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Medical Imaging > Facial Views, CT & MRI > Flashcards

Facial Views, CT & MRI Flashcards

1
Q

Why is it difficult to determine whether facial fractures are present?

A
  • there is often much bruising and swelling of the face
  • this can make it difficult to determine whether fractures are present
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2
Q

What are the 5 specific external signs that indicate facial fractures?

A
  • facial asymmetry
  • flattened cheek
  • “dish face”
  • deviation of the nose / flattened nasal bridge
  • pupils not level
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3
Q

What is “dish-face”?

A
  • occurs when the midface appears “dished-in”
  • this represents posterior and inferior displacement of the maxilla
  • the face can appear elongated as a result
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4
Q

Why are X-rays not often performed even though fractures of the nose are common?

A
  • X-rays are only indicated when there is an obvious deformity, such as deviation
  • if there is swelling, but no obvious deformity then the patient is advised to wait until the swelling subsides
  • if there is a deformity following disappearance of the swelling then imaging is performed
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5
Q

What does the pupils not being equal suggest?

A
  • this indicates fracture of the orbital floor and prolapse of orbital contents (such as inferior rectus)
  • the patient may have diplopia and impaired movements of the eye
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6
Q

When does a patient with facial injuries need a CT head or CT C-spine?

A
  • in severe blows to the face, the skull, brain and C-spine may have also sustained injury and a CT should be performed
  • an injury to the face is an injury to the head, so if any of the NICE criteria for CT scanning after head injury is met, it should be performed
  • bones of the face are sturdy (especially strong bones, such as the frontal bone), so fracture to these requires high force

in these situations, there is a high chance of a corresponding intracranial complication

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7
Q

Why can it be difficult to examine the eye and vision after facial injuries involving the orbit?

A
  • severe swelling and pain can make this examination impossible
  • eyelids and soft tissues can be so swollen that the patient cannot open the lids or withstand a gentle attempt to force them open
  • the eyes and the vision cannot be examined
  • a CT orbit can be performed in this situation
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8
Q

When are facial X-rays requested?

Why can they be difficult to interpret?

A
  • they are requested when the patient does not require a CT-head, but there is clinical suspicion of facial fractures
  • they are difficult to interpret as the anatomy of the facial skeleton is complex and there is superimposition of the bones of the face and skull upon each other
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9
Q

What are the 6 different types of facial fractures that can occur?

A
  1. simple nasal fractures
  2. LeFort fractures (“middle-third” fractures)
  3. zygomatic fractures
  4. orbital fractures
  5. frontal sinus fracture
  6. mandibular fractures + TMJ dislocation
  • in the case of mandibular fractures, there is likely to be more than one as the mandible is essentially a “ring structure”
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10
Q

What are the 2 views of the facial skeleton that are commonly taken?

A
  1. occipito-mental (OM)
  2. occipito-mental 30o (OM 30)
  • a standard PA facial view or PA mandible may also be taken
  • the patient details, date and technical adequacy must be assessed first
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11
Q

Why does a CT head not suffice for facial fractures?

A
  • a normal CT head will NOT cover the facial bones
  • the orbits and some of the maxillary sinuses may be visible
  • if the facial bones are specifically required, then CT facial skeleton must be requested to ensure they are included
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12
Q

How is an OM view taken?

What is this good for visualising?

A
  • the beam travels from the occiput to the jaw
  • this gives a good view of:
  1. frontal and maxillary bones
  2. zygomatic bone and zygomatic arch (“elephant trunk”)
  3. dens of C2
  4. frontal, ethmoid and maxillary sinuses
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13
Q

When is the OM 30 view preferred and how is this performed?

A
  • the head is tilted backwards by 30o before a beam is fired from the occiput to the jaw
  • this is good for visualising:
  1. maxillary sinuses
  2. inferior orbital rims
  3. features of the mandible
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14
Q

What are the features of this PA facial X-ray?

Which part of the temporal bone is visible?

