DMS100 Introduction To Imaging Physics

DEFINITION: An imaging technique that uses ionizing x-rays radiation to create an image of the internal body structures of a patient.

HISTORY: X-ray radiography is the oldest form of medical imaging. X-rays were discovered by Roentgen in 1895. He noticed that a cathode ray tube exposed a paper coated with barium compound placed at a distance.  

PHYSICAL PRINCIPLES: X-rays are generated in a cathode ray tube (x-ray tube). A beam of electrons generated from the cathode is accelerated towards a metal disk (anode) by a high voltage current.  Upon impact, the kinetic energy of the electrons is transformed into x-rays.

 The X-rays are passed through the body and captured by a detector placed behind the patient. The detector is usually a photo-sensitive film or a digital detector. Absorption of x-rays in the body varies, with dense bones absorbing more radiation and soft tissue allowing most radiation to pass through. This variation produces a contrast in the image, giving a 2D representation of the internal structures of the patient.

Medical Applications:

• Chest: to examine the lungs
• Skeletal: examination of bone structure and diagnosis of fractures, dislocations
• Abdomen: assessing abdominal obstruction, free fluid or free air within the abdominal cavity.
• Dental: to assess cavity and other dental pathology

Advantages: it’s not invasive technique

Disadvantage: 

• organs may obscure each other
• X-rays are ionizing and may damage tissues

Computed Tomography (CT Scan)

DEFINITION: Is a medical imaging technique that uses a rotating x-ray tube to take multiple x-ray projections from different angles to give detailed cross-sectional images of internal areas of the body.

PHYSICAL PRINCIPLES: When an x-ray beam passes through body tissues, it is progressively weakened and the final intensity recorded on the detector. The attenuation depends on the type of tissues it passes through. For each probe, a summary effect of beam attenuation by all tissues can be obtained. Collecting many images enables the reconstruction of radiation absorption by a specific point and thus, CT scans are able to give precise identification of the examined tissues and also reconstructs organ shapes.

Multiple scans of the same cross-section from different angles make it possible to create a 3-D visualization of the anatomic details of the patient.

Clinical applications:  

• imaging of soft tissues like brain,blood vessels, heart, lungs and abdomen,
• Diagnosis of tumors
• Pelvic scan

Advantages: non-invasive technique

Gives precise identification of examined body parts as there is no tissue obstruction (speedy diagnosis)

Disadvantages:  exposure to high doses of ionizing radiation (due to multiple scans) may damage tissues

DEFINITION: Medical imaging techniques that uses the interaction of radio-waves with a magnetic field to produce detailed real-time images of tissues and organs.

PHYSICAL PRINCIPLES: Some atomic Nuclei like Hydrogen (Proton) possess a spin and have magnetic moments. MRI Scan analyses the electromagnetic radiation generated by protons in the body. a strong magnetic field is applied which causes the nuclei spins become aligned and spatially oriented. An electromagnetic radiation (radio-waves) is then shone through the aligned nuclei, exciting them. The excited nuclei respond by oscillating in harmony, producing a signal that is recorded.  By monitoring the signal, it is possible to tell where the protons are abundant and where they are less.

Clinical Applications: Spinal MRI, Brain MRI,  Abdominal MRI for assessment of liver, kidneys, spleen etc., Cardiac MRI, Chest MRI and Angiography. 

Advantages:

• No exposure to harmful radiation. Radiowaves are safe.
• Non- invasive technique

Disadvantages:

• patients may experience claustropbobia during an MRI examination
• Cannot be used in patients who have metallic implants as exposure to the powerful magnets may interfere with the functioning of the implants
• Injuries (and even deaths) may result if metallic objects enter the MRI room (through projectile motion of these objects towards the magnets)

DEFINITION: The use of radioactive isotopes in medical diagnosis or treatment.

PHYSICAL PRINIPLES: Nuclei of radioactive isotopes (radionuclides) spontaneously emit radiations. The intensity of these radiations can be monitored continuously using appropriate detectors. Appropriate doses of Radionuclides are administered to the patient either through intravenous injections, oral administration or breathing in. the path of these nuclides in the body are monitored by recording the radiations emitted using appropriate detectors.  

For diagnosis, a radioactive isotope is introduced to a pharmaceutical that is specific for the organ being examined.  The radionuclide emits radiation which is monitored by a radiation sensitive detector that detects the distribution of the radionuclide in the body. The information collected is used to produce a digital image of the organ on a screen.

For treatment, a high dose of radioactive isotope is introduced into a pharmaceutical specific to the targeted organ. Once at the target organ, the emitted radiation illuminates the targeted cells killing them. This is usually used for treatment of tumorous cells.

Clinical applications: 

• Bone scans for assessing the bones’ metabolic activity for arthritis and oncology staging.
• My cardial Perfusion Scan for comparison of blood flows during rest and exercise (to differentiate infarction from ischaemia)
• Renal scan for drainage and perfusion of kidneys examination
• Lung scan for diagnosis of pulmonary embolism
• Thyroid scan for examining the thyroid gland functioning
• Chemotherapy for treatment of malignant tumors

Advantages:

• Provides anatomic and functional information that cannot be attained by other techniques.
• Can diagnose (and treat) malignant tumors therefore avoiding the need for costly surgery
• Can detect diseases in early stage even before symptoms start appearing

Disadvantages:

• Exposes patients to radiation that can damage body tissues.
• Images obtained may not be very sharp (since only low doses are administered)

DEFINITION:  Ultra-sound is an imagining technique that involves the use of high-frequency sound waves to produce cross-sectional images of body tissues. Vascular ultrasound is a specialized ultra-sound technique that evaluates the flow of blood through blood vessels to identify blockages and blood clots.

PHYSICAL PRINCIPLES:  The ultra-sound machine consists of a transducer (probe), processor, data storage and key-board and monitor. The transducer emits sound waves at a certain frequency, which penetrate the body tissues. The waves are reflected by body structures and bounce back. The frequency of the reflected sound wave is captured by the transducer which digitizes them to appear as dots (echoes) on the screen (monitor).

Clinical applications:

• Abdominal  and intraluminal ultrasound
• Pelvic ultrasound
• Cardiovascular ultrasound
• Obstetric ultrasound
• Transfontanelle ultrasound
• Small parts ultrasound for organs such as breast, scrotum, thyroid gland etc.

Advanrages:  

• Ultrasound is very safe with minimal if any adverse effects
• No exposure to radiation

Disadvantages: 

• May potentially cause cavitation (bubbles) within biological structures.

References

https://www.who.int/diagnostic_imaging/imaging_modalities/dim_plain-radiography/en/index5.html

https://www.medicalimaging.org/about-mita/medical-imaging-primer/