Imaging Modalities
What is the difference between MRI, CT, and Ultrasound?
MRI Scan
MRI Basics
Magnetic Resonance Imaging (MRI) scan is safe and uses no harmful radiation. The images are acquired using just radio waves, like those we use to listen to music on the radio, and alternating magnetic fields to create detailed images anywhere in the body. A coil, which is just a radio antenna, is placed adjacent to or surrounding the body part being imaged and the body part is positioned within is the center of the scanner where the magnetic field is most uniform. High field MRI scanners have a relatively long and narrow tube to create the strong, uniform magnetic field.
The scans take between 20 and 40 minutes. During the scan, multiple separate sequences, each taking between 2 to 5 minutes, are obtained in different imaging planes. Unlike CT or x-ray, the tissues on the MRI appear different on each sequence. For example, fluid can look white, grey, or dark, depending on the sequence. Because each sequence is acquired over a period of several minutes, patient must hold still to keep the images from blurring. New imaging techniques are being developed continually which can dramatically reduce scan times and even freeze motion altogether.
When to use MRI?
MRI scans have great soft tissue contrast and so they are ideal for imaging the joints and the spine.
MRI imaging of the joints is performed to diagnose:
- Meniscus tears
- Labral tears
- ACL and other ligament tears of the knee
- Bone bruises
- Osteonecrosis (AVN)
- Ligament injuries
- Tendon injuries
- Soft tissue masses (tumors) and cysts
- Arthritis
- Cartilage defects
- Muscle injuries
- Fractures and stress fractures
- Unexplained joint pain with normal x-rays
MRI is also best for imaging the spine to assess low back pain, sciatica, extremity weakness, and shooting pain (radiculopathy) down the arms or legs.
MRI imaging of the spine is performed to diagnose:
- Degenerative disc disorders resulting in arthritis
- Disc bulges
- Disc herniations (disc protrusions or extrusions)
- Lumbar spine or sacral fractures which are not visible or hard to see on X-rays
- Slippage (subluxation) of lumbar spine segments
- Pars defects and pars fractures
- Disc infections (discitis)
- Vertebral body infections (osteomyelitis)
- Tumors spine and soft tissues
- Abdominal aortic aneurysms
- Adenopathy (large lymph nodes) or masses around spine
- Stress fractures
MRI scans are safe, with no radiation, and have excellent soft tissue contrast which makes MRI the best modality to image the joints, the spine, and the brain.
CT Scan
CT Basics
A CT scan, which is a computerized form of x-ray, uses ionizing radiation to generate the images. The x-ray beam is shot through the body and the x-ray radiation that is not absorbed is captured on a receptor plates. The x-ray generators and receptor plates spin rapidly about the body within the circular housing as the radiation is continuously expelled and captured. A computer takes the data acquired and uses it to create detailed, high resolution images which can be viewed in any plane. The process of very rapid, taking only seconds for a small body part, to a few minutes for a larger area. Since CT is just a fancy x-ray, the soft tissues always look the same, similar to an x-ray, with bones appearing white, fluid and soft tissue appearing gray, and air appearing black. Bony detail is particularly good.
When to use CT?
CT scans have high spatial resolution and show fine anatomy of cortical bone to see nondisplaced fractures and lung parenchyma to see small nodules and interstitial lung disease.
CT scans are performed to diagnose:
- Cancer other disease of the lungs
- Large lymph nodes anywhere in the body
- Evaluation of the abdomen and pelvis with IV and GI contrast
- Complex fractures seen by x-ray
- Evaluation of any body part in the setting of trauma
- Bony anatomy of the middle and inner ear for hearing loss
- Evaluation of some bone tumors to supplement MRI
CT is super fast and has great spatial resolution making it the best choice to image bone injuries and the lungs. CT is also the best choice to image the brain, abdomen, and pelvis in the setting of acute trauma where fast scanning is essential.
Ultrasound
Ultrasound Basics
Ultrasound is safe, with no radiation. It uses sound waves that reflect off of soft tissues to create the images. The images are low resolution, but it is safe, portable, and inexpensive. Because of the safety and low cost, it is often used to evaluation of the abdomen and pelvis. It is the method of choice to image the gallbladder, female pelvic organs, testicles, and thyroid gland. Ultrasound can help differentiate between solid and cystic lesion.
The images are best if the soft tissues are close to the scanner.
When to use ultrasound?
Ultrasound is inexpensive, safe, and portable making it widely used to evaluate many parts of the body and can produce high resolution images when the probe is close to the body part being imaged, like the thyroid gland and scrotum. Also, ultrasound can detect flowing blood and is therefore used to evaluate the carotid arteries for atherosclerotic disease, renal arteries for aneurysms and stenoses, the aortic size, the peripheral arteries for narrowing, and the popliteal veins for clot/thrombosis.
Ultrasound scans are performed to diagnose:
- Masses in the solid organs of the abdomen and pelvis
- Gallstones and other pathology of the gallbladder and biliary tree
- Cysts and solid lesions of the thyroid gland and size of thyroid size
- Enlargement of the liver and spleen
- Renal tumors, cysts, renal obstructions, and renal parenchymal disease
- Abdominal aortic aneurysm
- Carotid artery plaque
- Deep vein thrombosis
- Peripheral artery disease
- Renal artery stenosis and aneurysm
- Rotator cuff pathology
- Endometrial thickening of uterus and uterine fibroids
- Free fluid in the abdomen or pelvis
- Congenital hip and brain abnormalities in neonates
Ultrasound uses sound waves and is inexpensive.