A
  • the petrous ridge is the part of the temporal bone that can be seen through the orbits
  • this is the dense / “rock-like” part of the temporal bone
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15
Q

Why can appearance of the mastoid differ between individuals?

What is the risk associated with the less common presentation?

A
  • mastoid aeration / presence of air cells is highly variable
  • some people have very dense mastoid processes that do not contain any air cells
  • these people cannot equalise the pressure in their ears as easily as those with air cells
  • they are more likely to develop retraction pockets in the ear drum and subsequent cholesteatoma
  • usually, the mastoid air cells appear “bubbly” as they are filled with air
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16
Q

What are the features of this OM view?

What type of fracture is this good for visualising?

A
  • the head has been tilted backwards, allowing the floor of the orbit and anterior wall of the maxillary sinus to be seen more clearly
  • this view is useful for visualising fractures of the orbital floor
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17
Q

Why can suture lines be mistaken for fractures?

How can this be avoided?

A
  • sutures are less dense than bone and appear as a small dark line
  • the suture between the frontal and zygomatic bone is often mistaken for a fracture
  • if in doubt, compare to the other side to see if they are symmetrical
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18
Q

What type of fracture seen in the OM view implicates 4 sutures and why?

How can this be recognised?

A

tripod / quadripod fracture

  • the zygomatic bone is attached to many other bones:
  1. temporal bone
  2. frontal bone
  3. maxillary bone
  4. pterygoid region
  • a severe fracture to the zygoma can rotate and implicate these 4 structures
  • look for the intact “elephant trunk” when assessing whether a zygomatic fracture is present
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19
Q

What are McGregor’s lines?

What type of fracture are they important in looking for?

A
  • in OM views of the face, it is important to systematically review the 3 sections of the face along 3 lines:
  1. over the upper orbits
  2. over the upper surface of the zygomatic arch, lower orbit and nose
  3. over the lower surface of the zygomatic arch and alveolar process
  • these are mainly related to LeFort fractures
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20
Q

How is integrity of the zygomatic complex determined in an OM view?

A
  • the integrity of the zygomatic complex is determined by tracing around the zygoma and upper and lower zygomatic arches
  • this is described as looking like an elephant’s head and trunk
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21
Q

Why can some fractures of the zygomatic arch be difficult to spot?

A
  • not all fractures appear dark on X-rays
  • lucency only appears if the bones are separated from each other in the fracture
    • the gap is less dense than the bones on either side
  • if the bones are overlapping each other in the fracture, it can appear more dense with no dark region
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22
Q

What are the features of the OM 30 view?

A
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23
Q

What lines should be traced in an OM 30 view?

A
  • lines should be traced over the zygomatic arches, nasal bridge and alveolar process
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24
Q

Describe the typical appearance of an orbital blowout fracture on facial X-ray

Why does this occur?

A
  • a blow to the orbit causes a transient rise in intra-orbital pressure that causes fracture of the thin orbital floor
  • there is a break in the cortex of the corresponding infraorbital rim
  • there is a fluid level present in the maxillary sinus on the same side
  • there is an opacity below the infraorbital rim, indicating prolapse of orbital contents into the maxillary sinus
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25
Q
A
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26
Q

What contents may herniate into the maxillary sinus following an orbital floor fracture?

Why is there a fluid level in the maxillary sinus?

A
  • contents that may herniate are periorbital fat and the inferior rectus muscle
  • a fluid level in the maxillary sinus suggests bleeding as a result of the fracture
  • usually the sinuses should be radiolucent and appear dark as they are filled with air
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27
Q

Why might someone with a blowout fracture suffer from diplopia?

A
  • the inferior rectus herniates into the maxillary sinus
  • if the inferior rectus becomes trapped, this can stop the eye from moving properly
  • the individual is unable to look up
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28
Q

How can an orbital blowout fracture be recognised on MRI?

