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CT and MRI imaging of normal anatomy at the level of the first cervical vertebra.
It’s summertime. The weather is scorching and most of us just can’t get enough of the water. It’s the perfect way to cool off from the summer heat, isn’t it? Cool water, with just a touch of adrenaline, what more could you need? But as easy it is for diving to give you an awesome thrill, one bad dive can just as easily leave you with a debilitating or sometimes even fatal spinal injury. These injuries often involve the cervical spine and, unfortunately, are a lot more common than many people think. So, why are we talking about diving injuries?
Well, comprehensive diagnosis of injuries to the vertebral column and the spinal cord most often require examination by means of medical imaging, namely CT and MRI scans. And that’s exactly what we’ll be studying today – medical imaging taken at the level of the C1 vertebra.
The head and the neck regions are without a doubt one of the most anatomically complex in the whole body. They’re packed with anatomic detail and they present a huge amount of structures which have very specific and intricate relationships with one another. Medical imaging has long been an extremely valuable means of characterizing these interrelationships as well as for diagnosing injuries and staging of conditions in this region.
In today’s tutorial, we’re going to be examining a number of sections taken at the level of the C1 vertebra by means of different imaging modalities. We’ll be exploring two different CT scans – CT being computed tomography as well as a comparative MRI scan which stands for magnetic resonance imaging – taken at a similar level. If you’re not already familiar with these types of medical imaging, suffice it to say that CT and MRI are completely different imaging modalities which rely on completely different principles for image acquisition, and we’ll shortly see that each has its individual roles and clinical applications as well as specific advantages and disadvantages in their usage and their results.
For instance, CT is useful to appreciate fine detail of bone, while MRI is generally more suitable for examining injuries to soft tissues such as the spinal cord, the nerves, the ligaments, and the tendon. That being said, with the right application, CT and MRI can complement each other exceptionally well in clinical practice. But a little bit more about all of this later in the tutorial.
So regardless of whether we’re looking at a cross-section, a CT or MRI scan, first and foremost, we need to be able to orientate ourselves when looking at any image, what’s anterior and posterior, what’s left and what’s right which, of course, is very important as I’m sure you can imagine.
So, a key point which may help you better understand any scan that we’re looking at is to remember that most scans are generated with the patient lying on his or her back. And this means that the lower parts of your scan or the image corresponds to the posterior side of the body, which of course means that this must be the anterior aspect of the body over here.
Another key points to keep in mind is that when we’re looking at scans, we should imagine that we’re observing them from the viewpoint of the patient’s feet and looking towards the patient’s head. So this means that the right side of the patient is to our left and the left side of the patient is to our right.
So now that we’ve orientated ourselves, it’s time to start exploring and comparing our CT and MRI images. So let’s begin with our CT scans.
So the first two images which we’re going to be exploring today are both CT scans and I’m sure you’ve already noticed that they don’t look exactly the same. So, why is that? In order to figure this out, we’re going to have to discuss a little bit of physics. But, don’t panic. It’s nothing that you can’t handle.
So, CT is all about density. In general, the denser the tissue, the brighter or whiter the tissue will appear on the final images. And the exact density measurement of each tissue is obtained using x-ray beams. The amount of x-rays absorbed by given tissue can be measured as what’s known as Hounsfield Units or HU for short. And these units are measured relative to the density of water which you can see is marked as 0 HUs on our diagram.
So to generate the CT image, HU measurements are translated into contrast, which to you and me, means somewhere between black and white on our image. And this is done by specific computer software, which produces the final image, which we see on our screens.
So, dense tissues such as bone appear white and are referred to as being radiopaque or radiodense due to their high absorption rates while tissues which are less dense such as your lungs appear dark and are said to be radiolucent as they have low absorption rates allowing x-rays to pass through the tissue without being absorbed.
You’ll notice, however, that within an open range of densities, it’s pretty difficult for our eyes to tell the difference in contrast between tissues of a relatively similar density; say, for example, water and muscle. It would be pretty difficult to tell these apart from one another. So, to overcome this issue and to visualize differences between structures within a relatively similar density range, a filtering technique known as windowing is used to help focus the final CT image to the tissues under examination. And you can think of this as a digital magnifying glass of sorts over a small range of densities.
For example, if the purpose of the scan was to examine soft tissues, the radiologist would select one from a number of windows known as soft tissue windows, which focus on a range of densities consistent with various soft tissues. So, when a window is applied, tissues below the range of density are displayed as black while tissues above the range of density, for example, bone, are displayed as white. And, likewise, if we wanted to focus only on bone and allow for a greater detail to be seen here, a window would move up to a higher range of densities allowing for greater detail of bone tissue to be seen. And since soft tissues are below the range of the bone window, these will appear as black on our scan, just as you can see here.
