The lemon sign

For my blog this week I wanted to learn more about spina bifida. Occasionally in the clinic we will see a case of spina bifida occulta when scanning a lumbar spine. However, I wanted to learn more about how more severe cases are diagnosed in utero. I found an interesting article which talked about the “lemon sign” on ultrasound. This sign is strongly associated with spina bifida. The image on the left shows the normal contour of a skull in an 18 wk fetus in utero. The image on the right is of a 20 week old fetus with spina bifida (note the lemon shape). The scans are transverse at the level of the ventricles. The lemon shape is caused by a loss of the normal shape of the frontal bones and it is suspected that a decrease in intracranial pressure causes the brain to shift downward, thus resulting in the depressed skull. For some reason the frontal bones are most sensitive to the pressure change. Interestingly this sign will disappear as the fetus matures so this sign is most useful in ultrasound before 24 weeks, especially in cases of open spina bifida. The lemon sign can also indicate other conditions but seeing this sign in an ultrasound prior to 24 weeks means the ultrasound tech should evaluate the spine more closely looking for other signs of spina bifida. In addition, the cranium needs to be evaluated for the 4 signs of spina bifida – ventriculomegaly, microcephaly, obliteration of the cisterna magna (w/absent cerebellum), and abnormal anterior curvature of the cerebellum.

Spina bifida occurs about seven times in every 10,000 live births. Folic Acid supplementation before and during pregnancy has been shown to decrease the risk of spina bifida in the fetus. Ultrasound or amniocentesis tests for spina bifida are usually ordered if alphafetaprotein (AFP) results are elevated in the mother’s second semester quadruple screen blood test. Effects of spina bifida on the fetus can be mild to severe and can include spinal abnormalities, hydrocephalus, paralysis, and urinary and/or fecal incontinence. Prenatal diagnosis is important, allowing the parents the option of in-utero surgery, surgery after birth, or pregnancy termination.

Images and References:

Spina Bifida Association. Accessed December 8, 2009 at http://www.spinabifidaassociation.org/site/c.liKWL7PLLrF/b.2642323/k.8E10/Spina_Bifida.htm

Thomas, M. (2003). The Lemon Sign. Radiology. RSNA Journals Online. Accessed December 8, 2009 at http://radiology.rsna.org/content/228/1/206.full

Thoracic Compression Fractures and Osteoporosis

For my blog this week I decided to write about thoracic vertebral compression fracture since this is a common reason for CT scans at my clinical site. Vertebral compression fractures are typically caused by osteoporosis and a severe fracture can cause a great deal of pain for the elderly patient. This pain can begin to interfere with the ability to perform tasks of daily living.

Vertebral fractures will affect 25% of postmenopausal women in the U.S. The risk increases with advancing age and rises to 40% by age 80. Of great concern is the mortality rate for these women. It is 15% higher than for the same age group without fractures.

Acute fractures usually occur with some trauma, like a fall, tripping over an object or a lifting injury. If the osteoporosis is severe something as simple as lying in bed or sneezing can cause a fracture. The force usually causes the anterior portion of the thoracic body to compress, forming a wedge. The fracture is considered “burst” if the entire vertebra compresses. Over time and with multiple fractures the patient will lose height and become kyphotic. Many fractures go undiagnosed because older patients may expect back pain as a sign of aging or arthritis.

Compression fractures in the thoracic area are usually in the T8 – T12 area. CT is usually used to confirm plain film and physical exam diagnosis, show spinal cord narrowing and assess the stability of a fracture. MR is used if the patient is showing symptoms of spinal cord impingement. Nuclear Medicine is also used for diagnosis because new fractures will appear “hot” on the scan but healed fractures will not.

Conservative treatment for stable fracture includes muscle relaxants, back brace, NSAID’s if well tolerated, and physical therapy. Mild exercise is encouraged once the fracture is fully healed. Some patients may require either vertebroplasty or kyphoplasty, where cement is injected into the collapsed vertebral body to increase stability. This CT image shows a thoracic compression fracture.

Image: http://www.urmc.rochester.edu/smd/Rad/neuroimages/CTT-spine.jpg accessed November 30, 2009

Reference: Old, J. & Calvert, M. (2003). Vertebral Compression Fractures in the Elderly. AAFP, January 1 2004. Accessed November 30, 2009 at http://www.aafp.org/afp/20040101/111.html

Odontoid Fracture

For my blog this week I wanted to learn more about Odontoid fracture. The facility where I work weekend option diagnostic x-ray is going to become a Trauma One facility in the next few years so we will be seeing more severe trauma. And we recently had a patient come into my clinical location for repeat scans post C-2 fracture. He had been in a car accident 8 months prior and sustained a severe whiplash injury to his neck. He told me that he had been in the back seat of the car when it was rear-ended at high speed. The fact that he wasn’t wearing his seat belt actually saved his life because he was only flung forward. He was told by his physicians that had he been caught by the seat belt and flung backward as well the force would have severed his spinal cord and killed him instantly.

I was amazed that he walked into the clinic. He told me he was put into a collar and onto a backboard at the accident scene. He had an immediate CT scan in the ER which showed the fracture. His first surgery was an anterior screw for stability similar to image #4 on the right. I forgot to ask him if he wore a halo after this surgery but I would guess that he did since this seems to be the conventional treatment. He told me that the initial screw started to migrate causing him pain and numbness so a second surgery was performed replacing the first screw with posterior hardware. We were scanning in CT that day to assess the healing and stability of the second set of hardware (similar to image #1 on the left). Unfortunately I don’t have his actual images to post here.