A

“teardrop sign”

  • the prolapse of orbital contents into the maxillary sinus appears as a teardrop on coronal MRI
  • this is caused by the herniation of extraconal fat into the sinus
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29
Q

Do orbital blowout fractures always affect the inferior orbital margin?

A
  • the lamina papyracea of the medial orbital wall can also fracture
  • this leads to herniation of orbital fat and medial rectus into the ethmoidal air cells
  • this is much less common
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30
Q

Why can a patient with an orbital floor fracture present with a numb cheek?

A

infra-orbital nerve injury

  • this occurs when there is a fracture of the orbital floor passing through the infra-orbital canal
  • this results in sensory disturbance in the region of the infra-orbital nerve (V2)
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31
Q

How can you look for lucency in the orbits?

What does this indicate?

A
  • an area of radiolucency appears as a dark shadow in the upper orbit
  • this is referred to as the “black eyebrow sign”
  • it indicates air in the orbit and the presence of a fracture somewhere
  • this is most commonly fracture of the roof of the frontal sinus, as fracture of the medial wall would cause air to be present more medially
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32
Q

What is shown in this image?

A
  • there is a fluid level in the right maxillary sinus (compare to left)
  • there is an area of radio-opacity below the right infra-orbital rim, suggesting blow-out fracture and prolapse of orbital contents
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33
Q

What are the 6 steps to interpreting facial X-rays (OM and OM 30)?

A
  • check patient details and projection
  • assess the technical adequacy of the film
  • describe any obvious abnormalities in as much detail as possible:
  1. look for symmetry / asymmetry by comparing left / right sides
  2. if one fracture is seen, look very closely for others
  • trace the specific “lines” along the facial bones to look for fractures
  • identify and assess the sinuses
    • they are air-filled so should be radiolucent
    • opacity or fluid-level suggests bleeding, and hence, fracture
  • look for signs of air in the orbit - “black eyebrow sign”
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34
Q

Why might a fluid level in the maxillary sinus not always be seen?

A
  • presence of the fluid level depends on the injury, as well as when the patient presents
  • if they present slightly later, the fluid level may not be seen
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35
Q

What terms are used to describe lesions within the orbit?

A

Extra-conal lesions:

  • this is used to describe lesions occurring within the muscle cone

Intra-conal lesions:

  • this is used to describe lesions occurring outside of the muscle cone
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36
Q

What is meant by a pre-septal and post-septal lesion in the orbit?

A
  • the septum is a line that runs across the midline of the globe
  • a post-septal lesion is more concerning
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37
Q

What is concerning in this image?

Why is immediate action required?

A
  • there is a retrobulbar soft tissue lesion on the right side
  • it is an extraconal lesion as it is occurring to the right of the lateral rectus
  • the lesion appears quite dense so it is likely to be blood (retrobulbar haematoma)
  • immediate action is required as the lesion is compressing the optic nerve
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38
Q

From what position are CT and MRI images viewed?

A
  • axial CT and MRI images are viewed from a caudal position
  • you are looking up from the feet towards the patient’s head
  • this means that the patient’s left is your right as you look at the image
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39
Q

What is attenuation / density in CT?

What unit is used to measure attenuation / density?

A
  • attenuation / density describes how bright or dark structures appear on CT
  • high density tissues / regions appear bright
  • low density tissues / regions are dark
  • the Hounsfield unit (HU) is a “measure” of the densities of different tissues:
  1. air is -1000 HU (black)
  2. water is 0 HU (grey)
  3. bone is +1000 HU (white)
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40
Q

Why is CT able to produce more detailed images than X-ray?

A
  • the HU scale is a measure of tissue density, represented by a number and corresponding shade of grey
  • this allows production of more detailed images and allows for tissues with similar densities to be differentiated from one another
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41
Q

How does a CT scan actually work to produce an image?