So let’s begin exploring our first CT scan now, which is displayed in the bone window. And since we’re using the bone window, it will come as no surprise then that the majority of structures which I’m about to identify a few will be bony or skeletal structures.
So, we’re going to start at our major point of reference for this tutorial, which is the atlas bone, also commonly known to us as vertebra C1. And the benefit of using the bone window in this particular CT scan is that it allows us to clearly define any variations in the density of the bone with the outer dense cortical bone visible here and with the inner less dense spongy bone visible here.
The bone window is also particularly useful when trying to identify fractures in the bone tissue. These features would not be near as well defined when using a non-bony or soft tissue window which we’ll have a look at a little bit later on.
So, let’s explore our CT now, and to begin, we’re going to focus a little more on the atlas bone. So, beginning anteriorly, we can identify the anterior arch over here which we know are flanked on either side but what are known as lateral masses. And projecting from each lateral mass, we have a transverse process, each of which presents a transverse foramen.
Continuing around the ring shape of the atlas, we come to the posterior arch of the atlas. At the anterior border of the vertebral foramen, we can see a separate round bony structure over here which is, of course, the dens or the odontoid process of the axis or vertebra C2. Projecting from the posterior arch of the atlas, we can also see the bifid or the two-headed spinous process of the axis bone which suggests that this section has been taken at an angle away from the axial plane.
Moving laterally now we can see another bony structure over here which is the mastoid process of the temporal bone and you’ll also notice these two inconspicuous specks of dense tissue here. These are the stylohyoid ligaments which attach to the styloid process of each temporal bone.
Continuing anteriorly now, we’re going to identify two structures belonging to the mandible. The first one is the condylar process which forms part of the temporomandibular joint as well as the coronoid process which serves as an attachment site for the temporalis muscle.
Next stop are these two bilateral structures here which are the zygomatic bones and these form a large part of what most of us know as the cheek bone.
Moving onto the anterior aspect of the head right now, we have these large paired facial bones which are the maxillae and these are separated anteriorly by the nasal bones. Within each maxilla, we can easily identify its maxillary sinus by these black areas just here which indicate air-filled spaces and along the anteromedial border of each maxilla, we can also identify the nasolacrimal ducts, which are also bounded by the small lacrimal bones which you can see here.
Continuing into the nasal cavity now, we can also see the nasal septum centrally which are this level is mostly composed by the perpendicular plate of the ethmoid bone and flanking the perpendicular plate are two more structures also belonging to the ethmoid bone and these are the middle nasal conchae.
Moving towards the posterior nasal cavity now, we can see these irregularly-shaped structures just here articulating with the posterior aspect of each maxilla, and these are the pterygoid processes of the sphenoid bone. You’ll also be able to identify the palatine bones, specifically their orbital processes. The posterior nasal aperture marks the posterior boundary of the nasal cavity which opens into the nasopharynx which is just here. And with that, we’ve identified all of the main structures of interest in the bone window of this CT scan.
So let’s move onto our next image which is also a CT scan, but as you can see, it looks somewhat different to our first example. The reason for this is that this image is filtered using a soft tissue window. As I mentioned earlier, soft tissues within the density range of this window appears various shades of gray whereas any tissues which are more dense than the selected range simply appear as white, meaning the specific detail of the bony structures is not as clear in this example, and this means that the specific detail of the bony structures is not as clear in this example.
Also important to note is that an iodine-based intravenous contrast has been administered to the patient here which is used to enhance the visibility of soft tissues; in particular, blood vessels. So, let’s quickly orientate ourselves one more time before we proceed to look at the bony structures in this CT scan.
I’m sure you have no problem identifying the atlas right here with its anterior and posterior arches as well as the lateral masses on either side. So, notice again, the tooth-like odontoid process or the dens is visible within the vertebral canal over here. Within the vertebral canal, we can just about make out a section of the spinal cord. However, you’ll see in just a short while that MRI imaging is much better differentiating between the spinal cord and the surrounding cerebrospinal fluid.
Moving anteriorly now, we can again identify the mandible in this section, specifically, each ramus as well as the alveolar processes of the maxilla, and you’ll notice a black space in the alveolar process of the right maxilla here, and this is the inferior most part of the right maxillary sinus.
Between the alveoli for the central incisors, we can see an oval-shaped opening here and this is the incisive fossa, which is also known as the nasopalatine foramen, or the anterior palatine foramen. This is the opening to the nasopalatine canal, which transits the greater palatine artery and vein from the oral to the nasal cavity and the nasopalatine nerve in the opposite direction.
And with that, we’re going to turn our attention from the bony structures of our CT scan to the soft tissues which were not identifiable in our other CT scan. So let’s begin anteriorly with this muscle which is the orbicularis oris, specifically, its superior portion and also notice the indentation of the upper lip which, us, geeky anatomists refer to as the philtrum.
