Lecture 13 - MRI Flashcards
MRI
magnetic resonance imaging
How do MRIs work?
Based on protons emitting radio waves in the presence of strong magnetic fields and pulses of radiofrequency energy
Benefits of MRI
No exposure to ionizing radiation
No significant biologic effects
Each set of images is from a series of radiofrequency pulses and variations in the magnetic field, called…
Pulse Sequence
Describe an MRI Scanner
Scanners are like a larger, thicker CT scanner
Some scanners are open at the sides or are vertical, which may assist claustrophobic patients.
Difference between open and closed scanners
Open scanners may be slower and not have the same image quality as closed scanners
How are scanners described?
Scanners described by the strength of their primary magnet
Typical magnet strength
1.5 Tesla
How are images produced?
Primary magnetic field is modified by additional fields, resulting in varied magnetic field strengths or gradients – these are used to produce the images
The fields affect the spin of hydrogen protons in the patient
Radiofrequency pulses are applied to these protons which give off radio waves that are detected by receiver coils (antennas).
Output from the coils creates the images.
Gradient fields and radio pulses chosen for each set of images (pulse sequence) creates the images but also the different appearance of tissues on the images
Most common pulse sequences
T1 or T2 spin echo T1 or T2 fast spin echo T1 or T2 gradient echo FLAIR sequence STIR sequence
Sequences are usually referred to as
T1 or T2 sequences (or T1-weighted or T2-weighted) sequences
called the longitudinal relaxation time and is the time it takes for the tissue to recover to its longitudinal state before the radiofrequency pulse was given
T1
the transversal relaxation time and is the time for the tissue to regain its transverse orientation before the radiofrequency pulse was given
T2
As soon as the pulse stops…
relaxation begins and the spinning nuclei release energy that is picked up by the receiver coil and produces an image
What appears dark on a T1 image?
calcification air chronic hemorrhage acute hemorrhage water (edema, CSF)
Have low signal and appears dark on the image.
What appears bright on a T2 image?
Lipids
Subacute hemorrhage
Fluid containing protein will have high signal and be bright.
What causes tissue to have a high signal?
Contrast will cause tissue with the contrast to have high signal
What appears dark on a T2 image?
fat calcification air early subacute hemorrhage chronic subacute hemorrhage chronic hemorrhage acute hemorrhage high protein
What will appear bright on a T2 image?
Water - Edema, CSF
Why do lipid containing tissues not affect not affect high signal?
T1 and T2 sequences can suppress the signal from lipids so that lipid containing tissues do not affect high signal from adjacent high signal fluid or tissue
Most common MRI contrast
Gadolinium
How is Gadolinium administered?
IV
Intraarticularly
How does Gadolinium work?
Enters the blood stream, enhances organ parenchyma, and is excreted by the kidneys
Causes a brighter signal on T1-weighted images – since fat is already bright the images are usually fat-suppressed to enhance the effect of the contrast
What structures light up with contrast?
Structures that light up with contrast are vascular (tumors) and inflammation and are described as enhancing
Who should not receive Gadolinium?
Pts with renal insufficiency
What can patients with renal insufficiency develop after Gadolinium?
Patients with renal insufficiency should not receive gadolinium as they are at risk for developing nephrogenic systemic fibrosis – produces fibrosis of skin, eyes, joints, and internal organs.
Patients with preexisting renal disease, especially dialysis patients, are at greatest risk
What has better soft tissue resolution?
MRI
MRI>CT for soft tissue resolution.
If contrast is indicated and CT and MRI are equivalent but contrast is contraindicated, what do you use?
unenhanced MRI is usually better than unenhanced CT
What must be removed from the patient before an MRI?
Has very strong magnetic fields, so metal objects are removed from the patient
Surgically implanted or imbedded metal devices and objects may be a problem.
If motion of the device or object may be harmful, then MRI cannot be done – cerebral aneurysm repair clips, vascular clips, surgical staples
What foreign bodies can cause an issue with MRIs?
