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Avascular Necrosis Diagnostic Imaging Methods
Submitted by joel on 21 March, 2009 - 23:58
Summary: 
Doctors use a number of diagnostic tools to detect and monitor the progress of Avascular Necrosis / Osteonecrosis. As a patient, it can be a little confusing trying to understand why different doctors make use of different diagnostic tools or draw seemingly detailed conclusions from something as simple as an X-ray image. This article will explain the different diagnostic tools in detail and discuss how they might be employed during the course of AVN treatment.
Detail: 
Diagnosing Avascular Necrosis
Doctors may utilize a number of diagnostic tools to confirm the presence of suspected Avascular Necrosis / Osteonecrosis. The most common methods for initial diagnosis of AVN are:
  • MRI (Magnetic Resonance Imaging)
MRI is a medical imaging technology used by radiologists to create high-contrast detailed internal views of the body. Used as a diagnostic tool for Avascular Necrosis, MRI imaging generates a series of two-dimensional cross-sections, or slices, through the bone. Each individual image slice is a view where weights and differences in tissues are represented via colorization or shading.
 
MRI offers significant benefits to Avascular Necrosis patients, especially when comparing the effectiveness of other current imaging techniques in diagnosing Avascular Necrosis. One significant difference between MRI and other diagnostic imaging techniques is that MRI does not expose the patient to radiation or other potentially harmful elements. Instead of radiation, an MRI machine utilizes three systems to capture and render images:
      • Static magnetic field (Aligns protons within water molecules of tissues).
      • Radio frequency transmitter and receiver (Excites protons in water molecules of tissues and interprets the reaction).
      • Orthogonal controllable magnetic gradients  (Used to determine the location of protons in 3D space).
When operated within set procedural guidelines, none of the three systems pose any known health risk. The only safety exception is that MRI cannot be used on patients with metallic or other non-MRI-safe implanted devices. Also, MRI is the best diagnostic method available to distinguish between living and dead (necrotic) bone, and thus it is the best diagnostic method to detect early stages of Avascular Necrosis.
 
The two primary drawbacks to MRI are high cost and long testing times. MRI equipment is a very expensive capital investment, requires continual specialized tuning and maintenance, and requires multiple highly-trained technicians to operate the machine. In addition, the process of generating diagnostic imaging of even small areas like a knee can average around 45 minutes. Add all these expense factors together and the total bill for a single MRI session can get quite high.
 
In the future, MRI technology will probably become faster and less expensive. MRI technology has already advanced such that newer machines are now able to scan multiple streams of data in parallel, reducing scan time and cost.
  • Bone Scan 
A bone scan is a medical imaging technology used by radiologists to detect changes or abnormalities in bones. A bone scan generates an x-ray-like image where high activity bone areas show as bright spots and low activity areas of bone show as darker spots. The concept behind a bone scan is that damaged, diseased, or cancerous bone will be in a state of active repair and thus will show as a high activity bright spot in the image. Conversely, other types of tumors or diseases that affect bones by slowing or stopping growth altogether, as is the case with Avascular Necrosis, will show as less active darker spots in the image.
 
Bone scan imaging has significant benefits over traditional x-ray radiography because bone scans can detect much smaller fractures or abnormalities and can produce an image with much more detail than an x-ray.  Although a bone scan can distinguish between living and dead bone and thus confirm the presence of fairly advanced Avascular Necrosis, the bone scan imaging is not sensitive enough to detect AVN if the disease in its early stages.
 
The primary potential concern about bone scans is that the process consists of injecting a radioactive fluid into a patient's bloodstream. After a few hours, this radioactive fluid collects within the bones and provides a detectable marker which is readable by a radioactivity-sensitive camera called a gamma camera. While the gamma camera does not expose the patient to any elements of concern, the radioactive marker fluid contains the equivalent radiation to that of 200 x-rays [3].
 
