Medical Imaging (Lecture 1)

Who founded the X-ray?

Wilhelm Rontgen

Radiographic film before an image is taken is ___________, which is why it appears white on a view box.

Transparent

The more X-rays that reach an area of film, the _________ the area will be.

Darker

If an object is very _________, less X-rays will reach the film, and the object will appear white on the radiograph.

Dense

If an object has very little density, it will appear ____________ on a radiograph.

Black

X-ray

Form of electromagnetic radiation

-Energy of very short wavelength

The ___________ the wavelength, the greater its energy & greater ability to penetrate various material.

Shorter

How are X-rays measured?

Electron volts

Briefly describe how X-rays are produced.

X-rays produced when an electron beam hits a metal target

-Beam & target are enclosed in an X-ray tube

-X-ray tube has a special window to allow X-rays to escape through small opening that regulates the size of the beam

-Beam is directed at X-ray film placed behind patient

What 2 factors does absorption of X-radiation depend on?

1. Density

2. Atomic weight

The lower an object's atomic weight or density, the more __________ it is on an X-ray.

Transparent

Attenuation

Degree to which x-rays are absorbed/deflected

In an x-ray of a hand, the bones _________ more radiation than the surrounding tissue, therefore, they cast heavier "shadows' on the scan.

Attenuate

What color are "shadows" on x-ray film?

White

Where x-rays have reached the film, the film is exposed & turns ___________.

Black

What are the 4 types of radiographic densities?

1. Gas (air)

2. Fat

3. Water

4. Bone

What are some factors that can affect image quality?

1. Thickness of body part

2. Motion

3. Magnification

4. Distortion

How can you overcome a motion artifact when taking an x-ray?

Shorten exposure time

-Use an intensifying screen (found in all film holders)

As an object moves toward the source of the x-ray beam, its shadow becomes ____________.

Larger

When does distortion occur?

When object is not perpendicular to the x-ray beam

Plain films

The most common diagnostic x-ray format

-Chest x-ray, plain films of abdomen, skeletal films

-Variations: fluoroscopy, tomography

Fluoroscopy

Uses an x-ray tube & fluorescent screen

-Used for watching real-time motion

-Used for viewing motion of the heart, diaphragm, esophagus, or abdomen

-Also used for guided placement of GI tubes

-Real time images on x-ray

Contrast exams

Often used with plain films to highlight adjacent structures of similar densities

-Contrast - radiopaque materials in various liquid prep

-Plain films - evaluates GI tract, urinary system, or blood vessels

-Also used with CT/MRI

What are some important considerations before administering contrast to a patient?

Allergy to contrast

-Kidney function

Barium

Many forms available - used most in GI studies

Water-soluble agents

Used in urography

Biliary contrast agents

Absorbed in the gut & excreted in bile for biliary studies

MRI enhancement agents

Gadolinium

Oil-soluble agents

For inhalation in bronchograms

GI tract contrast studies

Usually barium sulfate is used (swallowing/instilled through tube)

-Barium swallow: hypopharynx & esophagus

-Upper GI: esophagus, stomach, proximal small bowel

-Small bowel follow through: small bowel

Tomography

X-ray tube & film move synchronously around a focal point

-Objects in focal point remain in focus, other aspects are blurred

-Images are slices of the area scanned

-Useful in evaluating lungs, kidneys, & bony structures

What are the 2 types of tomography?

1. Computed tomography (CT)

2. Positron emission tomography (PET)

CT indications

Evaluates internal organs, infections, injuries, masses, & suspected tumors

CT disadvantages

More expensive than plain films, often needs IV contrast, & increased exposure to radiation

CT scan

X-ray beam & detector system move in 360 degree arc around the organ

-Fine x-ray beam directed through patient

-Detector system measures amount of radiation passing through patient

-Data analyzed & various shades of gray assigned to radiation levels

How is a CT scan read?

As if looking at a patient from feet lying on their back

What are some conditions that require a CT scan with contrast?

Recurrent/persistent pneumonia, abdominal masses, sinus anatomy, pulmonary nodules, lung masses, empyema, PE, soft tissue

What are some conditions that require a CT scan without contrast?

Acute cerebral bleed, cranial/facial fractures, renal stones

Spiral (helical) CT

3D reconstruction of images

What are some indications for nuclear imaging radionuclide scans?

Evaluate physiology of organs like bone, heart, lungs, thyroid, & kidneys

What's the radioisotope used in nuclear imaging, and what is its half-life?

Technetium-99 (half-life = 6 hours)

**Possible exam question**

True/false: After a nuclear imaging scan, a patient is theoretically not radioactive after 24 hours.

