CT (computed tomography) scans and MRI (magnetic resonance imaging) are vital diagnostic tools, especially in cancer care. Many patients are unsure about the distinctions between these imaging methods and why one might be preferred over the other. Here, drawing upon the expertise of Maria LaGratta, Director of Radiology at MSK Monmouth, we clarify common questions patients have regarding CT scans and MRIs.
CT Scan vs. MRI: Unveiling the Core Differences
What a CT Scan Reveals
A CT scan employs X-rays to generate comprehensive images of the body’s internal structures, including organs, bones, and various tissues. During a CT scan, the patient lies on a table that smoothly moves through a donut-shaped scanning device. This scanner collects data which is then processed to create detailed, three-dimensional images. These images are instrumental in identifying abnormalities within both bone and soft tissues. For instance, a CT scan can effectively detect pneumonia in the lungs, tumors within different organs, or bone fractures.
What an MRI Reveals
MRI, similar to CT scans, provides detailed visuals of the body’s internal landscape. However, instead of X-rays, MRI utilizes radio waves and a powerful magnetic field to produce these images. Like a CT scan, an MRI involves the patient lying on a table that slides into a donut-shaped apparatus, although the MRI machine’s “donut” is typically longer. The images generated by an MRI are adept at distinguishing between healthy and diseased tissues.
The Distinct Advantages of CT Scans
MSK radiation oncologist Maria LaGratta
Maria LaGratta
CT scans offer a significant advantage in speed. A CT scan can capture images spanning from the neck down to the thighs in mere seconds. This rapid imaging capability makes CT scans exceptionally valuable for various applications including cancer diagnosis and staging, monitoring for cancer recurrence, and assessing treatment effectiveness. CT scans are particularly effective for whole-body surveys to detect cancer spread, or metastases, to organs like the lungs, liver, or bones. In fact, CT scans are often the primary method for staging cancer. Beyond cancer, CT scans are frequently used to diagnose bone fractures, internal bleeding, blood clots, spinal and brain injuries, and a range of other medical conditions.
The Unique Strengths of MRI
MRI excels in visualizing certain conditions that may be challenging or impossible to detect with a CT scan. Specific cancers, such as prostate cancer, uterine cancer, and certain types of liver cancer, are often difficult to visualize or may be invisible on CT scans. MRI is also superior in detecting metastases in the bone and brain. Furthermore, MRI is widely utilized for diagnosing issues beyond cancer, including soft tissue and joint injuries, as well as conditions affecting internal organs like the brain, heart, and digestive system.
Understanding the Disadvantages of CT Scans and MRIs
A key concern with CT scans is their use of ionizing radiation. This radiation exposure carries a minimal risk of DNA damage and a slightly increased lifetime cancer risk. The U.S. Food and Drug Administration estimates that the added risk of developing a fatal cancer from a standard CT procedure is approximately 1 in 2,000. In contrast, MRIs do not utilize ionizing radiation, eliminating this particular risk. However, MRI procedures are considerably longer than CT scans, typically requiring patients to remain still within a confined space for 20 to 40 minutes. This can be problematic for individuals with claustrophobia. Additionally, MRI machines are known to be noisy, although ear protection is usually provided to mitigate this.
Both CT and MRI scans often involve the injection of a contrast dye to enhance the visibility of organs and tissues.
Patient Concerns Regarding CT Scans and MRIs
Radiation exposure is a common concern associated with CT scans, particularly when frequent scans are necessary. For instance, patients with certain early-stage cancers, even after successful treatment, might undergo regular CT scans for monitoring. In these scenarios, the question of alternative imaging methods arises. However, for detecting cancer recurrence throughout the body, a CT scan often remains the preferred choice over MRI. Radiologists adhere to the ALARA (As Low As Reasonably Achievable) principle, ensuring that the radiation dose used in CT scans is optimized to produce diagnostically valuable images while minimizing potential risks.
The decision to use CT scans, like all medical imaging, is guided by a risk-benefit assessment, ensuring the advantages of the test outweigh potential risks.
For MRI, claustrophobia and the requirement to hold still, sometimes even hold breath for abdominal scans, can be challenging for patients. While some MRI machines are designed to reduce claustrophobic feelings, medical implants pose another consideration. The radio waves used in MRI can heat up metallic implants, which could be a concern. However, modern medical devices are typically designed to be MRI-safe.
How Doctors Determine the Best Imaging Method: CT Scan or MRI
In most cases, CT scans are the initial imaging choice. However, if a tumor is known to be better visualized on MRI, or if the specific clinical question is better addressed by MRI, then MRI may be used first. Furthermore, if a CT scan reveals an unclear finding, an MRI might be recommended for further clarification. Conversely, if a patient struggles to remain still during an MRI, compromising image quality, a CT scan might be suggested as a more practical alternative. Ultimately, the selection between CT scan and MRI is rooted in the principle of balancing the benefits of the diagnostic information against the potential risks to the patient.
Key Takeaways:
- CT scans utilize X-rays to quickly create detailed cross-sectional images of the body.
- MRI employs strong magnetic fields and radio waves to produce images of internal body structures.
- CT scans are often the first-line imaging modality due to their speed and effectiveness for a wide range of conditions.
- MRIs are particularly advantageous for visualizing soft tissues and certain cancers that are not easily seen on CT scans.
- The choice between CT scan and MRI is always determined by a careful evaluation of benefits and risks for each individual patient.