  • CT Scan
 
A CT, or Computed Tomography Scan, is a medical imaging technology that, like a MRI, produces images of two-dimensional cross-sections, or slices, through the body. Each individual slice is an image with varying colorization or shading, proportional to densities and differences in tissues. The process of performing a CT Scan to detect Avascular Necrosis usually consists of first injecting a contrast fluid into the blood stream while the test is being performed. The CT Scanner directs x-rays (a type of ionizing radiation) through the body and any contrast fluids into a sensitized detector. Finally, a computer interprets and processes the data streams to render the final images.
 
CT Scans are not traditionally used for the initial diagnosis of Avascular Necrosis because a CT Scan cannot distinguish between living and dead bone and thus cannot detect the early stages of AVN. Instead, a CT Scan can be a helpful diagnostic tool in detecting the physical changes in bones and surrounding tissues inherent in the later stages of AVN. Also, a CT Scan is more sensitive than traditional X-ray and is noted as an effective tool at imaging tissues with weights that differ from the weights of surrounding tissues, such as bone, the vascular system, calcifications, and cartilage.
 
Another very important factor when considering the use of CT Scans for Avascular Necrosis patients is to consider risks and benefits associated with the high levels of radiation inherent in the CT Scan process. A CT Scan can expose the patient to as much as 500 times the radiation of an X-ray. [6]. In fact, many doctors argue that the limited benefits of a CT Scan for Avascular Necrosis patients does not justify the risk, especially when considering the cumulative tally of radiation exposure over the span of treatment.
 
  • X-Ray
 
X-Ray Radiography is a medical imaging process utilizing a type of ionizing radiation for creating images of the skeleton. X-ray machines work by emitting electromagnetic radiation in a wavelength shorter than ultraviolet rays through the body and into a sensitized detector medium. Sometimes contrast compounds containing barium or iodine are used to highlight specific structures. In optimal resolutions, an x-ray produces a contrast image with 128 shades of gray [4].  X-ray imaging has been in practice for decades since its investigation around 1875. Advances in computer digitizing techniques combined with newer and better quality films have drastically reduced the levels of radiation exposure required to create an image and thus have made the process far safer when used with care. [5]
 
X-ray Radiography cannot distinguish between living and dead bone and thus is not effective at detecting the early stages of Avascular Necrosis. X-ray is of most value in AVN treatment as a tool to monitor the changes in bone surfaces and surrounding tissues once the disease has already been confirmed by another diagnostic method such as MRI.
 
 
In summary, MRI is the only diagnostic imaging method capable of detecting the early stage of Avascular Necrosis. However, cost constraints and test time requirements limit the practicality and value in utilizing MRI much beyond initial diagnosis of AVN. Bone scans, CT Scans, and x-rays are other tools that offer significant value in the diagnosis and treatment of later stage AVN.
 
 
Monitoring Avascular Necrosis Progress
MRI is the best method of detecting early stages of AVN, and MRI renders the most detailed and complete images of the AVN-afflicted necrotic bone. Consequently, many patients wonder why doctors would employ any other diagnostic imaging methods at all during treatment. Yet, the unfortunate reality is that with health care costs skyrocketing, insurance companies impose a number of difficult restrictions on the use of the very expensive MRI procedure. Thankfully, monitoring an ongoing Avascular Necrosis patient case does not normally require the extreme precision and detail of an MRI.  After initial diagnosis of AVN, most doctors monitor the case to watch and see what type of degeneration or natural healing changes occur over time. Degenerative bone changes usually present by cracking, pitting, or flattening of the bone surfaces. X-ray radiography is more than capable of identifying those common signs of degenerative changes in bone surfaces. As a result, most doctors rely on the relatively inexpensive and readily available x-ray imaging to monitor the progress of an Avascular Necrosis patient case.
 
In summary, the use of x-rays alone to monitor a patient's case is not a sign that your doctor is being overly frugal. Modern x-rays are an effective tool in diagnosing the changes a doctor needs to watch for while monitoring the progress of AVN. Also, the use of x-ray imaging is, in fact, a sign of a cost conscientious and ethically responsible doctor.
 
References
 
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