Medical Imaging Fundamentals

Radiation and Imaging

• Doubling the distance from the X-ray source reduces exposure by 4 times.
• Closer to the light source, the image is larger, and closer to the wall, the image is sharper.

Contrast Media

• Radiopaque fluids like iodine and barium are used in medical imaging.
• They are administered through intravenous (IV), oral routes, or catheter into various cavities.
• These fluids enable the study of luminal or vascular organs.

Fluoroscopy

• Fluoroscopy is a 2D imaging technique using continuously emitted X-rays for real-time imaging.
• It is often used to assess the GI Tract.
• Provides insight into the morphology and motility of these organs.

Computed Tomography (CT)

• CT is an imaging technique using X-rays to create tomographic images.
• Patients lie supine on a scanner table and pass through the CT gantry (X-ray tube and detectors that rotate around the patient).
• A computer uses X-ray projections to reconstruct images.

Viewing CT and MRI Images

• CT images (and MRI) are viewed as if you are standing.
• Axial: viewed from the supine position, with the patient's feet at the top.
• Coronal: viewed from the front, facing the patient.
• Sagittal: viewed from the patient's left side.

MRI Scans

• MRI uses magnets and radio waves to create detailed cross-sectional images.
• The machine generates a strong magnetic field that excites the nuclei of atoms in the body.
• Radio waves temporarily disrupt this alignment, causing the nuclei to emit energy signals that vary in different tissues.
• These signals are detected by a coil around the body.
• MRI does not involve harmful radiation and is safe during pregnancy and for children.
• People with metal objects inside their body should not have an MRI due to the risk of injury.

Ultrasonography/Ultrasound

• Ultrasound uses ultrahigh frequency sounds that bounce off due to changes in density.
• High density = brighter image.
• Bone reflects nearly all ultrasound waves, making it unsuitable for CNS imaging.
• Ultrasound is operator-dependent.
• It cannot be used on the adult brain or lungs due to the presence of air.
• Gel is used to get better images and avoid interference.
• Doppler ultrasound measures the speed of blood flow using frequency shifts.
• For abdominal ultrasounds, having a full bladder helps by acting like a "window" and transmits sound through.

Nuclear Medicine Imaging

• Nuclear Medicine Imaging (NMI) reveals information about the spread and concentration of small amounts of radioactive substances in the body.
• A radioactive compound is attached to a substance that a specific organ absorbs.
• A detector displays where the radiotracer is in the body.
• NMI is used for diagnosis and to treat cancer, where radiotracers gather in tumors and emit particles that kill cancer cells.

Conventional Radiology (Plain Radiographs)

• Radiography uses X-rays for digital 2D image creation.
• X-ray images (radiographs) are obtained by passing X-rays through a patient onto a detector or film.
• Posterior Anterior (PA) projection involves the X-ray entering from the patient's posterior aspect and exiting anteriorly to reach the detector/film.
• AP radiograph is the opposite of PA.
• Radiographs are typically viewed with a standard orientation, as if the patient is facing you, with feet and hands appearing as if you are looking at your own.

Conventional Radiology - Radiopacity and Radiolucency

• Dense tissues absorb/reflect more X-rays, making them "radiopaque/radiosense" (bright area).
• Less dense tissues are "radiolucent" (allow more X-rays to pass through).
• Air appears black on the X-ray image.
• Fat is depicted as dark grey.
• Soft tissue appears as brighter grey.
• Bone appears white.
• Metal appears even whiter.