OpenCV(Version 3.2.0) - Extraction of ROI - c++

Question

I have been working on extracting the ROI from the image given below.
Fluroscopic Image
This code is supposed to work for images of all resolutions.
My approach is to:
Find the largest contour and store it's ID.
Use minEnclosingCircle() function to find the minimum bounding circle.
But, this circle is being offset to the top-left corner.
Here is my code:
#include<iostream>
#include<opencv2/opencv.hpp>
using namespace std;
using namespace cv;
int largestContourId(vector<vector<Point>> contourVec); //Function to find ID of largest contour
int main(int argc, char **argv )
{
if ((argv[1] != 0) && (argv[2] != 0))
{
String path = argv[1]; //Path of image to be given here
Mat img = imread(path); // Initialize and read image
Mat img_copy = img.clone(); //Make a copy of the orignal image
Mat img_gray; //Initialize grey image matrix
Mat thresh; //Initialize threshold mask
Mat result; //Initialize resulting image matrix
int maxAreaContourId; //This stores ID of the largest contour
Point2f center; //MEC center
float radius; //MEC radius
vector<vector<Point>> contours; // Defining contour vector
vector<Vec4i> hierarchy; //Defining contour hierarchy vector
cout << "Resolution of image: " << img.size() << endl;
cvtColor(img,img_gray,COLOR_BGR2GRAY); // Convert from colored to grayscale image
threshold(img_gray,thresh,5,255,THRESH_BINARY); //Apply threshold mask and convert to a binary image
imshow("Binary img",thresh); //Display binary image in B&W
findContours(thresh,contours,hierarchy,RETR_TREE,CHAIN_APPROX_NONE); //Find contours and hierarchy within using the Simple Approximation method
maxAreaContourId = largestContourId(contours); //Store largest contour ID
minEnclosingCircle(contours[maxAreaContourId],center,radius);
circle(img_copy,center,radius-20.0,Scalar(255,255,255),-1);
bitwise_not(img_copy,img_copy);
bitwise_xor(img,img_copy,result);
imshow("Orignal Image",img); //Show orignal image
imshow("Result",result); //Show the result of the plot
waitKey(0); //Wait for input
imwrite(argv[2],result); //Write the result to a file
destroyAllWindows();
return 0;
}
else
{
cout << "Please enter CLA in the format: *input_file* *output_file*" << endl;
return 0;
}
}
int largestContourId(vector<vector<Point>> contourVec) //Function to find ID of largest contour
{
double maxArea = 0; //Intialize area contour
int maxAreaContourId = -1; //Intialize largest contour ID
for(int j = 0; j < contourVec.size(); j++) //Cycle through contour vector
{
double newArea = contourArea(contourVec.at(j)); //Variable for comparing area
if (newArea > maxArea) //Comparison of areas
{
maxArea = newArea;
maxAreaContourId = j;
}
}
return maxAreaContourId; //Largest contour ID to be returned
}
This is my output:
Output Fluroscopic Image
I would like this code to work for all sorts of images even if the text is written on th e ROI itself.
Please let me know if my current approach is feasible or not.
Suggestions are always welcome.