A
  • as tissues have different densities, they attenuate X-ray beams to different degrees
  • an X-ray tube and multiple detectors rotate around the patient
  • a thin “slice” of body is exposed to the X-ray beam at any one time, and as the scanner rotates, the tissues in that slice can be viewed from multiple angles
  • the slice thickness is between 1 - 10mm
  • the slice is divided into a grid of voxels and the tissue density of each voxel is calculated
  • this allows for a corresponding grid of pixels and the final image to be constructed
42
Q

What are the 3 main advantages of using CT scans over X-ray?

A
  • CT scanner produces images with a much greater range of contrast, allowing tissues of similar densities to be distinguished from each other
  • 3D images can be produced
  • images can be manipulated in several ways after they have been taken to highlight specific areas of interest
43
Q

When might a CT thorax be performed in the acute setting?

A
  • to rule out / confirm pulmonary embolism (CTPA)
  • in chest trauma
  • in suspected thoracic aorta aneurysm rupture or dissection
44
Q

What is looked for in a CTPA?

What must be administered beforehand?

A
  • it requires administration of iodine-based contrast to the patient
  • CTPA does not show a pulmonary embolism directly, but a filling defect in the pulmonary vasculature
  • it can also show complications caused by PE, such as lung infarction
45
Q

When is contrast used in a CT thorax?

A
  • contrast involves putting a dye into blood vessels
  • it is used the majority of the time in CT thorax to look at the blood vessels and see whether structures in the lungs / lymph nodes are enhanced
  • contrast makes the vessels appear bright
  • this makes lesions easier to spot when they are against a bright background
46
Q

What are the following structures?

How could you use an image like this to look for pulmonary hypertension?

A
  • the diameter of the main pulmonary trunk should be less than the diameter of the aorta
  • the diameter of the pulmonary trunk is increased in pulmonary hypertension
47
Q

How can you tell that this scan has been taken early on after injection of contrast?

A
  • there is still contrast present in the SVC
  • contrast is injected into a vein in the antecubital fossa and then flows around the vascular system in a preditable fashion:
  1. pulmonary artery
  2. aorta (20 seconds)
  3. spleen / portal system (60 seconds)
  4. IVC (90 seconds)
  • this influences the timing that scans should be performed at
48
Q

What is represented by the coloured circles?

At what vertebral level was this taken?

A
  1. blue = right and left bronchi
  2. green = thoracic duct
  3. pink = azygos vein
  4. orange = oesophagus (contains air)
  • this scan is taken at around T4
    • most things happen at T4 in the thorax, so if in doubt, refer to this level
  • the azygos vein carries deoxygenated blood from the posterior chest/abdominal walls, passes over the top of the bronchus to drain into the SVC
  • (this scan later than the previous as contrast is less dense in pulmonary arteries than the aorta)
49
Q

What are the 3 most common reasons for requesting a chest CT?

A
  • if a suspicious mass is seen on CXR and malignancy investigation is needed
  • suspected pulmonary embolism (CT = gold standard)
  • suspected aortic dissection in patients with a suggestive history
50
Q

What is shown in this image?

What is the major concern with this mass?

A
  • there is a solid mass at the periphery of the lung on the right side of the chest
  • it is a solid mass as it does not contain air bronchograms, unlike consolidation
  • it is likely to be cancer** as the lesion is **spiculated (spikes on the outside)
  • as it is abutting the chest wall, there is concern that it might be invading into the chest wall
51
Q

When is a CTPA performed?

How is this performed and what is looked for on the scan?

A
  • CTPA is the gold standard assessment for imaging the pulmonary arteries and looking for pulmonary embolism
  • the patient is given an IV contrast agent and images are taken when this enters the pulmonary arteries
  • the contrast appears hyperdense (white) on CT scan
  • if there is PE obstructing a vessel, blood (and contrast) cannot fill this area
  • the clot is seen as a hypodense (dark) mass** and referred to as a **“filling defect”
52
Q

Other than the filling defect on CT, what are some other signs that are suggestive of pulmonary embolism?