Foreign bodies, such as bullets, shrapnel, and metal in the eyes, are also a problem
Patients with pacemakers, defibrillators, pain stimulator implants, insulin pumps, other implantable drug infusion pumps, and cochlear implants cannot have MRIs – devices can be deactivated by magnetic field
MRI Safety during pregnancy
Limited data on safety during pregnancy – American College of Radiology says pregnant patients can undergo MRI at any stage of pregnancy if benefit outweighs the risk. Should not be performed electively early in pregnancy, and gadolinium is not recommended in pregnancy.
MRI safety with metal objects
Some may still have an MRI depending on the metals used, the shape of the objects, and the location involved.
Issues with MRI
Very expensive
May be difficult for some patients due to psychological reasons
May not be ideal modality for all cases –bone has little or no MRI signal so CT is better
What images can MRI provide?
Axial
Sagittal
Coronal
Just like CT
MRI is used for…
Musculoskeletal Neurologic GI Endocrine/Reproductive GU
What does a Musculoskeletal MRI evaluate?
Bone marrow Menisci Tendons Muscles Bones Osteomyelitis Spine
What is a Musculoskeletal MRI used for?
Assess for meniscal tears Ligamentous or tendon injuries Contusions Occult or stress fractures Disk disease and marrow infiltration Differentiating scarring from prior surgery from new disease
A musculoskeletal MRI has a __________ negative predictive value if normal.
High
Neurologic MRI evaluates…
Brain - especially the posterior fossa Tumors Infarction Multiple Sclerosis Peripheral nerves looking for impingement or injury.
GI MRI evaluates…
Evaluates the liver – characterizes liver lesions, detects small lesions, cysts, hemangiomas, hepatocellular carcinoma, hemochromatosis, fatty infiltration
Evaluates the biliary system – MR cholangiopancreatography for strictures, ductal dilatation
Evaluates the small and large bowel – MR enterography, appendicitis in pregnancy
MRI Liver evaluation includes
Characterizes liver lesions Detects small lesions Cysts Hemangiomas Hepatocellular carcinoma Hemochromatosis Fatty infiltration
MRI Biliary system evaluation includes.
MR cholangiopancreatography for strictures
Ductal dilatation
MRI evaluation of the small and large bowel:
MR enterography
Appendicitis in pregnancy
MRI Endocrine/Reproductive evaluation includes:
Evaluates the adrenal glands – adenomas, adrenal hemorrhage
Evaluates the male pelvis – staging of rectal, bladder, and prostate carcinoma
MRI of the GU evaluates
Renal Masses
Cysts vs Masses
How does Ultrasound work?
High frequency sound waves are produced by a transducer while in contact with the skin.
Pulses of sound waves are reflected back (echoed) to the transducer at interfaces between and within body tissues
A white dot is placed on the monitor screen where the positions from which each echo occurred
The dots create a cross section image of the anatomy at the projected plane of the transducer on the body.
Describe an ultrasound within a fluid collection.
A fluid filled structure without debris will not have any interfaces within it to reflect sound, so the fluid will be anechoic – the image will not have any echoes within the fluid collection
Level of gray or brightness on an image
echogenicity
How is the echogenicity of soft tissue described?
The echogenicity (level of gray or brightness on the image) of soft tissue is described in relation to other tissues.
Tissue with the same echogenicity as the predominate tissue in the image is
isoechoic
Tissue with fewer and weaker echoes is darker than the reference tissue and is
hypoechoic
Tissue with many strong echoes (bright) is
hyperechoic
Sound travels unimpeded through fluid so echoes deep to a fluid collection are brighter than adjacent tissue
> acoustic enhancement
Anything that blocks the transmission of sound waves will cause
acoustic shadow
Use of ultrasound is limited when…
gas-containing or bony structures are in the field of view of a transducer or when the patient is obese
Ex. tail of pancreas may not be seen due to bowel gas shadowing, and chest or upper abdomen may have shadows from the ribs that may obscure parts of the spleen or liver.
