Comprehensive Guide to X-ray, Ultrasound, CT, MRI, and Nuclear Medicine Principles

Components for X-ray production

1. A source of electrons (Cathode) 2. A means to accelerate the electrons (high voltage) 3. A target to stop/decelerate the electrons (Anode)

Cathode

The negative electrode in the x-ray tube; it contains the filament and focusing cup.

Filament

A small coil of tungsten wire in the cathode that is heated by a low-voltage circuit.

Thermionic Emission

The process of 'boiling off' or releasing electrons from the filament when it is heated to a high temperature.

Focusing Cup

A negatively charged cup (made of molybdenum) that surrounds the filament; it condenses the electron cloud into a focused beam.

Anode

The positive electrode in the x-ray tube; it is the 'target' that the high-speed electrons strike.

Anode Target Material

Tungsten, because it has a high atomic number (for efficient x-ray production) and a high melting point (to withstand heat).

Rotating Anode

An anode disk that rotates at high speed to dissipate heat over a larger area (the focal track), allowing for higher exposures.

High Voltage (kVp) Application

The high negative charge at the cathode and high positive charge at the anode create a potential difference, accelerating the electron cloud from the cathode to the anode at high speed.

Kinetic Energy Conversion to X-rays

Only about 1% of the electron's kinetic energy is converted to X-rays.

Kinetic Energy Conversion to Heat

The other 99% of the electron's kinetic energy is converted into heat at the anode.

Bremsstrahlung Radiation

The primary type of x-ray. It's produced when a high-speed electron is 'braked' (slowed down and changes direction) by the positive nuclear field of a tungsten atom.

Characteristic Radiation

Produced when a high-speed electron collides with and ejects an inner-shell electron (e.g., K-shell) of a tungsten atom.

kVp (Kilovoltage Peak)

Controls the quality or penetrating power of the x-ray beam by controlling the speed/energy of the electrons.

mAs (Milliampere-Seconds)

Controls the quantity or number of x-ray photons produced by controlling the number of electrons boiled off (mA) and the duration of the exposure (s).

Role of Medical Imaging Modalities

A vital contribution to the overall diagnosis of the patient and help in the decision of an overall treatment plan.

Utilization of Medical Imaging

Increasing due to new technological advances in medical sciences.

Ultrasound (Sonography) Principle

Utilizes high-frequency sound waves (not ionizing radiation) to produce images.

Ultrasound Transducer (Probe) Function

Sends high-frequency sound waves into the body and then receives the returning echoes, which are processed into an image.

Ultrasound Image Creation

Based on the differences in acoustic impedance (the resistance to sound) between various tissues, which causes the sound waves to be reflected (echo).

Acoustic Coupling Gel Use

Used to eliminate air pockets between the transducer and the skin, ensuring a clear pathway for the sound waves.

Obstetrics

Clinical application of ultrasound for fetus imaging.

Cardiology

Clinical application of ultrasound for Echocardiography.

Abdominal Imaging

Clinical application of ultrasound for liver, gallbladder, and kidneys.

Musculoskeletal Imaging

Clinical application of ultrasound for musculoskeletal structures.

Doppler Ultrasound

Used to visualize blood flow and direction, often for vascular studies.

Abdominal Ultrasound Preparation

NPO (nothing by mouth) for several hours to reduce gas and ensure the gallbladder is full.

Pelvic Ultrasound Preparation

A full urinary bladder acts as an 'acoustic window' to better visualize the uterus and ovaries.

Computed Tomography (CT)

An imaging modality that uses x-rays and sophisticated computers to create detailed, cross-sectional images of the body.

CT Scanner Operation

An x-ray tube rotates around the patient, and detectors measure the attenuation values from different angles.

Hounsfield Units (HU)

Unit of measurement used in CT to represent tissue density.

CT Image Display

A computer reconstructs the 3D data set from all attenuation values, displayed in axial, sagittal, or coronal planes.

Contrast Media in CT

Used to enhance visibility and differentiation of blood vessels and soft tissue organs.

Key Clinical Applications for CT Scans

Trauma, Oncology, Vascular diseases, Neurological, and Abdominal/Pelvic assessment.

Critical Patient Preparation for CT with IV Contrast

NPO for 4-6 hours, and screening for contrast allergies, asthma, and kidney function.

Oral Contrast in CT

Used to opacify the GI tract so it can be distinguished from other abdominal structures.

Magnetic Resonance Imaging (MRI) Principles

Uses a powerful magnetic field, radiofrequency pulses, and a computer; does not use ionizing radiation.

Atoms Interacted with MRI

Hydrogen atoms (protons) in the body's water molecules.

MRI Step 1

The patient is placed in a powerful magnetic field, aligning hydrogen atoms.

MRI Step 2

A radiofrequency pulse is applied, knocking the aligned protons out of alignment.

MRI Step 3

Protons relax back to their original alignment, emitting signals detected by receiver coils.

Tissue Differentiation in MRI

Different tissues relax at different rates, producing different signal intensities and creating image contrast.

MRI Superiority

Superior for imaging soft tissues, ideal for Neurological, Musculoskeletal, and Cardiovascular imaging.