A
  • usually, the LV is larger than the RV on CT

reversal of the RV:LV ratio occurs due to the blockage increasing the pressure in the main pulmonary artery, which is fed back into the right side of the heart, causing dilation

  • usually, the interventricular septum is slightly deviated towards the right as the pressure in the heart is higher on the left side

as a result of an increase in RV pressure, the interventricular septum becomes straightened

53
Q

What is meant by a saddle embolus?

A
  • a large PE that occurs at the bifurcation of the pulmonary trunk and extends into both pulmonary arteries
  • this appears as a dark band extending across both pulmonary arteries
54
Q

Why is there a dark patch present in the SVC in this image?

A
  • this is NOT a thrombus!!!
  • as one brachiocephalic vein contains contrast and one does not, when they meet at the SVC there is mixing of opacified and non-opacified blood
  • this is called ad-mixing and it results in an artefact that resembles a thrombus
55
Q

What is significant about the ascending aorta in this CT scan?

A
  • when compared to the diameter of the descending aorta, it is noticeably dilated
  • it appears to have a line bisecting it, with differing tissue densities on either side of the line
56
Q

What condition is shown in this CT scan?

Why does the ascending aorta have this appearance?

A

aortic dissection

  • the line bissecting the aorta demarcates the true lumen** from the **false lumen
  • the true lumen is filled with contrast and appears more hyperdense (bright)
  • the false lumen appears more hypodense (dark)
  • the false lumen is a defect in the intima, which blood can penetrate to lift a flap of intima

it can still contain contrast and blood, but the true lumen contains denser, faster-flowing contrast so appears brighter

57
Q

What is shown in this CT scan?

A

aortic dissection in the descending aorta

  • it is difficult to determine which is the true and false lumen in this image as they appear as similar densities
58
Q

Why is CT abdomen performed in the acute setting?

Is contrast required?

A
  • it can be performed with or without contrast
  • it is performed in the acute setting to assess for serious / life-threatening intra-abdominal pathology, such as:
  1. ruptured / leaking AAA
  2. perforation of a hollow viscus
  3. acute pancreatitis
59
Q

How cna you identify the first lumbar vertebra in a CT abdomen?

A
  • as you scroll down, look for the ribs articulating with the transverse processes of the thoracic vertebrae
  • the first vertebra without a rib is L1
  • the following vertebral levels can be identified from there on
60
Q

What are the highlighted features on this axial CT abdomen?

A
  1. dark blue = rectus abdominis
  2. pink = liver
  3. blue dot = gallbladder
  4. green = stomach (with fluid level)
  5. star = duodenum
  6. bright blue = pancreas
  7. baby blue = right kidney
  8. purple = right crus of diaphragm
  9. red = aorta
  10. yellow = IVC
  11. orange = spleen
61
Q

Why is contrast usually preferred for CT abdomen?

A
  • it allows pathology to be seen more easily when structures / vessels are enhancing
  • in acute kidney obstruction / hydronephrosis, the affected kidney will not enhance as well as the healthy one
  • lesions within the liver can be seen more easily if the background liver is enhanced and the lesions do not enhance
62
Q

How does the gallbladder usually appear on CT?

A
  • it is low density as it contains bile, so usually appears hypodense (dark)
  • if there are gallstones within the gallbladder, these will appear bright (hyperdense)
63
Q

What structures can be seen at L2?

A
  • a smaller part of the abdomen is filled with the liver
  • the transverse colon, duodenum and small intestine are visible
  • the superior mesenteric artery and vein are visible here
    • at L1, we see the portal vein which is formed from the union of the superior mesenteric vein and the splenic vein
64
Q

What structures can be seen at level L3?

A
  • at this level, abdominal contents comprise mostly bowel
  • this is below the level of the kidneys, but the lower part of the liver can still be seen
  • the psoas muscles are visible
  • the ureters can be seen running over the psoas muscles on their course from the kidneys to the bladder
65
Q

Why does a CT-urogram need to be performed later than other types of CT scan?

