Abstract
Bone scan and PET are routinely used in follow-up of oncology patients to evaluate metastases. The availability of these imaging modalities is largely limited to developing countries. Moreover, these modalities expose the already weakened cancer-affected patient to potentially harmful radiation.
A preliminary study was undertaken to see if whole-body imaging with MRI is able to solve this dilemma, as MRI is usually more readily available and is a radiation-free modality. MRI helps in detecting metastatic lesions before changes in bone metabolism make them detectable on bone scan (1).
Diffusion-weighted whole-body MRI with background body signal suppression (DWIBS) was performed using 1.5 T Philips MRI on patients with malignancies and the following parameters were used: TR >5000 ms, TE <70 ms, EPI factors 47, SENSE factor 2, b value 1000 s
DWIBS images were displayed with inversion of grey scale. Tumors with high cellularity possess many cellular membranes leading to restricted diffusion. Therefore the region with restricted diffusion appears bright on DWI.
As the images are displayed with inversion of grey scale, they resemble bone scan or PET images, and are well accepted by treating doctors (2, 3). DWIBS images in a normal healthy man show normal low signal appearance in brain, spinal cord, stomach, spleen, pelvis, and testes.
The advantages of DWIBS over conventional diffusion weighted imaging (3) are thin slices can be obtained, images with multiple b-values including high b-values can be acquired, multiple signal averaging is possible, enabling volumetric (3D) image rendering in any plane. Moreover, no breath-hold is needed because as DWIBS employs single-shot EPI, the acquired phase-shift due to respiratory motion is equal in each phase-encoding step and hence does not affect image formation (3).
Our initial DWIBS results in seven women (aged >40 years) having breast carcinoma have proved satisfactory and have been well accepted by the referring doctors. Moreover, DWIBS is less expensive and less time-consuming than bone scan or PET.
One of the limitations of DWIBS (3) is that abscesses can mimic malignancies. Poor anatomical details are also noted. Normal non-pathological structures liver, gall bladder, spleen, kidneys, et cetera also show up, hence basic T1, T2WI and STIR remain indispensable to act as an anatomical reference frame for the DWIBS.
Applications of DWIBS in oncology (3) are mainly for staging and monitoring response to treatment. Persistent or recurrent tumor tissue will show a more restricted diffusion than treatment-related changes, mainly because of higher cellular density of the former. Hence it is possible to differentiate persistent or recurrent tumor tissue from non-tumoral post-therapeutic change using DWIBS, although further research is indicated for proving this concept.
Our initial results prove that DWIBS is a good alternative for whole-body imaging to assess metastases and help in staging of cancer patients. It is radiation-free, widely available, and cost-efficient. Therefore, we suggest that it could be used more often, although comparative studies with bone scan and PET would help in establishing statistical details about sensitivity and specificity of each modality.