (pneumonia).
  • Rotation: When the patient is not appropriately placed in front of the cassette, the structures are unevenly represented in the anatomical image.
    • The mediastinum and hilum mimic masses.
    • An image can be rotated to the right or to the left.
    • Inspect by checking the distance between the medial aspects of the clavicle and the spinous processes of the thoracic vertebrae.
Differences between expiratory and inspiratory chest X-ray

Differences between an expiratory and an inspiratory chest X-ray:
Notice that in the inspiratory X-ray, the posterior ribs and the pulmonary parenchyma are more easily seen, whereas in the expiratory X-ray, the parenchyma looks hazy and lacks definition.

Image by Hetal Verma.

Sequence

The quality inspection of the image must be included and should be done preferably before the following reading sequence:

  1. Foreign objects: tubes/lines
  2. Lung parenchyma
  3. Airways
  4. Mediastinal boundaries
  5. Surrounding soft tissue
  6. Bony structures (ribs and clavicles)
  7. Upper abdomen

Tubes and lines

The following elements should be checked for adequate placement:

  • Endotracheal tube
  • Tracheostomy tube
  • Feeding tubes:
    • Nasogastric tube 
    • Dobhoff tube
  • Central lines
  • Peripherally inserted central catheter
  • Swan-Ganz catheter
  • Chest tube

Lung anatomy

The following structures must be identified in a cephalocaudal manner and checked for abnormalities (e.g., cavitations, consolidations):

  • PA projection:
    • Right hemithorax:
      • Right upper lobe
      • Right lower lobe
      • Right costophrenic angle
      • Right cardiophrenic angle
    • Left hemothorax:
      • Left upper lobe
      • Left lower lobe
      • Left costophrenic angle
      • Left cardiophrenic angle
    • Trachea and carina
  • Lateral projection: 
    • The pulmonary lobes are identified by tracing a diagonal line and dividing the lung into:
      • An upper portion (2 upper lobes on the right, 1 upper lobe on the left)
      • A lower portion (1 lower lobe each on the right and left)

Heart and mediastinal anatomy

  • Mediastinum:
    • The area between the lungs and pleural cavities that stands in the middle of the thoracic cavity
    • Divided into the anterior, middle, and posterior mediastinum, this space houses every structure located medially to the lungs. 
    • This space contains:
      • Great vessels, such as the superior vena cava (SVC), inferior vena cava, pulmonary arteries, pulmonary veins, and the aorta
      • The thymus can be seen in the anterior mediastinum in children and young adults. 
  • The following structures must be identified in a cephalocaudal manner and checked for abnormalities in size or shape:
    • PA projection:
      • Trachea: should be on the midline
      • SVC
      • Ascending and descending aorta (aortic arch)
      • Pulmonary hilum (right and left)
      • Pulmonary artery
      • Right atrium (right heart border)
      • Left ventricle (left heart border)
    • To easily identify the structures on the left border in a cephalocaudal order, remember: 
      • 1st bump: aorta
      • 2nd bump: pulmonary artery
      • 3rd bump (largest): left ventricle
  • Lateral projection:
    • Retrosternal space
    • Right ventricle
    • Right hemidiaphragm
    • Aortic arch
    • Pulmonary hilum
    • Right atrium
    • Left ventricle
    • Left hemidiaphragm
    • Posterior cardiac space

Bones

The following structures must be identified in a cephalocaudal manner and checked for abnormalities (e.g., fractures):

  • PA projection:
    • Clavicles
    • Scapulae
    • Spinous processes of the vertebrae
  • Lateral projection: thoracic spine (evaluate vertebral body heights for compression fractures)
A PA projection of the chest identifying the major bony structures of the chest and the main structures of the upper abdomen

A posteroanterior projection of the chest identifying the major bony structures of the chest and the main structures of the upper abdomen

Image by Hetal Verma.

Upper abdomen

  • The clinician must be attentive for abnormal collections of air in this area.
  • The following parts of the upper abdomen are seen in a chest X-ray (PA and lateral projections):
    • Liver
    • Stomach
    • Ascending, transverse, and descending colon

Abdomen and Pelvis

Abdominal X-rays have low sensitivity for evaluating solid organs, which is why they have been replaced by CT scans and ultrasound examination.

Projections

X-ray images of the abdomen can be produced in the following projections:

  • AP, also known by the acronym KUB (kidneys, ureters, and bladder):
    • Upright and supine
    • Paired with a PA projection of the chest in acute abdomen
  • Lateral decubitus: used in patients who cannot stand up
  • Oblique: obtained when needed
Example of an abdominal X-ray

Abdominal X-ray showing an oval calculus projecting over the expected location of the right kidney/collecting system adjacent to the L3 transverse process

Image by Hetal Verma.

