Alternate Image Modality

What are the 4 alternate imaging modalities covered in this lecture?

Fluoroscopy, Computed Tomography (CT), Magnetic Resonance Imaging (MRI), and Ultrasound (US)

What is fluoroscopy?

An imaging technique commonly used to obtain real-time moving radiographic images of the internal structures

How does a fluoroscopy image differ from a standard radiograph?

Fluoroscopy produces a continuous, real-time series of images (a "movie"), while radiography produces a single static image

What is an esophagram used to evaluate with fluoroscopy?

Swallowing function and the esophagus in real time

What is a C-arm?

A portable fluoroscopy unit used for real-time imaging, commonly used in orthopedic procedures

Give two clinical uses of fluoroscopy shown in the lecture

Esophagram (swallowing study), collapsing trachea evaluation, angiocardiogram, and orthopedic procedures

What is ultrasound (US)?

An imaging modality that uses high-frequency sound waves reflected off internal tissues to create real-time images

What frequency range is audible sound?

15,000-20,000 Hz

What frequency range is used for diagnostic ultrasound?

2-20 million Hz (2-20 MHz)

What percentage of the time does an ultrasound transducer transmit vs receive sound waves?

Transmits about 1% of the time; receives (listens for returning echoes) about 99% of the time

How is an ultrasound image formed?

It is formed from the echoes returning from tissues; the machine measures the intensity and time of arrival of returning sound waves

At what speed does sound travel in soft tissue?

Approximately 1540 m/s

How is the distance to a reflector determined in ultrasound?

Based on the time of flight of the sound wave from the transducer to the reflector and back

How are frequency and wavelength related in ultrasound?

They are inversely related - higher frequency means shorter wavelength

How does transducer frequency affect penetration and resolution?

Lower frequency transducers penetrate farther but have lower resolution; higher frequency transducers do not penetrate as far but have higher resolution

What causes differences in how ultrasound interacts with tissues?

Differences in acoustic impedance (resistance to flow) between tissues, which alter the direction of the sound wave

Define anechoic on ultrasound

Black - produces no echo (e.g., bile, urine)

Define hypoechoic on ultrasound

Darker (dark gray) than the structure it is compared to

Define isoechoic on ultrasound

The same echogenicity as the structure it is compared to

Define hyperechoic on ultrasound

Brighter than the structure it is compared to (e.g., bone, gas, organ boundaries)

List, from anechoic to hyperechoic, the relative echogenicity of common abdominal structures

Bile/urine (anechoic) - renal medulla - muscle - renal cortex - liver - spleen - prostate - bone/gas/organ boundaries (hyperechoic)

What is the Doppler effect?

The perceived change in frequency that occurs as a result of motion between the sound source and the observer

What does the Doppler effect allow you to determine in ultrasound?

The velocity and direction of blood flow

In Doppler ultrasound, what act as the sound source and the observer?

Red blood cells are the sound source; the transducer is the observer

In color flow mapping, what do blue and red typically represent?

Blue represents flow away from the transducer; red represents flow toward the transducer (mnemonic: BART - Blue Away, Red Toward)

What is computed tomography (CT)?

A cross-sectional imaging modality in which an x-ray tube rotates around the patient while detectors measure the amount of radiation absorbed (attenuated) by the patient to reconstruct images

How does CT differ from conventional radiography in machine configuration and image type?

CT uses a rotating x-ray tube/detector gantry to produce true cross-sectional (tomographic) images, while radiography produces a single projectional image with superimposition of structures

What is the difference between single-slice and spiral (helical) CT?

Single-slice CT completes one full tube rotation, then the table moves before the next rotation; spiral CT has continuous tube rotation linked to continuous table movement

What is an advantage of multi-detector CT scanners?

Multiple rows of detectors allow faster scanning and thinner slices (e.g., 4, 16, 64, or even up to 320 detector rows)

What is a pixel?

A picture element with only two dimensions (x, y)

What is a voxel?

A volume picture element with three dimensions (x, y, z)

What determines the X and Y dimensions of a voxel?

The image matrix size

What determines the Z dimension of a voxel?

The slice thickness

What is an isotropic voxel?

A voxel in which the X, Y, and Z dimensions are all equal

Why are isotropic voxels important in CT?

They allow multi-planar reconstruction (MPR) and 3D volume rendering without loss of resolution

How does slice thickness affect multi-planar reconstruction quality?

Thinner slices (e.g., 0.6 mm) provide better detail for multi-planar reconstruction than thicker slices (e.g., 4.0 mm)

Approximately how many Hounsfield Unit (HU) values exist, and how does this compare to what can be displayed or seen?

There are more than 4,000 HU values, but a typical monitor can only display about 256 shades of gray, and the human eye can only differentiate about 20 shades of gray

What is the CT "window"?

The range of Hounsfield Units represented on a specific image

What is the CT "level"?

The central Hounsfield Unit value of the window being displayed

When is a wide window width (WW) used in CT, and what is the tradeoff?

