Introduction to Radiology

Gold Standard (GS) / Test of Choice (TOC)

The test that is the most accurate at providing a diagnosis among all the known tests. The MOST SENSITIVE AND SPECIFIC.

Radiation

Energy released by unstable atoms which are said to be "radioactive".

Inverse Relationship of Wavelength and Energy

Shorter the wavelength = higher energy, making it easier to penetrate various materials.

Scatter Radiation (Secondary Radiation)

Radiation produced from the primary beam emitted when it bounces off an object, causing undesirable dark density/haziness.

Attenuation

The process by which an object absorbs or blocks X-rays, resulting in image production.

Orthogonal Views

Two views taken ideally at 90 degrees to one another because a radiograph is a 2D representation.

ALARA

As Low As Reasonably Achievable; principle used to minimize radiation risk when medical benefits outweigh risks.

Three Ways to Decrease Radiation Exposure

Time, Distance, and Shielding.

Density

The degree of blackness in a film.

Contrast

The difference in radiographic densities on a film; represents the gray scale.

Radiopaque / Radiodense

Structures which block X-rays, produce less blackening, and show up gray to white (e.g., bone).

Radiolucent

Structures which allow X-rays to pass through, produce more blackening, and show up gray to black.

What are the 4 Natural Radiographic Densities (Least to Most dense)

1. Air, 2. Fat, 3. Soft tissue, 4. Bone

What are the 'non-natural' occurring densities?

Metal and Plastic.

Approach to reading an image to follow EVERY TIME

Check for Right Patient, Right Date, Right Study, and Right View by reading labels every time.

Image Orientation Rule

Orient the film as if facing the patient; the patient's Left will be on your Right.

PA (Posterior Anterior) Projection

X-ray projection where the beam travels through the body from back to front. (Most commonly used)

AP (Anterior Posterior) Projection

X-ray projection where the beam travels through the body from front to back.

Magnification & Distortion Rule

The further an object is from the image detector, the greater the magnification; place the area of interest closest to the image detector.

Adequacy

Verifying if all intended anatomy is included and if positioning/exposure is correct.

Alignment

Evaluating if the relationship between all structures is normal.

Rotation Check on CXR

Identified by measuring the spinous process distance from the clavicular heads (should be midline).

Ionizing Modalities

Radiography, Fluoroscopy, Contrast-enhanced imaging, CT, Isotope bone scan, DEXA scan, and PET scan.

Non-Ionizing Modalities

MRI, US (Ultrasound), and Doppler.

Fluoroscopy

Imaging technique using a continuous beam of X-rays to visualize internal structure movement in real-time.

Incidentalomas

Unexpected findings that have implications for further medical care.

Deterministic (Non-stochastic) Effects

Predictable radiation effects occurring when dose exceeds a threshold (e.g., skin burns, hair loss).

Probabilistic (Stochastic) Effects

Random occurrences at any dose with no threshold; probability increases as dose increases (e.g., cancer).

Most Common Contrast Agents

Iodine (Atomic number 53) and Barium (Atomic number 56).

Barium Sulfate

Contrast used to increase the density of structures we are examining (ie: the GI tract.)

Water-Soluble (Iodinated) Agents

Contrast agents that do not cause a desmoplastic reaction and are safely excreted by kidneys.

Metformin Contrast Rule

Stop 24 hours before and 48 hours after contrast due to risk of lactic acidosis.

How do you determine what imaging would be most appropriate?

Consider WHAT to order, WHEN to order it, and HOW to order it.

Why is radiology imaging helpful?

Allows us to establish or exclude disease, assists in differential diagnosis, monitors disease course, and helps select/adjust therapy.

What are the benefits of X-Ray?

Quick to take, widely used, non-invasive, and can be interpreted by a non radiologist.

What are the disadvantages of X-Ray?

Can be expensive, carries risk of morbidity/mortality (radiation, contrast anaphylaxis), and occurrence of incidentalomas.

What are "incedentalomas?"

Finding something you weren't looking for that now requires more testing and further evaluation/care.

What should your approach be to ordering tests?

Tests should only be ordered if they will affect patient management.

What are the two main imaging modalities?

Ionizing (Radiography, fluoroscopy, contrast enhanced imaging) and Non-Ionizing (MRI, US, Doppler)

What were the diagnostic techniques to identify a fracture before X-Ray?

Palpation and observation

Who "discovered" the X-Ray?

Dr. Wilhelm Conrad Roentgen

Characteristics of X-Rays:

Travels in short wavelengths (shorter = higher energy) and behaves like a particle (photons at speed of light).

An X-Ray Machine is essentially?

A camera

Absorption of the X-ray beam is affected by what?

The atomic mass of the object.

What does image production from an X-Ray result from?

The attenuation of those X-Rays by the object through which they pass.

What is attenuation?

The gradual loss of intensity, force, magnitude, or strength of something as it travels or spreads.

What order are these substances in, in terms of radiographic densities? ---> Metal, Bone, Muscle, Blood, Liver, Lung, Fat, Air

Most dense to least dense

A uniform beam of X-rays would expose the film in its entirety and the result would be what?

An entirely black image.

Depending on ____ to which the beam is absorbed (by different objects,) the film is increasingly lighter in contrast relative to full exposure black.

The Degree

What is a concern regarding ionization?

When it occurs in the tissues, it can lead to cellular damage.

Why is the risk of cellular mutation from radiation higher for younger people?

Rapidly dividing cells/growth, longer lifespan for cancer development, and varying age-group sensitivities.

When reading an image, what is the orientation of the patient?