A
  • the ureters will not usually appear dense as they contain urine
  • a late-phase image is required as the patient needs to have time to excrete the contrast in order for the ureters to appear hyperdense (bright)
66
Q

What are the 4 most common reasons for requesting a CT abdomen?

A
  • investigation of possible intra-abdominal malignancy
  • imaging the aorta in suspected or known AAA
    • this could be in the context of surgical planning or acute abdominal pain
  • patients presenting with acute abdominal pain with a history / examination suggesting serious intra-abdominal pathology
  • CTKUB is the gold standard for renal colic
67
Q

What is shown in this image?

A
  • the liver is taking up most of the space in the abdomen, with the abdominal aorta and spleen visible on the left side
  • several hypodense lesions within the liver are present (not enhanced as well as the rest of the liver)
  • they are likely to be metastases
  • they are low density and appear to be infiltrating into the liver tissue
  • they are not abscesses**, as these would form **ring-enhancing lesions with accompanying clinical signs
    • some mets can also be ring-enhancing so rely on clinical features
68
Q

What is shown in this image?

A

abdominal aortic aneurysm

  • there is a hyperdense lumen filled with contrast (bright)
  • and a thrombus within the aorta that is hypodense (dark)
  • this is different to a dissection, as it is crescentic in appearance rather than a line dissecting the lumen
  • contrast is required, as without this, you would not be able to tell apart the lumen from the thrombus
69
Q

When is CT-KUB performed?

What is significant about this type of scan?

A
  • it is the first-line investigation for suspected renal calculi
  • this scan is performed WITHOUT contrast
  • most kidney stones are radio-opaque and appear hyperdense (bright) on CT
  • if contrast was used, the ureters would also appear hyperdense and this would disguise the stone
70
Q

How does the choice of renal tract imaging change in the acute and non-acute setting?

Why is CT preferred in the acute setting?

A
  • CT is preferred in the acute setting a it can demonstrate other potentially life-threatening causes of acute intra-abdominal pain
  • e.g. ruptured / leaking AAA in at-risk groups
  • intravenous pyelogram (IVP) tends to be used in the non-acute setting
71
Q

What is shown in this image?

A
  • this is a CT-KUB showing a large stone in the left ureter
  • the left ureter is grossly dilated
  • there is also hydronephrosis as the left renal collecting system is also dilated
72
Q

What is shown in this image?

How do you know it was an incidental finding and what would the treatment be?

A
  • CT-KUB showing a large calculus in the hilum of the left kidney
  • the left kidney is not dilated and is comparable to the size of the right kidney
    • this is unlikely to be causing any symptoms currently
  • treatment is required before the stone enlarges further in situ and obstructs the flow of urine or passes into the ureter
  • if it passes into the ureter, this can cause severe pain and ureteric obstruction
73
Q

When is CT scanning the first-line imaging modality for the brain and why?

A
  • CT is first-line for imaging the brain and skull in the acute setting
  • it has good anatomical detail and is excellent for revealing:
  1. fresh intracranial bleeding (better than MRI)
  2. infarction
  3. skull fractures
74
Q

Why is non-contrast CT performed in suspected ischaemic stroke or intracranial bleeding?

A
  • contrast within the vascular tree will appear hyperdense
  • this will make it difficult to differentiate the contrast from the acute intracranial bleeding
75
Q

What is represented by labels A-G?

Has contrast been given for this scan?

A
  • A = falx cerebri
  • B = frontal lobe
  • C = body of lateral ventricle
  • D = splenium of corpus callosum
  • E = parietal lobe
  • F = occipital lobe
  • G = superior sagittal sinus
  • this is an example of a non-contrast axial CT head
76
Q

What is represented by labels A-I in this axial CT head?