I learned that there are 3 types of Odontoid fracture and most are caused by MVA’s and falls. 1/3 of C-spine injuries occur at C2 and ½ at C6-C7. The majority of fatal injuries occur at C1 or C2 and they tend to occur due to extreme flexion, extension or rotation. They are classified by their location on the dens. Type I goes through the very top of the dens and is very rare, only about 5% of cases. Type II is shown in the middle two images here and is through the base of the dens. This represents about 60% of dens fractures and is the most common type. Type III is through the body of C2 and doesn’t actually involve the dens and is about 30% of fractures. CT is better for demonstrating fractures and MR is used to show the extent of soft tissue, disc, ligament and spinal cord involvement.

Type I fractures are usually treated with a hard collar. Type II fractures are treated surgically as described above with the addition of a stabilizing halo for several months. My patient told me he just feels lucky to be alive. After seeing his images, I must say I agree!

References and Images:

http://www.learningradiology.com/caseofweek/caseoftheweekpix2007-1/cow248arr.jpg

http://www.ispub.com/ispub/ijss/volume_3_number_1/spinal_stenosis_c1_2_following_redo_surgery_for_failed_odontoid_screw_fixation_scrutinizing_the_odontoid_fracture_classification/spinal-fig2.jpg

http://www.thebarrow.org/stellent/groups/public/@xinternet_con_bni/documents/webcontent/bqjpg120.jpg

http://images.google.com/imgres?imgurl=http://www.learningradiology.com/caseofweek/caseoftheweekpix2007-1/cow248arr.jpg&imgrefurl=http://www.learningradiology.com/archives2007/COW%2520248-Dens%2520Fracture/densfxcorrect.html&usg=__9b3ipB8OObNWJB1UHLHw2svtEAA=&h=332&w=604&sz=38&hl=en&start=2&sig2=scLSLDfPMqpXROTBDOmmYA&um=1&itbs=1&tbnid=ip3Up3zf1tX1MM:&tbnh=74&tbnw=135&prev=/images%3Fq%3Dodontoid%2Bfracture%26hl%3Den%26sa%3DX%26um%3D1&ei=-gkMS_LfLIG8MPSC0dgC

CTA vs. MRA vs. Angiography

For my blog this week I decided to learn more about how physicians order neck imaging exams for signs and symptoms of vessel stenosis or occlusion. When do they order CTA, MRA, vascular ultrasound, or conventional angiography? I wanted to learn more about this topic because I didn’t know which modalities are considered better or when they are used. So here is what I learned. In general, conventional angiography is now used mainly for treatment and CTA, MRA and ultrasound are used for diagnosis. This is a shift from the past when conventional angiography was used for both diagnosis and treatment. In the absence of specific symptoms these tests are usually ordered after some type of abnormality is shown in a CT or MR scan. The tests will be ordered directly if the patient comes into the emergency room showing symptoms of occlusion. CTA, MRA and ultrasound are used for diagnosis because they are less invasive and do not involve a femoral stick and catheter placement, thus they are less risky for the patient. In addition, they can be done by a technologist rather than a physician.

I did some reading on the web about vascular ultrasound and discovered that this is a good initial screening test. Advantages are it is non-invasive, inexpensive and can be done at the bedside. Disadvantages are it is better for surface vessels but less accurate for deeper vessels. It is also fairly operator dependent so the technologist needs to be highly skilled in order to get an accurate result. If abnormal results are noted then a CTA or MRA is usually ordered. The CTA or MRA can usually give enough information so that surgery decisions can be made although some physicians like to have both a CTA and MRA done before surgery.

So when should a CTA vs. a MRA be ordered? Obviously, any patient who is allergic to iodinated contrast should have an MRA. A patient with a lot of metal in their body should be scheduled for a CTA. A patient with severely impaired renal function could have a MRA with 2D time of flight images (no contrast needed). In general, the cardiology community prefers MRA for evaluation of the carotids and CTA for evaluation of the aorta (although I found another study which indicates MRA is now preferred for the aorta). MRA is better for showing perfusion and function. CTA would be better if there was also suspected lung or airway involvement (i.e. pulmonary embolism in conjunction with stroke). For critically ill patients CTA would be faster than MRA. If a patient required repeat studies over time then MRA would be preferred (less radiation dose). CTA tends to be better in showing the coronary arteries in young children.

I thought this was a worthwhile blog for the week. Now I feel I could answer a patient question if I were asked why a CTA had been ordered vs. another type of imaging study. The image above is of an MRA of the neck.

References and Image:

Gallo, B (2008). Magnetic Resonance Angiography (MRA) and Computed Tomography Angiography (CTA): The new gold standards in vascular imaging. http://www.frost.com/prod/servlet/market-insight-top.pag?docid=144155345

http://www.tramedicalimaging.com/assets/news/TRA_News_12_03.pdf

http://radiologycme.stanford.edu/2008mdct/handouts/tue/1505-Chan-MRA_CTA.pdf

http://www.anthem.com/medicalpolicies/guidelines/gl_pw_a053801.htm

http://www.sikermedical.com/services/mri/mra_carotids/

http://www.imaginggroupde.com/images/mr_angiography_neck.jpg

All studies accessed via web on November 17, 2009