CT-MRI

A Novel Technique

Within a single MR setting, a variety of clinical questions can be answered which, until recently, required multiple and distinct examinations, usually on different days

Dr Natasha Nanda Centre Head- Piramal Diagnostic Centre

Whole-body MRI (WBMRI) is a fast, reliable, safe and accurate means of detecting disease throughout the body— a novel technique for examining the body, from head-to-toe. Within a single MR setting, a variety of clinical questions can be answered which, until recently, required multiple, distinct examinations, usually on different days.

WBMRI can show various disease processes with accuracy almost equal to that of a variety of comparison gold standard diagnostic tests. Many technical advances have rendered WBMRI a viable option. No patient handling is required due to remote movement of the imaging table from the imaging console. A rolling table platform, also called a 'bodysurf, allowing a broader view.

Ultra-fast imaging-multiple input channels allowing simultaneous use of specialised surface coils that generate high-resolution images of multiple regions of the body, without the delay of coil exchange and set-up.

How it Works

The technique is simple. Patients are placed on the table and made to move through the isocentre of the magnet bore. The thorax and the abdomen are imaged using fast breath-hold sequences in the coronal plane. After intravenous paramagnetic contrast administration, 3D gradient-echo data sets are collected in five stations from the skull to the knees.

The result is that the entire body can be imaged in a matter of 10 to 15 minutes, making the technique very suitable for rapid, highly accurate whole-body imaging in an easily tolerable time frame.

Research has shown that WBMRI had a sensitivity of 100 per cent, a specificity of 95 per cent and an accuracy of 97 per cent. The principal application of WBMRI is in detecting skeletal metastases as an alternative to skeletal scintigraphy. Given the gravity of the diagnosis of cancer, a thorough, effective and expedient means to screen for metastases is of utmost importance.

In contrast to scintigraphy, where localisation of tumor deposits is indirect and requires tumor induced activity in osteoblasts, the abundance of protons in the matrix of the tumor allows direct visualisation at MRI. In effect, rather than multi-modality staging integrating skeletal scintigraphy with CT of the chest, abdomen and pelvis at the expense of radiation dose, a single whole body MRI scan may facilitate assessment of total tumor burden, particularly in patients whose tumors spread preferentially to brain, bone and liver, such as breast and lung tumors.

Of the patients who present with skeletal metastatic disease, 15 per cent have no known primary tumour. In these patients, despite an extensive search integrating serological tests, endoscopy and imaging, a primary tumour is likely to be found in only one in five patients at a substantial cost. As an alternative, total morphological assessment of the body, as afforded by WBMRI, may allow the detection of a primary tumor as often as the other described costly approaches.

In the uncommon event of a neoplasm developing in a pregnant woman, staging information can be gained from a scan obtained by WBMRI without the ionising risk to the foetus that is incurred by both traditional approaches and PET.

MRI is a natural candidate for screening, a term that refers to the search for occult disease - a disease that has not yet become symptomatic (secondary prevention). The aim of screening is detection of disease in an early stage, which allows for more efficient therapy and may result in reduction of morbidity and mortality. MRI fulfills many requirements for a screening technique - no ionising radiation, high diagnostic accuracy and high-patient acceptance.

Benefits

WBMR angiography facilitates the visualisation of the entire arterial system from head-to-toe (with the exception of the coronary arteries). A fairly comprehensive combined protocol has been developed that achieves the depiction of the brain, the heart and the peripheral arteries from the carotids to the ankles. The implementation of 'bolus chase' techniques allows extending the coverage to visualise the entire run-off vasculature, including the pelvic, femoral, popliteal and trifurcation arteries with acquisition times of less than two minutes. However, it is noteworthy that the outlined technique, although referred to as WBMRA, does not include the intracranial or coronary arteries. Nevertheless, WBMRA offers the opportunity to integrate dedicated imaging protocols for the cerebral vasculature and additional incorporation of a cardiac imaging algorithm, but with prolongation of the study by 30 minutes.

Multiple Myeloma is characterised by neoplastic marrow infiltration. WBMRI has proven to be superior to Radiological Skeletal Survey (RSS) in staging patients with Plasma Cell Neoplasms (PCN) as well to evaluate the therapeutic impact.