A
  • A = anterior horn of lateral ventricle
  • B = head of caudate
  • C = anterior limb of internal capsule
  • D = putamen & globus pallidus
  • E = posterior limb of internal capsule
  • F = 3rd ventricle
  • G = quadrigeminal cistern
  • H = vermis of cerebellum
  • I = occipital lobe
  • mainly the cerebellar hemispheres are seen, but a part of the cerebellar vermis is seen due to the plane the scan is taken in
    • the cerebellum can be recognised by the folia
77
Q

What appears darker on CT, white or grey matter?

Why?

A
  • grey matter contains few axons and a high density of cell bodies
  • white matter contains many myelinated axons
  • myelin is a fatty substance, so it is of relatively low density compared to the cellular grey matter
  • white matter appears darker than grey matter as it is lower density
78
Q

What is a sign of raised intracranial pressure involving the quadrigeminal cistern?

A
  • shrinking of the quadrigeminal cistern is a sign of raised intracranial pressure
  • this is represented as the “bum moving onto the toilet seat”
  • the ambient cisterns form the V-shape adjacent to the quadrigeminal cistern
79
Q

What is the quadrigeminal plate?

A
  • the quadrigeminal plate consists of the superior and inferior colliculi
  • the quadrigeminal cistern sits behind the quadrigeminal plate
  • the cerebral aqueduct, which connects the 3rd to 4th ventricle, sits in front of the plate
80
Q

In the acute setting, when might CT of the C-spine be performed?

A
  • if C-spine fracture is seen on plain X-ray
  • if a patient with history of injury to the C-spine has neurological signs/symptoms
  • if an XR is equivocal or normal, but there is ongoing clinical concern
    • a normal XR cannot rule out pathology when there are ongoing concerns / symptoms / signs
  • as part of a trauma series in major trauma
81
Q

When is MRI usually performed?

What are the 2 exceptions to this?

A
  • MRI is usually performed in the non-acute setting
  • the exceptions are in suspected spinal cord compression and cauda equina syndrome as these are both surgical emergencies
  • these arise as a consequence of compression by a prolapsed intervertebral disc or a SOL
  • urgent imaging reduces the risk of irreversible paralysis of the lower limbs and loss of sphincter control
82
Q

What is the basic principle behind how an MRI image is generated?

A
  • when hydrogen nuclei are all spinning in synchrony (“phase”), a signal is generated and an image can be created
  • when the RF pulse is switched off, the hydrogen nuclei relax and desynchronise, and the strength of the signal decreases
  • relaxation and desynchronisation does not happen at the same rate in all tissues as different tissue types have different densities of nuclei
  • when the RF pulse is removed, different tissue types generate different signals as their nuclei relax and desynchronise at different rates
  • different signals are represented as differing shades on a grey scale
83
Q

How is the appearance of structures on MRI referred to?

A
  • the appearance of structures is referred to in terms of signal intensity
  • bright tissues are said to have a high signal
  • dark tissues are said to have a low signal
  • the appearance can vary depending on whether T1 or T2 images are being viewed
84
Q

What is the difference between a T1 and T2 scan?

A
  • the T1 signal is a representation of how quickly hydrogen nuclei in different tissues relax after the RF pulse is removed
  • the T2 signal is a representation of how quickly the hydrogen nuclei desynchronise when the RF pulse is removed
85
Q

What is the difference in the appearance of CSF in a T1 and T2 scan?

A
  • CSF appears dark in a T1 scan** and **bright in a T2 scan
86
Q

What is a FLAIR sequence and when might this be used?

A
  • it is similar to a T2 image, but the high signal from tissues with a high water content (e.g. CSF) is suppressed
  • this is useful for identifying lesions that lie close to CSF spaces
  • suppression of the signal that would have been generated by CSF allows other lesions (e.g. blood) to stand out more clearly
87
Q

What is meant by diffusion-weighted imaging (DW-MRI) and when might this be used?

A
  • this is used to detect an acute from an old infarct
  • in acute ischaemia, the cell membrane of glial cells becomes dysfunctional and allows sodium and water to move into them and cause oedema
  • this region will generate a high signal on DW-MRI
88
Q

What is represented by numbers 1 to 12 in this image?