Medical risk to the patient with ultrasuond
No medical risks to a patient from an ultrasound exam with little or no patient discomfort
Ultrasound vs MRI & CT
Less expensive that CT or MRI, also portable
Tends to be more difficult to interpret than CT, MRI, or radiography – display of structures depends on orientation and angulation of transducer – examiner uses whatever position that maximizes the visualization of the structure being studied.
Ultrasound orientation
Images are rarely oriented along strict anatomic planes, so interpretation depends on recognizing patterns of structures
Ultrasound units can identify, characterize, and quantify moving fluid, usually blood, using
Doppler Principles
Doppler can look at
Can look at tubular structures based on direction and velocity of fluid flow, and abnormal flow can be identified.
Doppler is useful for
vascular studies
Where are ultrasound units becoming more prevalent?
ED
Special Clinics
Primary Care
Ultrasound images are very…
Very operator dependent – identifying free fluid in the abdomen after trauma may be pretty easy, but identifying a rotator cuff tear is much more difficult
What can have an effect on ultrasound image quality?
body habitus
Images close to being in the axial plane should be viewed in the same orientation as
axial CT images
Images close to the sagittal plane are shown as if
the viewer is looking at those sections from the right side of a supine patient with the cranial end of the patient to the viewer’s left and the caudal end to the viewer’s right.
Types of Ultrasound
A-Mode B-Mode M-Mode Doppler Duplex
Simplest type of ultrasound
A-Mode
spikes along a line represent signal amplitude at a certain depth – used mainly in ophthalmology
A-Mode Ultrasound
mode most often used in diagnostic imaging – each echo depicted as a dot, can show real time motion
B-Mode Ultrasound
used to show moving structures such as blood flow or motion of heart valves
M-Mode Ultrasound
assesses blood flow, so used in vascular ultrasound
Doppler
used in vascular studies – uses both gray-scale and color
Duplex Ultrasonography
_____________
to visualize structure and flow within a vessel and spectral waveform Doppler to quantitate flow
Combines studying the morphology of the blood vessels with a recording of the velocity of flow displayed by the Doppler spectral waveform
Duplex Sonography
a graph of velocity of flow over time within a given area
Doppler Spectral Waveform
Doppler Spectral Waveform
Flow towards the transducer is displayed
above the baseline
Doppler Spectral Waveform
Flow away from the transducer is displayed
below the baseline
Color flow Doppler imaging adds
superimposed moving blood shown in color over a gray scale image of the structure – easier to identify abnormalities
Study of choice for noninvasive assessment of atherosclerotic disease
Carotid Ultrasonography
_________ accounts for more than 1/2 of strokes
Carotid Disease
Used to evaluate bruits, as preop screening prior to other major vascular surgery, and to assess the patency of the vessel after endarterectomy
Carotid Ultrasonography
Carotid stenosis causes elevations in velocity of flow when >_______ narrowing of the lumen
50%
Significant stenosis alters the Doppler waveform proximal, at and distal to the stenosis
Venous Duplex
Ultrasound for evaluating for DVT is performed by examining
the leg along the common femoral vein proximal deep femoral vein greater saphenous vein popliteal vein
What is most sensitive in symptomatic patients who have symptoms above the knee
Venous Duplex
Veins with thrombi will not
Compress
Normal venous structures will easily be compressed and completely collapsed by the transducer
Will also try to visualize the echogenic thrombus itself
Ultrasound of the Heart
Echocardiography
Echocardiography provides information on
cardiac great vessel anatomy
Echocardiography + Doppler provides information on
cardiac and great vessel blood flow.
Echocardiography can assess
ventricular function valvular heart disease myocardial disease pericardial disease intracardiac masses aortic abnormalities
Echo + Doppler can evaluate
cardiac chamber function
valvular function
shunts seen in congenital heart disease
Benefits of echocardiography
Noninvasive, relatively inexpensive, no radiation, can be performed at bedside in critically ill patients
Results are immediately available
How can echocardiography be done if you cannot get a good image?
Transesophageal
Echocardiography + Stress testing can assess
ischemia by wall motion analysis of LV function