Moving now to the lateral aspect of the maxilla, we have two muscles here, both of which belong to the muscles of mastication. The first is the buccinator muscle which contributes in forming the lateral walls of the oral cavity and the second is the masseter muscle, which is a larger muscle of mastication which we know works to elevate, protract, and adduct the mandible.
While we’re in this area, let’s identify another soft tissue structure over here. And do you have any idea what this is? It’s not a muscle, so can you think of what else it might be? That’s right. It’s the parotid gland. Medial to the mandible is another muscle of mastication – the medial pterygoid muscle.
And moving around to the lateral and posterior aspects of the atlas bone, we can see a number of muscles of the cervical region, so beginning with the posterior belly of the digastric muscle which has its origin in the mastoid process of the temporal bone, followed by the obliquus capitis inferior muscle which attaches to the posterior aspect of the lateral mass of the atlas, and the rectus capitis posterior major muscle, which extends between the axis and the occipital bones.
We can also identify some of the larger more superficial muscles of the neck such as the sternocleidomastoid which we can see now in green as well as the splenius capitis muscle seen here. And finally, deep to that, we have the semispinalis capitis muscle.
Of course, muscles are not the only soft tissues to be seen in this CT scan. There are several blood vessels which we can identify here too. So, if you look closely, you’ll notice that there are somewhat lighter shade of gray in our image relative to the surrounding muscle tissue, and this is due to the iodine based intravenous contrast which I mentioned earlier. And let’s see what we can identify.
So, we’re going to focus our attention around the general area anterior to the atlas here since it’s a nice point of reference for us to use in identifying vessels. And beginning here anterior to the transverse processes, we can easily identify these two structures which are, of course, the internal carotid arteries. And as you would expect, they’re flanked on either side, of course, by the internal jugular veins.
I hope you haven’t forgotten your orientation when examining CT scans, so remember we’re looking from the feet upwards, meaning that this is the right internal jugular vein and this is its counterpart on the left. Moving laterally, we can also identify the external carotid arteries over here just posterior to the mandible.
So if we look closely within the transverse foramina of the atlas bone, we should also be able to identify the vertebral arteries which are contained within here. And lateral to the tips of the transverse processes of the atlas bone, we can identify the occipital arteries. And if we continue more towards the posterior aspect of the atlas, we’ll see their venous counterparts – the occipital veins. Close to the occipital veins, we can also identify the deep cervical veins, and both of these vessels are tributaries from the external jugular vein which forms at a more inferior level of the neck.
And with that, we have examined our two CT scan examples – one using the bone window and another set to better display the soft tissues.
So this brings us nicely to the next section of this tutorial where we’re going to be comparing and contrasting our two CT scans with an MRI scan taken at the same level.
So, let’s begin our exploration of our MRI scan which is a T2-weighted image, and we’re going to first examine any bony detail visible on our image, of course, beginning first with the atlas bone.
So, as you would expect, the bony structures in our section are not near as obvious or well-defined as we observed within our CT scan, however, there’s still some detail for us to extract here. If we look closely, we can see some interesting detail in regards to this bone. So, you’ll notice a well-defined black border around the bone, and this is cortical bone. And since cortical bone tissue contains relatively little water, it does not appear bright as we saw in our CT scans. Spongy or trabecular bone, however, contains bone marrow, which is why we can visualize the lateral masses a little bit better.
Once again, we’re also able to identify the odontoid process of the axis as well as the vertebral canal and this time, however, we’re clearly able to see the difference between the cerebrospinal fluid and the spinal cord. As this is a T2 weighted image, the cerebrospinal fluid appears hyperintense relative to the spinal cord within.
Posterior to the nasal region, we can see the horizontal portion of the maxilla which contributes to the hard palate, and this is known as the palatine process of the maxilla. We should also be able to identify a section through the ramus of the mandible at this level if you can spot it. Can you spot it? Have a close look. It’s right here, but certainly not as easy to pick out compared to the previous CT scans. In MRI lingo, we describe this as being hypointense.
So MRI is all about examining soft tissues so it should be no surprise that many of the muscles and blood vessels which we identified in the earlier CT; for example, the masseter and the medial pterygoid muscles of mastication, will also be seen here. In addition, we can also identify the lateral pterygoid muscle which assists in protraction and lateral deviation of the mandible.
Continuing posteriorly, we again can identify the several muscles of the neck namely the longus capitis muscle found posterior to the oropharynx, the posterior belly of the digastric muscle seen here deep to the parotid gland, the sternocleidomastoid muscle seen here along the lateral aspect of our image, the obliquus capitis inferior muscle which is a deep muscle of the neck and located close to the atlas, and finally, the semispinalis capitis muscle which is seen here.
So now that we are familiar with both CT and MRI imaging of the spine at the level of the C1 vertebra, let’s have a look at why medical imaging can be useful in clinical practice.
