Monday, April 30, 2012

How To Read A Head CT

A noncontrast CT of the head has become the dominant modality for an initial assessment of intracranial pathology, especially in the setting of trauma. Many approaches to reading a head CT exist, with the method described below simply being one example. As they say, there are many ways to skin a cat. Of course, the real question in my mind is: why is anyone skinning a cat in the first place? Anyway, um, back to head CTs: no matter how you read a study, it is important that you:
  • Cover all the relevant areas of the study
  • Go through your system in a consistent manner
Sounds simple enough, but it is very very important to set up a good system and then go through it each time. On some studies, a major finding may jump out at you and distract from other areas, which may contain even more significant findings. This phenomenon has been termed search satisfaction, which is where you note a finding and therefore stop searching for more. One way to avoid this is to try to make at least two findings on every study, no matter how trivial, just to force yourself to cover the entire study.

Hopefully you are not satisfied with this post... yet. Getting back to head CTs, there are some unique features of the brain that assist in one's reading of this type of study. The main feature is symmetry: the two hemispheres of the cerebrum and cerebellum should generally look the same. Unlike the abdomen or even the chest, symmetry in the head allows for easy comparison across the midline whenever a possible abnormality is questioned. Another helpful feature is that the cranium is a confined space, so if there are masses (or abnormalities with mass effect), neighboring structures will typically be compressed rather than merely pushed out of the way. Practically this means that even small lesions can have a significant impact on the appearance of the brain.

A Systematic Approach

Keeping those general principles in mind, here is one general 10 step approach to assessing a head CT.
  1. Posterior fossa
    Check to see if the cerebellum is symmetric. If the cerebellar fissures are prominent, this may represent volume loss. If the tonsils are low-lying (visible below the foramen magnum), this may represent a normal variation or an Arnold-Chiari malformation.

  2. Ventricles
    Ensure that the four ventricles are in their expected positions and are not enlarged. The lateral ventricles should be symmetric.

  3. Midline structures
    The midline structures should be... midline. Specifically, the septum pellucidum should be exactly in between the lateral ventricles. To check deviation, draw a line from the apex of the inner table of the calvarium anterior to posterior. At the level of the globus pallidus, check for deviation of the septum from the line.

  4. Extra-axial spaces
    Look for any collections between the inner table of the calvarium and the brain parenchyma. Blood, CSF fluid, air, and pus can potentially collect here. For bleeds, biconvex collections represent epidural bleeds; lenticular bleeds represent subdural bleeds and can cross suture lines. Subarachnoid bleeds will line the sulci. The base of the skull is a good place to look for bleeds after trauma due to the sharp edges there.

  5. Brain parenchyma
    Check for symmetry, gray-white differentiation, areas of hyper- or hypo-attenuation, and any architectural distortion.

  6. Orbits
    Check for symmetry, propotosis, lens position and thickness, the optic nerve, artery, and vein. Use soft tissue windows. Make sure no masses are seen behind the globe. This is also a good time to check the lamina papyracea to rule out any fractures.

  7. Paranasal Sinuses
    Look for mucosal thickening or polyp formation. In trauma, fluid and blood can collect in the sinuses.

  8. Mastoid Air Cells
    Check to make sure that fluid is not accumulating in the air cells, especially in the setting of trauma.

  9. Bones
    Fractures can be hard to spot in the skull. The key to identifying an acute fracture is to look for overlying soft tissue swelling, lack of sclerotic margins, and lack of symmetry. Vascular channels are plentiful in the skull and easily confused for a fracture. However, they will have sclerotic margins. Sutures should be symmetric.

  10. Soft Tissues
    Look for soft tissue swelling or masses. Outside of the setting of trauma, the soft tissues should generally be unremarkable.

While this is not a fully comprehensive approach to reading a head CT, it provides a framework for assessing the critical structures. By being systematic and thorough each time, it is highly unlikely that you will miss a significant finding. Review your neuroanatomy thoroughly to help you localize a lesion once you do spot an abnormality. For pediatric patients, the head CT may be preceded by a neonatal head ultrasound, which we will cover in a future post. 

Normal Head CT
Normal Head CT
Source: Wikipedia


Elbow Ossification Centers

The elbow ossification centers ossify at different ages as a child develops. Knowing the timeline of ossification is very important when assessing whether a child has a fracture in their elbow. A helpful mnemonic for remembering the ossification centers of the elbow is the six-letter made-up word of CRITOE. I suppose it rhymes with 'elbow', but other than that - I don't see the relationship, but it is memorable enough for our purposes:

Ossification SiteAge (years)
RRadial Head3
IInternal (medial) epicondyle5
EExternal (lateral) epicondyle11

Of these, the I and T are the most important: the trochlea ossification center should not appear before the medial epicondyle one. I found the image below to be helpful, given the superimposed drawing: 

Ossification Centers of the Elbow
Ossification Centers of the Elbow
Source: Radiology Assistant

Sunday, April 29, 2012

How To Differentiate Neuroblastoma and Wilms Tumor

Renal masses are fairly common in the pediatric age group. Two of the most common are neuroblastoma and Wilms Tumor.

