X-ray, MRI, CT

Relationship between radiodensity and radiographic density

inverse

Define radiolucent

increased radiographic density (blackness)

X-ray advantages

Procedure is quick

How do you improve contrast resolution on a CT?

Thicker slices and larger pixels (pg. 127)

How do you improve spatial resolution on a CT?

Thinner slices and smaller pixels (pg. 127)

CT disadvantages

1. Average volume effect: the computer's applying average values to a small volume of tissue and thus displaying it in one shade of gray even though it contains more than one type of tissue. For instance, when there is a tumor present that has the same radiodensity as the muscle surrounding it, it may be missed. (pg. 27 and 120) 2. High level of radiation.

MRI advantages

1. Can detect changes in fluid content, ischemic, edema, and hemorrhage more accurately than CT. 2. Functional MRI (fMRI) can display changes in blood flow based on the paramagnetic properties of hemoglobin or through the use of contrasts. fMRI has become the key method in mapping neuronal activity in the brain. 3. Uses a magnetic field so there is no radiation to the patient. 4. Sensitive for detecting changes and variation in bone marrow which is important in diagnosing bone tumors, stress fractures, and avascular necrosis. 5. Displaying soft tissue detail. Great for diagnosing sports injuries including partial tears of tendons and ligaments from complete tears. 6. Best modality for evaluation of disk herniations and other potential causes of nerve root impingement. 7. Ability to stage neoplasms in bone and soft tissues as well as evaluate the extent of tissue invasion prior to surgery. 8. No ionizing radiation 9. Greater contrast resolution for soft tissue imaging 10. Greater ability to image organs surrounded by dense bone structures. 11. Less risk of missing disease processes 12. Excels at demonstrating areas of inflammation.

Brain Characteristics on T2 MRI

1. Gray matter gives rise to higher signal intensity (brighter) than white matter. 2. CSF: bright white 3. Cranial bones: dark (pg. 122)

Brain Characteristics on T1 MRI

1. Gray matter gives rise to lower signal intensity (darker) than white matter. 2. Cranial bones: dark 3. Subcutaneous fat: bright (pg. 122)

Brain Characteristics on a CT

1. Gray matter is more radiodense (brighter) than white matter. 2. CSF is more radiolucent (darker) than both white and gray matter. 3. Cranial bones: bright 4. Subcutaneous fat: dark (pg. 122)

MRI disadvantages

1. Length of time needed to produce an image and length of time of imaging. 2. Cost 3. Claustrophobia 4. Limited in imaging of cortical bone because of low signal intensity. 5. Cannot be used on patients with tattoos, Ferromagnetic surgical clips, orthopedic hardware, pacemakers. 6. Patients may need to be sedated especially children.

T2

1. Long TR and TE times 2. Measures energy late in the decay of transverse relaxation and selectively images strictest that do not readily give up energy, such as water. 3. Water: bright 4. Grainer and display less spatial resolution. 5. Particularly valuable for detecting inflammation.

T1

1. Short TR and TE times 2. Measures energy from structures giving off high energy levels early in the process of longitudinal remagnetization. 3. Fat: bright 4. Images of good anatomic detail, displaying images in a fairly balanced manner.

Radiographic Density

Amount of blackening on the radiograph.

Images of a CT

Axial slices based on multiple radiographic projections from different angles.

What can you see in an X-ray?

Bone and fractures in the bone. Some infections and soft tissue swelling maybe visible.

What can you see in a CT?

Bone, soft tissue tumors, subtle or incomplete fractures, intra-articular abnormalities, the detection of small bone fragments, degenerative changes, serious trauma, spinal stenosis, condition of intervertebral discs, and quantitative bone mineral analysis. (pg. 27)

What is radiation?

Energy that is transmitted through space or matter. (pg. 5)

Radiodensity

Combination of physical qualities of an object that determine how much radiation it absorbs from the X-ray beam. Determined by composition and thickness.

Define radiopaque

Decreased radiographic density (white)

Quantitatively, what is the difference between T1 and T2?

Different time to echo (TE) and time to repetition (TR) values. (pg. 131)

What are X-rays?

Form of ionizing electromagnetic radiation, similar to visible light but of shorter wavelength. (pg. 6) In order to take an image, you must superimpose the body to acquire the desired view or angle.

What is the standard protocol in trauma for the immediate assessment of intracranial bleeding?

Head CT (pg. 122)

How is an MRI image produced?

Interaction of tissue with radiofrequency waves in a magnetic field. Image obtained is based on a patient's remission of absorbed radiofrequencies while in the magnetic field. (pg. 31)

CT advantages

Less time consuming than MRI or ultrasound, less expensive than MRI, less problematic for patients with claustrophobia, greater availability, faster imaging times, thinner slices, greater power of resolution when imaging cortical bone, easier imaging of individuals with metal implants, allows for accurate measurements of osseous alignment in any plane (pg. 120 & 139)

What is an MRI?

Magnetic resonance image is a cross-sectional imaging technology that uses a magnetic field and radiofrequency signals to cause hydrogen nuclei to emit their own signals, which then are converted to images by a computer. Process by which nuclei, aligned with an external magnetic field, absorb and release energy. Energy is emitted by protons during their re-alignment with the main magnetic field (pg. 129 and 141)

What is a CT?

Merges x-ray technology with the computer to provide detailed digital cross-sectional images (axial slices) of the body relatively free from superimposition of the different tissues. (pg. 113)

When is a CT the modality of choice?

Neuroimaging in acute settings and in case of trauma. (pg. 127)

T1 and T2 phenomena

Occurring simultaneously, are different processes related to the return of the protons to alignment with the main magnetic field. (pg. 130)

X-ray disadvantages

Radiation, "one view is no view", superimposition of the body.

Define T2

Relaxation of transverse magnetization toward equilibrium (pg. 130)

Define TI

Return to longitudinal magnetization (pg. 130)