Alternate Imaging Modalities

What is fluoroscopy + how is it different from a radiograph?

Imaging technique to obtain real-time moving radiographic images of internal structures

Video vs image, opposite: bones are black, air is white

What are some examples of fluoroscopy?

• Esophagram: barium paste to visualize swallowing, collapsing trachea

• Angiocardiogram: iodine injected to see patent ductus arteriosus, occluder to close

• Ↄ-Arm: portable fluoroscopy

• Orthopedic procedures: acetabular prosthesis

Basic principles of CT scanning

• Highest radiation exposure

• Cross-sectional imaging without superimposition, no overlying structures

• Tissue attenuates least radiation = white, most radiation = black

• X-ray tube spins around patient, detectors determine amount of radiation absorbed by patient

Single slice vs spiral

• Single: complete rotation of tube then table moves before next rotation of tube

• Continuous rotation of tube linked to continuous movement of table

What is a multi-detector CT?

• New scanners have multiple rows of detectors

• 4, 16, 64, 128, 320 slice detector spiral scanner

Pixel vs Voxel

• Pixel: picture element, has only two dimensions (x, y)

• Voxel: volume picture element, has three dimensions (x, y, z)

What do voxel values mean?

• Value of voxel = attenuation coefficient

• Thicker slice = bigger volume, have to take average of volumes

• Thinner voxels produce a higher resolution

What is an isotropic voxel?

X, Y, and Z dimensions are equal —> allows for multi-planar reconstructions and volume rendering without loss of resolution

What are Hounsfield Units used for in CT?

• Measure of radiodensity to describe how much an x-ray beam is absorbed by different tissues

• > 4,000 HU values

• Typical monitor displays only 256 shades of gray

• Human eye can differentiate only 20 shades

• Fat negative, absorbs least, bone positive, absorbs most

What is a window and level in terms of HU?

Window: range of HU represented on a specific image

Level: central value of HUs displayed

*All post-CT scan processing

When are wide and narrow windowing best used?

• Wide windowing (400-2,000 HU) best for tissue types that vary greatly —> see all various tissues in one image

• Ex: lung - both low-density parenchyma and high-density vascular structures

• Narrow windowing (50-400 HU) best for tissue types with similar densities

• Ex: brain - white + gray matter

Describe hyperattenuation on CT scan

Tissue is more dense = absorbs more radiation and blocks x-rays

Areas appear white —> bone, metal

Highest HU values

Describe hypoattenuation on CT scan

Tissue is less dense = absorbs less radiation and allows more X-rays to pass through

Areas appear black —> fat, fluid, air

Lowest HU values

Difference between x-ray and CT

X-ray provides quick, flat 2D image

CT scan combines multiple X-ray angles to create detailed 3D cross-sections of bones, organs, and soft tissues

Advantages to CT

• More detailed view of anatomy without superimposition

• Faster and less expensive than MRI

• Best for bone, lung, abdomen

Disadvantages to CT

• Higher radiation exposure

• More expensive than radiographs

• Poor soft tissue contrast when compared to MRI

What is different about an MRI scan compared to an x-ray or CT?

No ionizing radiation

How does image formation work on MRI? (full)

1. Atoms with an odd number of protons possess a magnetic moment

2. Hydrogen (abundant in body) contains 1 proton and thus acts like a tiny magnet

3. Magnetic moment allows hydrogen atoms to function as millions of tiny magnets

4. Normally they are randomly aligned, which cancels out any net magnetic moment

5. Some hydrogen nuclei line up in direction of magnetic field and other hydrogen nuclei line up opposite to direction of magnetic field

6. More nuclei align with magnetic field than against it —> takes less energy to align with than against

7. Transiently a radiofrequency coil applies energy

8. RF energy causes non-cancelled out H to flip to opposite direction

9. After a short period RF pulse is stopped

10. Energy released from H as it returns to its normal orientation

11. This energy release induced current in receiver coil

How does image formation work on MRI? (shortened)

1. Hydrogen protons in tissue align with magnetic field

2. Transient RF pulse shifts protons out of alignment

3. RF pulse stops; protons realign and emit energy

4. Emitted energy induces a signal in receiver coils to construct image

What is weighting in MRI and what are the main weights??

• Method of adjusting machine settings to make specific tissue properties stand out in the final image

• T1-weighted and T2-weighted

How does T1 weighted appear?

• Fluid is hypointense (black)

• Fat is hyperintense (white)

• Great for showing clear anatomical structures

T1 relaxation times

• Fat: short relaxation times (hyperintense - white)

• Tissues: intermediate relaxation times (intermediate intensity - gray)

• Free water, pure liquids like CSF: long relaxation times (hypointense - black)

How does T2 weighted appear?

• Fluid AND fat are hyperintense (white)

• Muscle is hypointense (black)

• Great for spotting cysts, swelling like edema

MRI terminology: hypointense

Less bright than tissue you compare to

MRI terminology: isointense

Same brightness as tissue you compare to

MRI terminology: hyperintense

Brighter than tissue you compare to

Advantages to MRI

• Better soft tissue contrast and detail

• Noninvasive

Disadvantages to MRI

• Long scan times

• General anesthesia

• Cost

Comparing CT and MRI

CT

• Better bone detail

• Good for thorax and abdomen

• Heavy sedation or general anesthesia

• Faster scan time: seconds/minutes

• Safety issues: ionizing radiation

MRI

• Better soft tissue detail

• Good for neuroimaging and musculoskeletal

• General anesthesia

• Longer scan time: 1-2 hours

• Safety issues: projectiles, metal implants

Ultrasound wave frequency

• Higher than humans can hear

• Audible sound: 15k-20k Hz

• US: 2-20 million Hz (2-20 MHz)

Ultrasound transducers transmitting vs receiving

Transmits 1% of time —> propagates sound waves into tissues

Receives 99% of time —> measure intensity and time of arrival of returning sound waves

Sound is a vibration that transmits energy by…

alternating regions of high pressure (compression) and low pressure (rarefaction)

Relationship between frequency and wavelength on US

Frequency inversely related to wavelength (w = 1/f)

Lower frequency transducers penetrate further —> lower resolution

Higher frequency transducers do not penetrate far —> higher resolution

How to measure distance to object on US

• Image formed using echoes returning from tissues

• Sound travels at a relatively constant velocity in soft tissue (1540 m/s)

• Distance of reflector from transducer is based on time of flight

US terminology: anechoic

Black

US terminology: hypoechoic

Darker (dark gray) than compared structure

US terminology: isoechoic

The same echogenicity to compared structure

US terminology: hyperechoic

Brighter than compared structure

Echogenicity of body tissues and substances from anechoic to hyperechoic

• Bile, urine

• Renal medulla

• Muscle

• Renal cortex

• Liver

• Spleen

• Prostate

• Bone, gas, organ boundaries

Doppler Effect in terms of US and function of it

• Perceived change in frequency —> result of motion between sound source and observer

• Allows determination of velocity and direction of blood flow —> RBCs sound source, transducer observer

Describe color flow mapping in US

• Signals from moving RBCs displayed in color as a function of their motion towards or away from transducer

• BLUE - away

• RED - towards