What type of MRI scan is this?

A
  1. pons
  2. corpus callosum
  3. pituitary gland
  4. thalamus
  5. midbrain
  6. hypothalamus
  7. quadrigeminal plate
  8. dens
  9. 4th ventricle
  10. lateral ventricle
  11. tentorium cerebelli
  12. sphenoid sinus
  • this is a para-sagittal MRI (not quite in the midline as the turbinates can be seen in place of the nasal septum)
  • it is a T1 weighted image (CSF appears dark)
89
Q

What are the structures on this T2 weighted image?

What is meant by peritrigonal high signal?

A
  • the trigones of the lateral ventricles can be seen in this view
  • the occipital and temporal horns emerge from the trigones
  • high signal around the trigones suggests white matter changes in small vessel disease
90
Q

When is CT better than MRI?

A
  • CT is better than MRI for revealing acute / fresh bleeding
  • CT is usually performed in the acute setting as it is much quicker, and usually sufficient to give a definitive diagnosis
91
Q

What is the most common reason for undertaking CT head in the acute setting?

A
  • to identify whether a patient has had an intracranial bleed (spontaneous or secondary to head trauma) or cerebrovascular accident (stroke)
  • strokes can be haemorrhagic or ischaemic, and these appear very different on CT
92
Q

What is the difference in the way fresh intracranial bleeding and areas of ischaemia appear on CT?

What is the issue with this?

A

New intracranial bleeding:

  • appears bright white (hyperdense) on non-contrast CT

Areas of ischaemia:

  • appear hypodense and darker than the surrounding brain tissue on on-contrast CT
  • very soon after a stroke this chance can be very subtle or not visible at all
  • the hypodensity (“low attenuation lesion”) evolves over time
93
Q

What type of lesion is shown here?

Why is it likely to be an elderly patient?

A
  • this is an infarct in the territory of the middle cerebral artery
  • this is ischaemic as the infarct appears hypodense and darker than the surrounding brain tissue
  • it is likely to be chronic as it is not causing much mass effect relating to its large size
    • if the brain was swollen, there would be pressure on the ventricles and they would change shape
  • it is likely to be an elderly patient as there is cerebral atrophy anteriorly
94
Q

Why does this image represent an example of cystic encephalomalacia?

A
  • the neurones within the infarcted area die, but the support structures are still there
  • volume is lost from the infarcted area, but it is replaced with cystic changes and gliosis
  • the glial cells form a scar, which can be seen on MRI
95
Q

Is this lesion acute or chronic?

Why might their be visual disturbances?

A
  • this is likely to be acute as there is some mass effect, shown through compression of the ventricles
  • this is a posterior circulation infarct affecting the visual cortex within the occipital lobe
  • the lesion is hypodense, demonstrating an area of ischaemia
96
Q

Where is the lesion?

Is this acute or chronic and how could you confirm this?

A
  • focal hypodensity of the left thalamus
  • there is not much mass effect, but it is difficult to determine whether it is acute or chronic
  • this could be confirmed by performing a DW-MRI
97
Q

How can CT be used in the acute and non-acute setting to investigate if a patient has a space-occupying lesion?

A
  • if a SOL is suspected, a non-contrast CT is performed first
  • if there is a region of suspicion, a CT with contrast is then taken
  • contrast enhances SOLs so they can be assessed more easily
98
Q

What is the difference between these images?

A
  • image 2 is a contrast-CT, which allows the metastases to be more easily seen
99
Q

What is the difference between these 2 images and what do they show?

A
  • MRI images are used to assess SOLs in the non-acute setting
  • the image on the left does not use contrast, whereas the image on the right does
  • MRI gives excellent visualisation of SOLs, which is enhanced even further when contrast is used
100
Q
A
101
Q
A

Medical Imaging (12 decks)

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