The patient is "looking at you."

When might we use distortion with an X-Ray?

When attempting to get a view blocked by structures, like seeing lung apices past collarbones for TB.

What do you need to ensure anatomically when examining an X-ray image?

Ensure the entire structure being evaluated is in frame (e.g., hip to knee for a femur).

Why is rotation important to note in radiology?

It can cause abnormal distortion of structures, like making the heart appear larger or trachea off midline.

When is Barium CONTRAINDICATED?

If there is suspicion of GI integrity (perforation).

What happens if Barium is leaked into the peritoneum, or other unintended body cavities?

It can have a desmoplastic reaction; use gastrografin, omnipaque, or iodine-based contrast instead.

When are iodinated (Iodine) contrasts CONTRAINDICATED?

If the patient has a potential aspiration risk or elevated BUN/Creatinine levels.

What are the 5 "basic" densities we look for on a CT scan?

Air, fat, muscle, bone, metal.

What is the hounsfield density for water?

0

What does a higher hounsfield unit indicate?

A denser structure.

What does a lower hounsfield unit indicate?

A less dense structure.

Which type of scan is "safer" for pregnancy?

MRI (but still not recommended)

Which type of scan is better for soft tissue imaging? (Think brain.)

MRI

Ionizing Advanced Imaging Modalities

CT, DEXA scan, Isotope bone scan (Nuclear medicine), and PET scan.

Non-Ionizing Advanced Imaging Modalities

MRI, US, and Doppler.

Conventional X-ray Limitation

Soft tissues all have essentially the same grayscale, making it difficult to distinguish pathology.

Hounsfield Units (HU)

The digitized numbers representing the attenuation (density) of the X-ray beam by an object in a CT scan.

Air HU Value

-1000 HU (appears as a black density).

Fat HU Value

-40 to -120 HU (simplified as -100 HU).

Water HU Value

0 HU (assigned as the middle of the grayscale, appears gray).

Simple Fluid HU Value

-10 to 20 HU.

Soft Tissue HU Value

+20 to +100 HU (simplified as 30 to 45 HU; liver is 100 HU).

Acute Blood (Clotted) HU Value

60 to 90 HU.

Iodinated Contrast HU Value

100 to 500 HU (depends on concentration).

Trabecular Bone HU Value

300 to 800 HU.

Cortical Bone HU Value

+400 to +600 HU (simplified as >1000 HU; appears as a white density).

Metal HU Value

+1000 HU or higher.

CT Windowing

Post-processing manipulation of pixel gray scales to better evaluate specific organ systems (Soft tissue, Lung, Bone) without re-exposing the patient to additional radiation.

CT Windowing: Soft Tissue Window

Differentiates soft tissues better; used for the mediastinum and abdomen.

CT Windowing: Lung Window

Allows the viewer to see lung parenchyma easier.

CT Windowing: Bone Window

Differentiates the bone cortex from the medullary canal.

CT Limitations

Subject to motion artifact, unable to detect acute ischemic stroke in the first 1-2 days, unable to detect demyelination, and has poorer spatial resolution compared to MRI.

Contraindications to CT Contrast

History of anaphylaxis to contrast media, and renal dysfunction.

CT Imaging Views

Axial (transverse plane viewed from feet upward), Sagittal, and Coronal.

MRI Mechanism

Uses a magnet to align hydrogen nuclei (mostly water) in the body, disturbs them with radiofrequency (RF) waves, and calculates images based on the unique RF signal returned by different cells.

T1 Weighted Sequence

The "Anatomy scan" where receivers listen "early" at the start of the scan; water/infection appears dark/hypointense, and fat appears white/bright/hyperintense.

T2 Weighted Sequence

The "Pathology scan" where receivers listen "late" to search for inflammation; water/pathology lights up white/bright/hyperintense, and low water content is dark.

MRI Absolute/Key Contraindications

Cerebral aneurysms clipped by ferromagnetic clips, cardiac pacemakers (unless compatible), inner ear implants, and metallic foreign bodies in/around the eyes.

MRI Disadvantages

Expensive, long scan times (45 mins/plane), prone to motion artifact, induces claustrophobia, and produces a loud jackhammering or thumping noise.

DEXA Scan Function

Measures bone mineral density (BMD) using spectral imaging to screen for osteoporosis, utilizing 1/10th the radiation of a chest X-ray.

DEXA Screening Indications

All women over age 65, men with non-traumatic long bone or vertebral fractures, and patients on long-term corticosteroid treatment.

DEXA T-score

Compares a patient's BMD to a healthy 35-year-old female BMD; used to define osteopenia and osteoporosis.

DEXA Z-score

Compares a patient's BMD to an age and sex-matched control population.

Osteopenia Criteria

A DEXA T-score between -1 and -2.5.

Osteoporosis Criteria

A DEXA T-score below -2.5 (Standard deviation).

DEXA Fracture Risk Rule

For every standard deviation below normal, the risk of an insufficiency fracture doubles (T-score -1 = 2x risk; T-score -2 = 4x risk).

Technetium 99

The most commonly used radioactive isotope in nuclear medicine; emits gamma radiation and has a 6-hour half-life.

Organ-Specific Nuclear Tracers

Thyroid takes up Iodine; Brain utilizes glucose; Kidney excretes medications; Bone utilizes phosphates; Lung traps certain particle sizes.

Nuclear Medicine Safety Guardrails

Avoid in pregnant or breastfeeding mothers. To limit technologist/other exposure to the patient (the source), utilize decreased time, increased distance, and shielding.