Neuroblastoma is a neuroendocrine tumor that usually develops in the adrenal glands, but can develop in any neuroectodermal tissue. One unique feature of disease is that a subset of children with metastatic disease located only in the liver, bone marrow, and skin, will demonstrate spontaneous regression. This Stage IV-S (S for special) does not require any form of chemotherapy.

Wilms tumor (also called nephroblastoma) is the most common renal malignancy in children. It develops from the embryologic metanephric blastema. The eponym was coined for German surgeon Max Wilms who first described the entity.

Since both tumors can be commonly found in the abdomen, they can easily be confused for each other radiographically. Here is a table that helps distinguish the two:

FeatureNeuroblastomaWilms Tumor
AgeLess than 2 yrsPeak at 3 yrs
Ca++Yes (stippled)No
Growth pattenSurrounds and engulfs vesselsDisplaces vessels
Effect on kidneyInferior displacement and rotationArises from kidney (claw sign)
Lung metsNoYes (20%)
Vascular invasionNoRenal vein, IVC, right atrium

Adapted from Pediatric Imaging: The Fundamentals, 1e by Lane Donnelly. 

Wilms Tumor with Claw Sign
Wilms Tumor with Claw Sign
Source: Radiopaedia

To summarize, Wilms tumor occurs in slightly older children, is centered in the kidney, and has vascular spread. Neuroblastoma is typically younger and more agressive, metastasizing by direct spread to liver and bone.

Friday, April 27, 2012

Wormian Bones: Differential Diagnosis #6

Wormian bones are ossified structures that are found within the sutures. Their incidence varies widely among different ethnic groups, with a slightly greater prevalence among females. Some authors only consider them abnormal if they number 10 or more. The differential for Wormian bones is broad and includes many entities, which can be remember by the mnemonic PORKCHOPS:

PPyknodysotosisassociated with osteosclerosis and acro-osteolysis
OOsteogenesis imperfecta
KKinky hair syndromeaka Menkes disease
CCleidocranial dysostosismay have absent clavicles
HHypothyroidism / Hypophosphatasia
OOtopalatodigital Syndrome
Primary acro-osteolysis (Hadju-Cheney)

SSyndrome of Downs

Example of Wormian bones (Normal Skull in top left)
Source: Wikipedia

The most common location of Wormian bones is the lambdoidal suture.  The most common cause is actually idiopathic.


Thursday, April 26, 2012

Cyanotic Heart Disease: Differential Diagnosis #5

If a child has congenital heart disease, the blood may bypass the pulmonary system. If so, the blood is not fully oxygenated, causing the baby to have a dusky hue (appear cyanotic). Depending on the degree and type of malformation, the child may present anytime from birth up til late adolescence. Five common causes of cyanotic heart disease are the 1-2-3-4-5 Ts:

TTruncus arteriosus1 vessel
TTransposition of great arteries2 vessels switched; "egg on a string" heart
TTricuspid atresia3 leaflet valve
TTetralogy of Fallot4 findings: Pulmonary stenosis, RVH, VSD, aortic override; "boot shaped heart"
TTotal Anomalous Pulmonary Venous Return 5 vessels involved; "snowman" heart

In TAPVR, the pulmonary veins do not drain into the left atrium. In the most common form Type I, the vessels form the left vertical vein and drain superior to the heart to the brachiocephalic, SVC or azygos vein. The left vertical vein forms the snowman's head. In Type II, the pulmonary veins drain to the coronary sinus. In Type III, they drain subdiaphragmatically to the IVC.

TAPVR Type I (Supracardiac)
Source: LearningRadiology

Tuesday, April 24, 2012

Bowel Obstruction in Children: Differential Diagnosis #4

Intestinal obstruction is a common concern in the pediatric population. While neonates often have congenital causes of obstruction such as Hirschsprung's disease or meconium ileus, older children above the age of infancy are more likely to have acquired causes. They may also have late presentations of the entities found in neonates. An easy way to remember the common causes of bowel obstruction in children is the mnemonic where you take (AIM)^2 or AAIIMM:

IIncarcerated inguinal hernia
MMeckel's diverticulum Rule of 2s for Meckel's: 
2% of the population
2 feet from the ileocecal valve
2 inches long
2 types of ectopic tissue (gastric and pancreatic)
2 years old at presentation.
2 times more common in boys
MMalrotation / midgut volvulusCorkscrew appearance

Midgut Volvulus
Midgut Volvulus
Source: Radiopaedia

The mnemonic comes from Donald R. Kirks, MD as described in Lane Donnelly's Pediatric Imaging: The Fundamentals.