So as I mentioned at the beginning of this tutorial, one of the most frequent causes of C1 fracture is diving. So, injuries to the C1 vertebra are unsurprisingly considered to be extremely serious because damage incurred at this level has the potential to cause injury to the spinal cord. This may impede or even sever communication between the brain and the rest of the body below this point resulting in quadriplegia or even proving fatal. So, fractures of the atlas most often occur after excessive axial loading on this bone, such as that often sustained during a diving accident.
And fractures of the C1 vertebra generally fall within one of four patterns of injury known as Jefferson classifications. So, type 1 fractures relate to fractures of the posterior arch only which is usually caused by axial loading combined with extension. Type 2 fractures are the opposite of type 1, meaning that they refer to fractures of the anterior arch caused by axial loading with flexion.
Type 3 fractures present fracture of both the anterior and the posterior arches, and this is known as a Jefferson burst fracture, and it is usually sustained by a direct axial loading during a fall. Just a quick note, the Jefferson burst fracture is perhaps the most well-known fracture pattern of the atlas and it was first described way back in 1920 by the British neurosurgeon, Sir Jeffrey Jefferson. And, finally, type 4 fractures refer to the fracture of one or both of the lateral masses of the atlas bone which can occur when lateral flexion of the neck occurs with axial loading.
So, since this tutorial is all about medical imaging, let’s explore how fractures to the C1 spine might appear when examined by a CT scan.
So we know by now that CT scans are particularly well suited for examining bone, so it’s no surprise that this modality is extremely useful in diagnosing fractures. So, this CT scan shown here is of a Jefferson burst type fracture of the atlas, and we can clearly identify the fractures which usually involve both the anterior and posterior arches. This fracture pattern fortunately, however, is not generally associated with neurological injury as any broken fragments of the atlas bone tend to radiate away from the spinal canal.
We also can use this type of scan to determine whether there has been damage caused to the transverse atlantal ligament which holds the odontoid process of the axis in place. Injury to this is recorded by means of an increase in the typical length of the atlantodental interval which is usually around three millimeters. Anything above six millimeters would suggest damage to this ligament. When this ligament is damage, more aggressive treatments most often surgical are usually required as the joint is considered liable to causing further damage.
So, Jefferson fractures are typically treated by means of hard collar immobilization provided that the transverse atlantal ligament is still intact. And the primary concern, of course, with all treatment plans concerning C1 fractures is to protect and maintain the stability of the cervical spinal cord.
So we saw in our CT scan that the vertebral arteries are transmitted via the transverse foramina of the C1 vertebra and this is important to note as injuries or fractures sustained to the C1 vertebra can also potentially result in neurological damage as damage to one or both vertebral arteries may interrupt arterial supply to the cerebellum and the brainstem.
And that’s it! We’ve reached the end of our tutorial on CT and MRI imaging at the level of C1 vertebra. I hope you’ve enjoyed comparing these two types of medical imaging with me, but before I leave you, I just want to quickly summarize some of the major topics which we discussed today.
So, we began our tutorial by first comparing two different CT scan which were taken at the C1 vertebral level and we discovered that the first image was filtered using a bone window, meaning it’s specifically optimized for exploring the fine detail of bony structures. Of course, our main point of reference was the atlas bone and we were able to identify the anterior and posterior arches, as well as the lateral masses and transverse foramina. Within the vertebral foramen we spotted the dens or the odontoid process of the axis bone which serves as a point of rotation for the head. Other bony features which we discovered included the maxillae with the large maxillary sinuses clearly visible within.
We later moved onto our second CT scan which instead was filtered using a soft tissue window with an iodine-based contrast used for additional definition. And this time, the bony skeleton at this level appeared bright or hyperintense and we were able, however, to define several of the soft tissues present here; for example, the masseter, the sternocleidomastoid muscle, the splenius capitis muscle, and the semispinalis capitis muscles.
In addition, we also managed to identify some of the major arteries and veins at this level such as the internal carotid arteries as well as the internal jugular veins.
And this brought us nicely onto our final section which allowed us to compare our CT scans with an MRI scan taken at the same level. And this time, we were able to appreciate the superior definition of soft tissues afforded by MRI scans.
So starting in the region of the atlas, our attention was drawn to the vertebral foramen and when we were there, we could clearly identify the spinal cord surrounded by a hyperintense border of cerebrospinal fluid which is a classic feature of a T2 weighted MRI image like this. Once again, we explored our section and we were able to define the borders of several muscles and other soft tissues such as the masseter muscle, the sternocleidomastoid muscle, the parotid gland, and the internal carotid artery.
And there you have it! Another wonderful Kenhub tutorial has come to an end. Please be sure to explore our atlas sections and challenging quizzes on these images to test your knowledge.
So thanks for watching, see you next time, and happy studying!