Sequence

The quality inspection of the image is done preferably before the reading sequence for abdominal X-rays:

  1. Lung bases 
  2. Free air
  3. Bowel-gas pattern
  4. Solid organs
  5. Soft-tissue masses
  6. Calcifications
  7. Bones

Bowel gas

  • The most radiolucent finding in the abdomen
  • The largest quantities are seen in the stomach and colon.
  • Small bowel:
    • Stacked-coin appearance
    • Large amounts of gas in the small intestine should be considered abnormal.
    • > 3 air-fluid levels in distended small intestine indicative of functional ileus or mechanical obstruction
  • Large bowel:
    • Located in the periphery of the abdomen
    • The haustra separate gas into larger segments.
    • No air-fluid levels should be seen due to fluid absorption.
    • Stool is seen as small bubbles of gas in the expected trajectory of the colon.
  • Gas in the peritoneal cavity is indicative of post-operative status or pneumoperitoneum.

Soft tissues and fat shadow

  • Soft tissues:
    • The abdomen is occupied predominantly by soft tissue.
    • Liver (RUQ)
    • Spleen (LUQ)
  • Fat shadow:
    • Fat deposits delineate organs.
    • The flank stripe (properitoneal fat stripe) can be seen on the lateral abdominal walls.
      • The flank stripe can be identified by following the course of the ascending or descending colon.
      • Widening of the space between the stripe and the colon suggests the presence of fluid.

Bones

  • The most radiopaque finding in the abdomen
  • Ribs, lumbar and thoracic spines, and pelvis
  • Calcifications (e.g., calcified arteries, urinary calculi, prostatic calculi, pancreatic calcifications)
Abdominal X-ray showing relevant structures

Abdominal X-ray with the relevant structures identified

Image by Hetal Verma.

Head and Spine

Imaging of the spine and spinal cord using X-rays was widely used to study the contents of the cranial vault and the bones of the spine before the advent of CT and MRI.

Projections

  • X-ray images of the skull are produced in the following projections:
    • PA
    • Lateral
    • Waters’ view (occipitomental)
  • X-ray images of the spine are produced in the following projections:
    • AP
    • PA
    • Lateral 
    • Oblique 
    • The open-mouth (odontoid) view allows for visualization of the odontoid process of the axis (C2 vertebra).
    • Panoramic
Orbital X-ray (Waters view)

Waters’ view of the skull:
This particular patient shows diffuse prominent mucosal thickening in the right maxillary sinus and mild mucosal thickening in the left maxillary sinus.

Image: “Orbital X-ray (Waters’ view)” by Erhan Erdogan, Vural Fidan, and Ersem Giritli. License: CC BY 4.0

Bones

  • In the skull, large quantities of X-rays are absorbed by the bones, making it difficult to visualize skull contents and soft tissues.
  • Spinal structures:
    • Vertebral bodies
    • Facet joints
    • Disk spaces
    • Pedicles
    • Laminae
    • Transverse and spinous processes
    • Neural foramen
  • A panoramic view of the spine may be obtained, but the following visualizations are possible:
    • The thoracic spine using a chest X-ray
    • The lumbar spine using an abdominal X-ray

Spinal alignment

  • The spinous processes, pedicles, and laminae of the vertebrae must be checked for adequate positioning.
  • Vertebral lines should be parallel:
    • Anterior vertebral line: connects the anterior margins of the vertebral bodies
    • Posterior vertebral line: connects the posterior margins of the vertebral bodies
    • Spinolaminar line: connects the posterior margins of the spinal canal
    • Interspinous line: connects the tips of the spinous processes

Limbs and Joints

X-rays are used to assess the bones and joints of the extremities in suspected fractures, joint problems, and soft-tissue abnormalities (inflammation, edema, or gas/emphysema, as seen in the cases of necrotizing fasciitis).

Projections

  • Specific projections to be requested depend on the bone or joint to be studied. 
  • Commonly used projections include:
    • Frontal view
    • Lateral
    • Oblique: a frontal view with 15 degrees internal rotation (usually requested for the study of the ankle joint)
  • Adequate imaging of bones and joints relies heavily on orthogonal imaging.

Joints

The following joints are commonly studied using conventional radiography:

  • Shoulder joint
  • Elbow joint
  • Wrist joint
  • Sacroiliac joint
  • Hip joint
  • Knee joint
  • Ankle joint

Fractures

The diagnosis of fractures can be made based on an X-ray of the affected limb or joint, usually utilizing 2 or more projections:

  • Clavicle fractures
  • Hip fractures 
  • Distal radius fractures
  • Pelvic fracture
AC Separation XRAY (enhanced)

X-ray of a separated acromioclavicular joint (gray arrow)