Saturday, April 21, 2012

Differential Diagnosis #3: Bilateral Upper Lung Disease

Today's Daily Diff is bilateral upper lung disease. When evaluating chest x-rays, this pattern of disease is fairly common. Having a handy list of possible diagnosis can help you narrow down possible etiologies when framed in an appropriate clinical context. One handy mnemonic to help remember this differential is CASSETTE PRR:

CCystic fibrosis
AAnkylosing spondylitis
EEosinophilic granuloma / Langerhans cell histiocytosis Young male smokers
TTumors (metastases)
EEverything else
PPneumocystis Pneumonia (PCP)Immunocompromised
RRadiation fibrosis Head and neck cancers
RRheumatoid lung

In general, keep in mind that the upper lung zones have increased aeration and decreased perfusion relative to the lower lung zones due to the effects of gravity.

Source: Radiology Notes

Thursday, April 19, 2012

The Lauge-Hansen Ankle Fracture Classification

Ankle fractures are a very common entity in musculoskeletal radiology (and for many a weekend warrior). Two main classification systems are used to group the various ankle fractures: the Weber and the Lauge-Hansen (LH) classification schema. While Weber is much easier to apply, LH is the standard used by radiologists and orthopedic surgeons as it offers a higher level of detail about the mechanism of injury. However, the main drawback is... it's confusing! However, misery loves company as the quote below from an ortho resident shows that the orthopods also find it confusing:
For whatever reason, I have a difficult time wrapping my head around the Lauge-Hansen (LH) ankle fracture classification.  The Weber classification is a little more straightforward, but doesn't impart as much information about the injury as the LH classification.  I'm going to go through ankle fractures like I did with pelvic fractures and hopefully, in attempting to understand the LH classification, impart some knowledge on everyone else.  As usual, I'm stealing my images from the AAOS Comprehensive Orthopaedic Review.  Information is borrowed from this text and the Handbook of Fractures. 
To begin, let's take a quick look at the anatomy of the ankle joint (picture below).  The ankle is made up of articulations between the tibia, fibula and talus.  The joint is maintained by a variety of ligaments.  On the lateral side, the anterior inferior tibiofibular ligament, posterior inferior tibiofibular ligament and the inferior transverse ligament help to prevent eversion of the ankle.  The lateral collateral ligaments of the ankle (anterior/posterior tibilfibular ligaments, calcaneofibular ligaments) help to prevent inversion and anterior translation of the fibula.  Medially, the strong deltoid ligament, which has a short and thick deep layer covered by a more superficial layer help to resist inversion of the foot.
To see the rest of the post which attempts to clarify the situation a bit, including the anatomy and fracture diagrams, check out the post on the blog Bone Broke? Me Fix!

The challenge for the radiologist is a little different from the orthopedist, who is more concerned about treatment. From the plain x-ray, the radiologist has to infer which type of injury has occurred. The following framework applies to the most common categories but is not all inclusive. Remember: in an ankle fracture x-ray, is the fibula fractured?
  • Yes: Is the fracture above the tib-fib syndesmosis, or at/below it?
    • High: If there is only a medial malleolus fracture, this is a pronation-external rotation type III; if there is a medial and posterior malleolus fracture, this is a P-ER type IV.
    • At/Below: If there is a posterior malleolus fracture also, this is a supination-external rotation type III; if there is only a medial malleolus fracture also, this is a S-ER type IV.
  • No: Is there a medial malleolus fracture?
    • Yes: This is a P-ER type II or III fracture.
    • No: This is a S-ER type I fracture (anterior tib-fib ligament sprain)
Like anything, getting used to this classification scheme takes practice. Review ankle fracture studies several times until it becomes comfortable. 

Tuesday, April 17, 2012

Differential Diagnosis #2: Cavitary Lung Disease

Today's Daily Diff is for cavitary lung disease. Cavitation can occur in the lung for many reasons. To remember the causes, think about big groups first, and just remember what you are looking at - a CAVITY:

CCancer (squamous cell carcinoma, melanoma, sarcoma mets) 
AAutoimmune (Wegener's granulomatosis, rheumatoid lung)
VVascular (ie bland or septic emboli)
IInfection (TB, fungal (coccidioidomycosis, aspergillosis), bacterial (Staph, strep, Klebsiella))
YYouth (ie congenital: bronchogenic cyst, sequestration)

To help narrow your differential diagnosis, consider the thickness of the cavity's wall. If the thickness is greater than 10 mm, this favors a metastatic process; thinner walls are more likely to be secondary to the other causes, with infection being the most common. 

Cavitary disease in pulmonary TB
Source: Radiopaedia

Source: Radiopaedia

Sunday, April 15, 2012

Differential Diagnosis #1: Cystic Lung Disease

Today's Daily Diff is for cystic lung disease. Specifically, for cases with multiple lung cysts. One mnemonic is to remember whenever you see a lung cyst, refer to your LIST:

LLymphangioleiomyomatosis, Lymphocytic interstitial pneumonia, Langerhans Cell Histiocytosis
IInfection (TB, Staphylococcus, PCP)
TTuberous sclerosis

Cystic lung disease (Ex. lymphangioleiomyomatosis)
Source: Dr. Andrew Dixon

(Partial) Reference: Radiopaedia Cystic Lung Disease