Used for imaging tissue types that vary greatly in density (e.g., lung) so all tissues appear in one image; the tradeoff is that subtle density discrimination is lost

When is a narrow window width (WW) used in CT, and why?

Used for imaging tissue types with similar densities (e.g., brain white vs gray matter) because it provides greater density discrimination and contrast

In CT terminology, what does hyperattenuating mean and what does it look like?

Structures with the highest HU values (e.g., bone, metal) - they appear WHITE

In CT terminology, what does hypoattenuating mean and what does it look like?

Structures with the lowest HU values (e.g., air) - they appear BLACK

What terms describe density on radiographs vs on CT?

Radiography uses "opacity"/"attenuation"; CT uses "hyperattenuating" and "hypoattenuating"

What are the advantages of CT?

More detailed view of anatomy without superimposition, faster and less expensive than MRI, and best for bone, lung, and abdomen

What are the disadvantages of CT?

Radiation exposure, more expensive than radiographs, and poor soft tissue contrast compared to MRI

What is magnetic resonance imaging (MRI)?

An imaging modality that uses a strong magnet, radiowave transmitter/receiver, and computer to image hydrogen protons in the body without ionizing radiation

What are the four "ingredients" needed for MRI?

A magnet, a radiowave transmitter and receiver, a computer, and a patient containing hydrogen protons

How does MRI fundamentally differ from radiography and CT?

MRI forms images based on hydrogen protons aligning in a magnetic field rather than on x-ray attenuation, so it does not use ionizing radiation

Why does hydrogen act like a tiny magnet in the body?

Because atoms with an odd number of protons possess a magnetic moment, and hydrogen (abundant in the body, e.g., in water) has 1 proton

In a magnetic field, do more hydrogen nuclei align with or against the field?

More nuclei align with the magnetic field than against it, because it takes less energy to align with it than against it

What happens when a radiofrequency (RF) coil applies energy during MRI?

The RF energy causes the non-cancelled-out hydrogen protons to flip to the opposite direction

What happens after the RF pulse is stopped in MRI?

Energy is released from the hydrogen protons as they return to their normal orientation, and this energy release induces a current in the receiver coil, generating the signal used to form the image

What is "weighting" in MRI?

The use of certain parameters to optimize differences in relaxation rates of different tissues in order to provide contrast in the image

What are the two main MRI weightings discussed?

T1-weighted and T2-weighted

On a T1-weighted image, how do fluid and fat appear?

Fluid is hypointense (black); fat is hyperintense (white)

On a T1-weighted image, how do fat, intermediate tissue, and free water/CSF compare in relaxation time and signal?

Fat has short relaxation time (hyperintense/white); tissue has intermediate relaxation time (intermediate/gray); free water and pure liquids like CSF have long relaxation time (hypointense/black)

On a T2-weighted image, how do fluid and fat appear?

Fluid is hyperintense (white); fat is hyperintense (white)

On a T2-weighted image, how do liquid, intermediate stiffness tissue, and firm solid tissue compare in signal?

Liquid material is hyperintense (white); intermediate stiffness tissue is intermediate intensity (gray); firm solid tissue is hypointense (black)

What is the "Terminator" mnemonic for remembering T1 vs T2 water signal?

T1 - Terminator 1: Arnold is bad, so water is dark (hypointense); T2 - Terminator 2: Arnold is good, so water is light (hyperintense)

Define hypointense in MR terminology

Less bright (darker) than the tissue you are comparing it to

Define isointense in MR terminology

The same brightness as the tissue you are comparing it to

Define hyperintense in MR terminology

Brighter than the tissue you are comparing it to

What are the advantages of MRI?

Better soft tissue contrast and detail, and it is noninvasive

What are the disadvantages of MRI?

Long scan times, need for general anesthesia, and high cost

Comparing the images: which modality gives better bone detail, and which gives better soft tissue detail - CT or MRI?

CT gives better bone detail and is good for thorax and abdomen; MRI gives better soft tissue detail and is good for neuroimaging and musculoskeletal imaging

Comparing image acquisition: how do sedation needs and scan times differ between CT and MRI?

CT typically requires heavy sedation or general anesthesia and has a fast scan time (seconds to minutes); MRI requires general anesthesia and has a longer scan time (about 1-2 hours)

What is the primary safety concern with CT?

Ionizing radiation

What are the primary safety concerns with MRI?

Projectiles from ferromagnetic objects and interactions with metal implants

Why is it important to remember that "the magnet is ALWAYS on" in MRI?

Ferromagnetic objects can become projectiles at any time, and anything that can be demagnetized (e.g., credit cards, ID cards) should be removed before entering the room

What should be done before a patient or object enters the MRI room?

A screening form should be completed to check for pacemakers, aneurysm clips, piercings, and other implants, which must be confirmed MR-safe or MR-compatible before entry

How is a nuclear medicine study performed?

A radionuclide is administered to make the patient "radioactive," and images are created using a large radiation detector

What do nuclear medicine images represent, as opposed to CT, MRI, US, or radiographs?

Physiologic function, not morphology (anatomic structure)