Prostate cancer continues to be leading disease in cancer-related mortality. Early diagnosis of bone metastasis is important for the therapy regime and for assessing the prognosis. The standard method is bone scintigraphy. WBMRI can detect significantly more bone metastasis. Further advantages of WBMRI are additional information about extra-osseous tumor infiltration and their complications, for example stenosis of spinal canal or vertebral body fractures.

WBMRI represents an alternative to CT in the staging of lymphoma with its additional ability to evaluate for the presence or absence of disease spread to bone marrow. In contrast to CT, lymphadenopathy at MRI can be characterised on the basis of both size and signal characteristics.

People with Neurofibromatosis 1 (NF1) have multiple benign neurofibromas and a 10 per cent lifetime risk of developing Malignant Peripheral Nerve Sheath Tumours (MPNSTs). Most MPNSTs develop from benign plexiform neurofibromas, so the burden of benign tumors may be a risk factor for developing MPNST. Whole-body imaging of young NF1 patients may allow those at highest risk for developing MPNST to be identified early in life.

Obesity is increasingly recognised as a major health problem, being a known risk factor for hypertension and cardiovascular disease, cerebrovascular disease, type 2 diabetes and other forms of cancer. DEXA and CT are widely available, quick and because of the unique reproducibility of fat attenuation, suitable for automated image analysis. However, the prohibitive dose of radiation precludes its use in a general population. WBMRI allows for assessment of total and compartmental adipose tissue as well as quantification of muscle mass. The system generates results in a matter of minutes, allowing for an initial assessment to be performed, immediately after the completion of an MRI scan. All MRI tables have tabletop weight restrictions and limited bore size.

WBMRI Vs PET-CT

WBMRI appears to be a valuable alternative to PET-CT in children or in FDG-negative tumors. Unlike the latter two methods, it is without exposure to radiation and thus gaining increasing importance in paediatrics. Emerging applications of WBMRI include the evaluation for osteonecrosis, chronic multifocal recurrent osteomyelitis, myopathies, and generalised vascular malformations. For young patients in particular, in whom extensive longitudinal follow-up is anticipated, WBMRI may offer an alternative non-ionising method of disease surveillance.

WBMRI has been performed in human corpses, rendering valuable additional information on the cause of death. WBMRI could play a role in examining cases without consent to autopsy or in case of distinct risk of infection, in the gross assessment of the corpse, helping to identify sites suitable for percutaneous biopsy, particularly in immunocompromised hosts.

In the future, WBMRI could also play a role in the assessment of systemic diseases such as polymyositis or muscular dystrophy or as a screening tool for detecting axial and peripheral manifestations of spondyloarthritis. Multifocal areas of synovitis and inflammation in RA before formation of erosions, considered irreversible damage, can be studied. Role of WBMRI in the assessment of child abuse as a single non-ionising tool may be used to assess for brain, visceral and skeletal contusions.

Challenges

One of the challenges for which the radiologist must be prepared in WBMRI protocols is a dramatic increase in image data, which might result in an increase in false negative findings. Also the radiologist cannot restrict to assessing only the target structures, the 3D nature and the large field-of-view of WBMRI necessitate the work-up of all visible structures. The radiologist must assume responsibility for chance findings and the patient must consider the possibility that indistinct findings may be made and that these might have to be assessed with further, potentially invasive tests.

The Hindsight

WBMRI is poor at detecting lung nodules, root of mesentry nodes or small lymph nodes (1-6 mm) and lack adequate information on small bowel, breast, prostate and coronary arteries.

The application of a WBMRI screening protocol in a large group of presumably healthy volunteers has resulted in a very low detection rate of arterial and organ pathologies. This highlights the need for adequate pre-selection of patients. Screening studies should be tailored to look for the most common lethal diseases that afflict the general population or the types of disease that the person is at risk of acquiring.

The final disadvantage of WBMRI is cost and availability. The methods we use are not available on all systems, although most of the major vendors now offer some type of whole-body imaging capabilities. As MRI becomes more widely available, the cost of whole-body studies may lessen significantly. WBMRI, although a relatively quick technique, is still relatively slow compared with MDCT. Gating and bowel preparation, which will improve diagnostic performance, will undoubtedly add time.

natasha.nanda@pds.piramal.com