Image: “AC Separation XRAY (enhanced)” by Root4(one). License: CC BY 2.5

Other Imaging Modalities

  • Imaging of the CNS (brain, spinal cord, and vertebral column): 
    • Radiography is often used to evaluate fractures of the vertebral column. 
    • CT is a good choice to evaluate head trauma and exclude intracranial hemorrhage.
    • MRI provides more detailed images of the brain and spinal cord, allowing for the identification of infarction, tumors, disc herniation, and demyelinating disease.
  • Pulmonary radiology and imaging of the mediastinum: 
    • Radiography is the preferred initial imaging study for viewing lung pathology. 
    • CT provides more detailed views of the lung parenchyma, mediastinal structures, and vasculature. 
    • Although MRI is not often used, it could be ordered for evaluating malignancies and cardiac disease.
    • Ultrasound can be used for the rapid assessment of bedside trauma and for guiding procedures (thoracentesis).
  • Breast imaging: 
    • Mammography is often the initial choice to screen for breast cancer. 
    • MRI can be used to further evaluate and stage breast cancer. 
    • Ultrasound is helpful in evaluating lymph nodes and guiding biopsy.
  • Imaging of the abdomen and renal imaging: 
    • Radiography is often used to evaluate for kidney stones, bowel obstruction, and pneumoperitoneum. In addition, barium may be used to assess swallowing and bowel function. 
    • CT and MRI provide more detailed assessments of the abdominal viscera and vasculature. 
    • Nuclear medicine can be used to assess gallbladder function, gastric emptying, and GI bleeding.
  • Imaging of the uterus and ovaries: 
    • Ultrasound is the most commonly used modality to evaluate the ovaries and uterus, including pregnancy assessment and determining the causes of abnormal uterine bleeding. 
    • CT and MRI provide more detailed views and are often useful in assessing cysts, malignancies, and benign masses.
  • Imaging of the musculoskeletal system: 
    • Radiography is often used to exclude fractures. 
    • CT is more sensitive to bone pathology, including osteomyelitis. 
    • MRI is preferred for soft-tissue evaluation, such as assessing for malignancy and myositis. 
    • Bone scan can be useful in determining occult fractures, osteomyelitis, and metabolic bone disease.

References

  1. Berger, M., Yang, Q., Maier, A. (2018). X-ray Imaging. In: Maier, A., Steidl, S., Christlein, V., et al., Editors. Medical Imaging Systems: An Introductory Guide [Internet]. Cham (CH): Springer. https://www.ncbi.nlm.nih.gov/books/NBK546155/
  2. Chen, M.M. (2011). Chapter 8. Plain film of the abdomen. Chen, M.M., Pope, T.L., Ott, D.J. (Eds.), Basic Radiology, 2e. McGraw-Hill. https://accessmedicine-mhmedical-com.ezproxy.unbosque.edu.co/content.aspx?bookid=360&sectionid=39669017
  3. Dixon, R.L., Whitlow, C.T. (2011). Chapter 2. The physical basis of diagnostic imaging. In Chen, M.Y.M., Pope, T.L., Ott, D.J. (Eds.), Basic Radiology, 2e. New York, NY: The McGraw-Hill Companies. accessmedicine.mhmedical.com/content.aspx?aid=6668091
  4. Guha-Thakurta, N., & Ginsberg, L.E. (2011). Chapter 13. Imaging of the spine. Chen, M.M., Pope, T.L., Ott, D.J. (Eds.), Basic Radiology, 2e. McGraw-Hill. https://accessmedicine-mhmedical-com.ezproxy.unbosque.edu.co/content.aspx?bookid=360&sectionid=39669023
  5. Miner Haygood, T., Sayyouh, M.H. (2011). Chapter 6. Musculoskeletal imaging. Chen, M.M., Pope, T.L., Ott D.J. (Eds.), Basic Radiology, 2e. McGraw-Hill. https://accessmedicine-mhmedical-com.ezproxy.unbosque.edu.co/content.aspx?bookid=360&sectionid=39669014
  6. Wasserman, P.L., Pope, T.L. (2011). Chapter 7. Imaging of joints. Chen, M.M., Pope T.L., Ott D.J. (Eds.), Basic Radiology, 2e. McGraw-Hill. https://accessmedicine-mhmedical-com.ezproxy.unbosque.edu.co/content.aspx?bookid=360&sectionid=39669015
  7. Zaer, N.F., Amini, B., Elsayes, K.M. (2015). Overview of diagnostic modalities and contrast agents. In Elsayes, K.M., Oldham, S.A.A. (Eds.), Introduction to diagnostic radiology. New York, NY: McGraw-Hill Education. Retrieved from accessmedicine.mhmedical.com/content.aspx?aid=1115257266
  8. Zapadka, M.E., Bradbury, M.S., Williams, D.W. III. (2011). Chapter 12. Brain and its coverings. Chen, M.M., Pope, T.L., Ott, D.J. (Eds.), Basic Radiology, 2e. McGraw-Hill. https://accessmedicine-mhmedical-com.ezproxy.unbosque.edu.co/content.aspx?bookid=360&sectionid=39669022
  9. CDC. (2020). Radiation and Pregnancy: A Fact Sheet for Clinicians. Retrieved April 19, 2021, from https://www.cdc.gov/nceh/radiation/emergencies/prenatalphysician.htm

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