MRI Patient Safety Screening

Screening for any metal in or on the body due to the powerful magnet.

Patient preparation for MRI

Screening for metal, removing all jewelry/piercings, and screening for claustrophobia (as the machine is a long, narrow tube).

Contrast agent used in MRI

Gadolinium-based contrast agents.

Primary purpose of Nuclear Medicine

To evaluate the function (physiology) and metabolic activity of organs, rather than just their anatomy.

How Nuclear Medicine creates images

Small amounts of radiopharmaceuticals (tracers), which are radioactive materials administered to the patient.

Creation of nuclear medicine image

The patient emits gamma rays from the tracer that has accumulated in the target organ. A gamma camera detects these rays to create an image (a scintigram).

SPECT

Single Photon Emission Computed Tomography; a rotating gamma camera creates 3D functional images (slices).

PET

Positron Emission Tomography; uses tracers that emit positrons to map metabolic activity. It is often used with an FDG (glucose) tracer to find metabolically active areas, like cancer.

Common clinical applications for Nuclear Medicine

Bone scans (for metastasis), Cardiac perfusion (heart function/blood flow), Thyroid uptake scans (for hyper/hypothyroidism), and Renal scans (kidney function).

Patient preparation for Nuclear Medicine

May include NPO, hydration, specific dietary restrictions (e.g., for PET scans), and checking for allergies to the radiopharmaceutical.

Mammography

A specialized, low-dose x-ray imaging modality used specifically for breast tissue.

Primary goal of mammography

To detect breast cancer and other abnormalities (like microcalcifications) at their earliest, most treatable stage.

Importance of breast compression in mammography

1. Spreads out overlapping tissue. 2. Reduces the thickness of the breast (which lowers the required radiation dose). 3. Immobilizes the breast to prevent motion blur and improve image quality.

Difference between screening and diagnostic mammography

Screening is done on asymptomatic women to detect early cancer.

Diagnostic

Evaluation of specific symptoms (like a lump, pain, or discharge) and may include extra views or ultrasound.

Digital Breast Tomosynthesis (DBT)

Also known as '3D Mammography'; the x-ray tube moves in an arc, taking multiple low-dose images from different angles to create 'slices' that reduce the problem of overlapping tissue.

Key patient preparation for a mammogram

Schedule the exam for the week after the menstrual period (when breasts are less tender). Do not wear deodorant, talcum powder, or lotion on the chest/underarms (as they can mimic calcifications on the image).

Angiography

An x-ray imaging modality used to visualize blood vessels (arteries and veins) after injecting a contrast media.

Conventional angiography

A catheter is inserted into an artery (like the femoral artery) and guided under fluoroscopy (live x-ray) to the area of interest. Contrast is then injected to make the vessels visible.

Digital Subtraction Angiography (DSA)

A technique where a 'mask' image (pre-contrast) is taken first. The computer then subtracts this mask from the contrast-filled images, leaving only the vessels visible with high contrast.

Interventional Radiology

A subspecialty that uses imaging (like angiography and fluoroscopy) to perform minimally invasive treatments, such as angioplasty (opening a vessel), stenting (placing a tube), or embolization (blocking a vessel).

Common clinical applications for angiography

Detecting aneurysms, stenosis (narrowing), occlusions (blockages), vascular malformations, or internal bleeding.

Key patient preparation for an angiography

NPO, check for contrast allergies, check kidney function (creatinine), and possibly stop taking blood-thinning medications.

Fluoroscopy

An imaging modality that provides 'live' or 'real-time' x-ray imaging, allowing for the visualization of movement and physiological function.

Difference between fluoroscopy and standard radiography

Radiography produces a static (still) image. Fluoroscopy produces a dynamic (moving) image, like an 'x-ray movie,' using a continuous or pulsed stream of low-dose x-rays.

Key components of a fluoroscopy unit

An x-ray tube (often under the patient) and an Image Intensifier or Flat-Panel Detector (over the patient) that captures the x-rays and converts them to a visible light image.

Function of an Image Intensifier

It electronically amplifies or brightens the very faint fluoroscopic image, making it bright enough to be viewed on a monitor.

Common contrast media used in fluoroscopy

Barium sulfate (for GI tract studies, e.g., UGI, Barium Enema) and Iodine-based contrast (for vascular or urinary studies).

Common clinical applications for fluoroscopy

GI studies (Upper GI, Barium Enema, Swallowing studies), Orthopedic procedures (setting fractures, joint injections), Interventional procedures (catheter placement, angiography), and observing diaphragm motion ('sniff test').

Radiographer's role during a fluoroscopic procedure

Educate the patient, prepare the contrast media, assist the radiologist, operate the equipment, and ensure strict radiation safety (ALARA).

3 cardinal rules of radiation safety (ALARA)

Minimize Time, Maximize Distance, and Use Shielding (lead aprons).

Fluoroscopy features used to reduce patient dose

Pulsed fluoroscopy (rapid 'snapshots' instead of a continuous beam) and Last-Image Hold (displays the last captured image on the monitor without continuous exposure).