True

Nuclear imaging

Depends on selective uptake of certain compounds by different organs of the body

-Organs are 'labeled' with a radioactive substance of sufficient energy level to allow detection outside the body

-Look for areas with radiation accumulation

When should you be cautious of ordering nuclear medicine studies?

On pregnant/lactating women

Bone scan

Looks at the skeletal system

-Radioactive phosphate coupled to technetium-99 (taken up by osteoblast & incorporated into bone --> scanned 3 hours later)

When should you order a bone scan?

Bone metastasis, osteomyelitis, trauma/fractures, metabolic bone diseases, avascular necrosis, arthritic disease

PET scan

Measures important body functions, like blood flow, O2 use, & glucose metabolism

-Used to evaluate tumors & cancers, myocardial function, & brain function

True/false: A PET scan can be used alone or in conjunction with a CT scan.

True

When should you order a tagged RBC study?

For an occult bleed

Tagged RBC study

Tag RBCs with Tc99m-pertechnetate

-Once inside the cell, the isotope is reduced & cannot diffuse back out

Thyroid scan

Used for any suspicion of thyroid disease (hypo/hyper)

-I131 injected --> active thyroid tissue takes up iodine & scan determines activity

What's an important consideration before ordering a thyroid scan on a patient?

They must not have done a CT scan with (iodine) contrast within the past 8 weeks!

Cold thyroid nodules

Areas of decreased function --> more likely to be cancerous!

-No iodine uptake in a thyroid scan

Hot thyroid nodules

Increased uptake --> sign of Graves/toxic multinodular goiter

When would you order a renal scan?

ARF, agenesis, congenital anomalies, decreased kidney function, ectopic kidney, HTN, obstruction, ureteral reflux

Renal scan

Kidney absorbs 99m-Tc & is cleared by filtration

Thallium stress test

Used to assess regional blood flow of coronary arteries (blood supply to heart muscle)

-Inject thallium (K+ analog); exercise vs. rest

-Stop BB & CCB for 72 hours

-No caffeine/nitrates for 24 hours

-Used to assess atypical CP, known CAD, extent of infarction

Ventilation/perfusion scan

Used to evaluate PE, R to L shunt, lung function before surgery

-Uses argon gas --> ventilation

-Tags RBCs --> perfusion

Gastric emptying study

Suspected delayed gastric emptying

-Radioactive labeled food (eggs)

-Delayed if > 50% of food remains in stomach after 2 hours

Hepatobiliary scan

Used to view gallbladder & bile ducts

-Fast for 4 hours

-Inject HIDA (hepatobiliary iminodiacetic acid)

Multiple gated acquisition scan (MUGA)

Radioactive substance "tags" or "labels" the RBCs & a gamma ray camera takes pics of the heart as the tagged RBCs circulate

-Looks at flow in the heart (how effective is the pump)

What are MRI's used for?

Looks at soft tissues, organs, & CNS

What type of contrast is used in MRI's?

Gadolinium

List some advantages of MRI's.

No ionizing radiation, non-invasive, & produces detailed images

List some disadvantages of MRI's.

Artifact (patient motion), no ferrous objects near the magnet, expensive, much slower than a CT

MRI mechanism

Apply a magnetic field to the body (aligns atoms)

-When atoms are released, radio waves are generated

-Frequency depends on type of tissue

MRI T1 images

Fat is white

Water is dark

MRI T2 images

Fat is dark

Water is white

MRI flair images

Similar to T2, but CSF is dark

True/false: Bone is well visualized on MRI's.

False

Nephrogenic systemic fibrosis

Risk of receiving MRI contrast

-Pt develops large areas of indurated skin with fibrotic nodules & plaques (similar to scleroderma)

-Gadolinium found in tissue samples (can be deposited in almost any tissue in the body)

NSF pathophysiology

Related to exposure of patients with renal insufficiency to gadolinium in imaging studies

-Mechanism unknown

-Pt could become wheelchair-bound because of contractures

What are diagnostic ultrasounds used for?

View organs & evaluate motion (cardiology, OBGYN, GI, & vascular medicine)

Diagnostic ultrasound

Echoes of the US beam bounce off interfaces between tissues with different acoustic properties

-Sends high-frequency sound into patient & assesses strength & time of returning echoes

-Echoes caused by changes in density

What are ultrasounds used for?

Obstetrics, cardiology, vascular, abdominal

US advantages

Inexpensive, non-invasive, no radiation

US limitations

Quality is technician-dependent

-Pt body habitus

Ultrasounds can also be used for procedure guidance. Give 2 examples of US-guided procedures.

1. Thoracentesis

2. Biopsy (needle placement)

What's an echocardiogram used to evaluate?

Valves, pericardial fluid, pressures, wall motion, ejection fraction, & masses